Effects of rhIGF-I administration on bone turnover during short-term fasting

IGF signaling pathway in bone and cartilage development, homeostasis, and disease

The skeleton plays a fundamental role in the maintenance of organ function and daily activities. The insulin-like growth factor (IGF) family is a group of polypeptide substances with a pronounced role in osteoblast differentiation, bone development, and metabolism. Disturbance of the IGFs and the IGF signaling pathway is inextricably linked with assorted developmental defects, growth irregularities, and jeopardized skeletal structure. Recent findings have illustrated the significance of the action of the IGF signaling pathway via growth factors and receptors and its interactions with dissimilar signaling pathways (Wnt/β-catenin, BMP, TGF-β, and Hh/PTH signaling pathways) in promoting the growth, survival, and differentiation of osteoblasts. IGF signaling also exhibits profound influences on cartilage and bone development and skeletal homeostasis via versatile cell-cell interactions in an autocrine, paracrine, and endocrine manner systemically and locally. Our review summarizes the role and regulatory function as well as a potentially integrated gene network of the IGF signaling pathway with other signaling pathways in bone and cartilage development and skeletal homeostasis, which in turn provides an enlightening insight into visualizing bright molecular targets to be eligible for designing effective drugs to handle bone diseases and maladies, such as osteoporosis, osteoarthritis, and dwarfism.

Functional hypothalamic amenorrhea: Impact on bone and neuropsychiatric outcomes

Functional hypothalamic amenorrhea is a state of reversible hypogonadism common in adolescents and young women that can be triggered by energy deficit or emotional stress or a combination of these factors. Energy deficit may be a consequence of (i) reduced caloric intake, as seen in patients with eating disorders, such as anorexia nervosa, or (ii) excessive exercise, when caloric intake is insufficient to meet the needs of energy expenditure. In these conditions of energy deficit, suppression of the hypothalamic secretion of gonadotrophin-releasing hormone (with resulting hypoestrogenism) as well as other changes in hypothalamic-pituitary function may occur as an adaptive response to limited energy availability. Many of these adaptive changes, however, are deleterious to reproductive, skeletal, and neuropsychiatric health. Particularly, normoestrogenemia is critical for normal bone accrual during adolescence, and hypoestrogenemia during this time may lead to deficits in peak bone mass acquisition with longstanding effects on skeletal health. The adolescent years are also a time of neurological changes that impact cognitive function, and anxiety and depression present more frequently during this time. Normal estrogen status is essential for optimal cognitive function (particularly verbal memory and executive function) and may impact emotion and mood. Early recognition of women at high risk of developing hypothalamic amenorrhea and its timely management with a multidisciplinary team are crucial to prevent the severe and long-term effects of this condition.

Low bone mineral density in anorexia nervosa: Treatments and challenges

Anorexia nervosa, a psychiatric disease predominantly affecting women, is characterized by self- induced starvation and a resultant low-weight state. During starvation, a number of hormonal adaptations - including hypothalamic amenorrhea and growth hormone resistance - allow for decreased energy expenditure during periods of decreased nutrient intake, but these very same adaptations also contribute to the medical complications associated with chronic starvation, including low bone mass. Almost 90% of women with anorexia nervosa have bone mineral density (BMD) values more than one-standard deviation below the mean of healthy women at peak bone mineral density and this disease is associated with a significantly increased risk of fracture. Although multiple therapies have been studied for the treatment of low bone mass in anorexia nervosa, there are currently no approved therapies and few promising long-term therapeutic options. This review will outline the mediators of low bone mass in anorexia nervosa, discuss therapies that have been studied for the treatment of low BMD in this disorder, and highlight the important challenges that remain, including the differences in bone modeling in adolescents with anorexia nervosa as compared to adults, necessitating that potential therapies be tested in these two populations separately, and the paucity of long-term therapeutic strategies for treating bone loss in this disorder.

Changes in marrow adipose tissue with short-term changes in weight in premenopausal women with anorexia nervosa

OBJECTIVE

In anorexia nervosa, a psychiatric disease characterized by self-induced starvation and a model of chronic undernutrition, levels of subcutaneous (SAT) and visceral (VAT) adipose tissue are low, whereas marrow adipose tissue (MAT) levels are elevated compared to normal-weight women. The reason for this paradoxical elevation of an adipose tissue depot in starvation is not known. We sought to understand changes in MAT in response to subacute changes in weight and to compare these changes with those of other fat depots and body composition parameters.

DESIGN AND METHODS

We conducted a 12-month longitudinal study including 46 premenopausal women (n = 26 with anorexia nervosa and n = 20 normal-weight controls) with a mean (s.e.m.) age of 28.2 ± 0.8 years. We measured MAT, SAT, VAT and bone mineral density (BMD) at baseline and after 12 months.

RESULTS

At baseline, SAT (P < 0.0001), VAT (P < 0.02) and BMD of the spine and hip (P ≤ 0.0002) were significantly lower and vertebral and metaphyseal MAT (P ≤ 0.001) significantly higher in anorexia nervosa compared to controls. Weight gain over 12 months was associated with increases not only in SAT and VAT, but also epiphyseal MAT (P < 0.03). Changes in epiphyseal MAT were positively associated with changes in BMD (P < 0.03).

CONCLUSIONS

In contrast to the steady state, in which MAT levels are higher in anorexia nervosa and MAT and BMD are inversely associated, short-term weight gain is associated with increases in both MAT and BMD. These longitudinal data demonstrate the dynamic nature of this fat depot and provide further evidence of its possible role in mineral metabolism.

Effects of Anorexia Nervosa on Bone Metabolism

Anorexia nervosa is a psychiatric disease characterized by a low-weight state due to self-induced starvation. This disorder, which predominantly affects women, is associated with hormonal adaptations that minimize energy expenditure in the setting of low nutrient intake. These adaptations include GH resistance, functional hypothalamic amenorrhea, and nonthyroidal illness syndrome. Although these adaptations may be beneficial to short-term survival, they contribute to the significant and often persistent morbidity associated with this disorder, including bone loss, which affects >85% of women. We review the hormonal adaptions to undernutrition, review hormonal treatments that have been studied for both the underlying disorder as well as for the associated decreased bone mass, and discuss the important challenges that remain, including the lack of long-term treatments for bone loss in this chronic disorder and the fact that despite recovery, many individuals who experience bone loss as adolescents have chronic deficits and an increased risk of fracture in adulthood.

Bone metabolic responses to low energy availability achieved by diet or exercise in active eumenorrheic women

PURPOSE

We aimed to explore the effects of low energy availability (EA)[15 kcal·kg lean body mass (LBM)^-1·d^-1] achieved by diet or exercise on bone turnover markers in active, eumenorrheic women.

METHODS

By using a crossover design, ten eumenorrheic women (VO_{2 peak}: 48.1 ± 3.3 ml·kg^-1·min^-1) completed all three, 3-day conditions in a randomised order: controlled EA (CON; 45 kcal·kgLBM^-1·d^-1), low EA through dietary energy restriction (D-RES; 15 kcal·kgLBM^-1·d^-1) and low EA through increasing exercise energy expenditure (E-RES; 15 kcal·kgLBM^-1·d^-1), during the follicular phase of three menstrual cycles. In CON, D-RES and E-RES, participants consumed diets providing 45, 15 and 45 kcal·kgLBM^-1·d^-1. In E-RES only, participants completed supervised running sessions (129 ± 10 min·d^-1) at 70% of their VO_{2 peak} that resulted in an exercise energy expenditure of 30 kcal·kg LBM^-1·d^-1. Blood samples were collected at baseline (BASE) and at the end of the 3-day period (D6) and analysed for bone turnover markers (β-CTX and P1NP), markers of calcium metabolism (PTH, albumin-adjusted Ca, Mg and PO₄) and hormones (IGF-1, T₃, insulin, leptin and 17β-oestradiol).

RESULTS

In D-RES, P1NP concentrations at D6 decreased by 17% (BASE: 54.8 ± 12.7 μg·L^-1, D6: 45.2 ± 9.3 μg·L^-1, P < 0.001, d = 0.91) and were lower than D6 concentrations in CON (D6: 52.5 ± 11.9 μg·L^-1, P = 0.001). P1NP did not change significantly in E-RES (BASE: 55.3 ± 14.4 μg·L^-1, D6: 50.9 ± 15.8 μg·L^-1, P = 0.14). β-CTX concentrations did not change following D-RES (BASE: 0.48 ± 0.18 μg·L^-1, D6: 0.55 ± 0.17 μg·L^-1) or E-RES (BASE: 0.47 ± 0.24 μg·L^-1, D6: 0.49 ± 0.18 μg·L^-1) (condition × time interaction effect, P = 0.17). There were no significant differences in P1NP (P = 0.25) or β-CTX (P = 0.13) responses between D-RES and E-RES. Both conditions resulted in reductions in IGF-1 (-13% and - 23% from BASE in D-RES and E-RES, both P < 0.01) and leptin (-59% and - 61% from BASE in D-RES and E-RES, both P < 0.001); T₃ decreased in D-RES only (-15% from BASE, P = 0.002) and PO₄ concentrations decreased in E-RES only (-9%, P = 0.03).

CONCLUSIONS

Low EA achieved through dietary energy restriction resulted in a significant decrease in bone formation but no change in bone resorption, whereas low EA achieved through exercise energy expenditure did not significantly influence bone metabolism. Both low EA conditions elicited significant and similar changes in hormone concentrations.

Food Versus Pharmacy: Assessment of Nutritional and Pharmacological Strategies to Improve Bone Health in Energy-Deficient Exercising Women

PURPOSE OF REVIEW

The review aims to summarize our current knowledge surrounding treatment strategies aimed at recovery of bone mass in energy-deficient women suffering from the Female Athlete Triad.

RECENT FINDINGS

The independent and interactive contributions of energy status versus estrogen status on bone density, geometry, and strength have recently been reported, highlighting the importance of addressing both energy and estrogen in treatment strategies for bone health. This is supported by reports that have identified energy-related features (low body weight and BMI) and estrogen-related features (late age of menarche, oligo/amenorrhea) to be significant risk factors for low bone mineral density and bone stress injury in female athletes and exercising women. Nutritional therapy is the recommended first line of treatment to recover bone mass in energy-deficient female athletes and exercising women. If nutritional therapy fails after 12 months or if fractures or significant worsening in BMD occurs, pharmacological therapy may be considered in the form of transdermal estradiol with cyclic oral progestin (not COC).

Biochemical investigations in diagnosis and follow up of acromegaly

Measurements of human growth hormone (GH) and insulin-like growth-factor I (IGF-I) are cornerstones in the diagnosis of acromegaly. Both hormones are also used as biochemical markers in the evaluation of disease activity during treatment. Management of acromegaly is particularly challenging in cases where discordant information is obtained from measurement of GH concentrations following oral glucose load and from measurement of IGF-I. While in some patients biological factors can explain the discrepancy, in many cases issues with the analytical methods seem to be responsible. Assays used by endocrine laboratories to determine concentrations of GH and IGF-I underwent significant changes during the last decades. While generally leading to more sensitive and reproducible methods, these changes also had considerable impact on absolute concentrations measured. This must be reflected by updated decision limits, cut-offs and reference intervals. Since different commercially available assays do not agree very well, method specific interpretation of GH and IGF-I concentrations is required. This complexity in the interpretation of hormone concentrations is not always appropriately reflected in laboratory reports, but also not in clinical guidelines reporting decision limits not related to a specific analytical method. The present review provides an overview about methodological and biological variables affecting the biochemical assessment of acromegaly in diagnosis and follow up.

Immunoexpression of IGF1, IGF2, and osteopontin in craniofacial bone repair associated with autogenous grafting in rat models treated with alendronate sodium

OBJECTIVES

The aim of this study was to verify the likely influence of presurgical administration of low doses of alendronate sodium in craniofacial bone repair and correlate the histological frame found on reparative tissue to the immunohistochemical presence of IGF1, IGF2, and osteopontin (OP).

MATERIALS AND METHODS

In total, 120 rats were randomly allocated into four groups: group C (control), group OA (autogenous bone), group B (bisphosphonates), and group OA-B (autogenous bone + bisphosphonates). Groups B and OA-B received alendronate sodium (ALN) 0.01 mg/kg subcutaneously on alternate days for 4 weeks. Groups C and OA received saline solution. Critical 5-mm defects were created in rat calvaria, which were filled with blood clot in groups C and B and with autogenous bone in groups OA and OA-B. The animals were euthanized at 15 or 30 days postoperatively. Histological analysis and immunohistochemistry of IGF1, IGF2, and OP proteins was performed. Immunohistochemistry evaluated the expression in cells and extracellular matrix.

RESULTS

Groups C and B revealed healing predominantly characterized by connective tissue. In groups OA and OA-B, healing of connective tissue and neoformation of compact bone was observed. Expression of IGF1 an OP was present in all specimens. IGF1 expression in cells was more pronounced in groups OA and OA-B 15 days postoperatively. The expression of IGF2 was only observed in groups OA and OA-B, with greater intensity in group OA-B 30 days postoperatively. OP expression was only observed in cells and not in the extracellular matrix and was more pronounced in group OA 15 days postoperatively.

CONCLUSIONS

The application of systemic ALN at a dose of 0.01 mg/kg did not improve cranial bone matrix deposition. Nevertheless, the expression of IGF1 and OP and a slight marking of IGF2 were observed especially in groups OA and OA-B in the wound healing process. Future studies should assess higher doses of ALN to verify its influence on bone repair.

CLINICAL RELEVANCE

The systemic use of ALN 0.01 mg/kg on alternate days 4 weeks prior to surgery did not interfere with bone repair.

Skeletal effects of growth hormone and insulin-like growth factor-I therapy

The growth hormone/insulin-like growth factor (GH/IGF) axis is critically important for the regulation of bone formation, and deficiencies in this system have been shown to contribute to the development of osteoporosis and other diseases of low bone mass. The GH/IGF axis is regulated by a complex set of hormonal and local factors which can act to regulate this system at the level of the ligands, receptors, IGF binding proteins (IGFBPs), or IGFBP proteases. A combination of in vitro studies, transgenic animal models, and clinical human investigations has provided ample evidence of the importance of the endocrine and local actions of both GH and IGF-I, the two major components of the GH/IGF axis, in skeletal growth and maintenance. GH- and IGF-based therapies provide a useful avenue of approach for the prevention and treatment of diseases such as osteoporosis.

Insulin-Like Growth Factor-I is a Marker for the Nutritional State

Measurement of the serum concentration of insulin-like growth factor-I (IGF-l) is generally used as a screening investigation for disorders of the growth hormone (GH)/IGF-I axis in children and adolescents with short stature. IGF-I concentration is sensitive to short-term and chronic alterations in the nutritional state, and the interpretation of IGF-I measurements requires knowledge of the child's nutritional status. In this review, we summarize the effects of nutrition on the GH/IGF-I axis, and review the clinical implications of these interactions throughout childhood, both in under-nutrition and over-nutrition.

FGF21 and the late adaptive response to starvation in humans

In mice, FGF21 is rapidly induced by fasting, mediates critical aspects of the adaptive starvation response, and displays a number of positive metabolic properties when administered pharmacologically. In humans, however, fasting does not consistently increase FGF21, suggesting a possible evolutionary divergence in FGF21 function. Moreover, many key aspects of FGF21 function in mice have been identified in the context of transgenic overexpression or administration of supraphysiologic doses, rather than in a physiologic setting. Here, we explored the dynamics and function of FGF21 in human volunteers during a 10-day fast. Unlike mice, which show an increase in circulating FGF21 after only 6 hours, human subjects did not have a notable surge in FGF21 until 7 to 10 days of fasting. Moreover, we determined that FGF21 induction was associated with decreased thermogenesis and adiponectin, an observation that directly contrasts with previous reports based on supraphysiologic dosing. Additionally, FGF21 levels increased after ketone induction, demonstrating that endogenous FGF21 does not drive starvation-mediated ketogenesis in humans. Instead, a longitudinal analysis of biologically relevant variables identified serum transaminases--markers of tissue breakdown--as predictors of FGF21. These data establish FGF21 as a fasting-induced hormone in humans and indicate that FGF21 contributes to the late stages of adaptive starvation, when it may regulate the utilization of fuel derived from tissue breakdown.

Bone metabolism in adolescent girls with eating disorders and weight loss: independent effects of weight change, insulin-like growth factor-1 and oestradiol

Adolescents with eating disorders (ED) are at risk of developing osteoporosis if weight is not recovered. Previous investigations do not separate the effects of weight change per se from those of concomitant hormonal changes. In this investigation serum osteocalcin (OC), C-terminal telopeptide of collagen (CTX), insulin-like growth factor-1 (IGF-1) and oestradiol were measured at assessment of 498 girls with ED and during weight gain of 59 girls. At assessment, OC concentrations were associated independently with weight (change), IGF-1 and oestradiol. Low weight, a high rate of weight loss and the hormone concentrations were associated with low OC. Low weight and high rate of weight loss were associated with high CTX concentrations but there were no associations independent of weight (change) with the hormones. During weight recovery, OC and CTX were independently and positively associated with weight, weight gain, IGF-1 and oestradiol. Bone metabolism markers are related to weight change independently of IGF-1 and oestradiol during both weight loss and weight gain. During weight gain, when pubertal development and growth are resumed there is an additional independent positive association between the markers and IGF-1 and oestradiol. These relationships are strongest in premenarcheal girls.

Osteocyte-derived insulin-like growth factor I is not essential for the bone repletion response in mice

The present study sought to evaluate the functional role of osteocyte-derived IGF-I in the bone repletion process by determining whether deficient expression of Igf1 in osteocytes would impair the bone repletion response to one week of dietary calcium repletion after two weeks of dietary calcium deprivation. As expected, the two-week dietary calcium depletion led to hypocalcemia, secondary hyperparathyroidism, and increases in bone resorption and bone loss in both Igf1 osteocyte conditional knockout (cKO) mutants and WT control mice. Thus, conditional disruption of Igf1 in osteocytes did not impair the calcium depletion-induced bone resorption. After one week of calcium repletion, both cKO mutants and WT littermates showed an increase in endosteal bone formation attended by the reduction in osteoclast number, indicating that deficient Igf1 expression in osteocytes also did not have deleterious effects on the bone repletion response. The lack of an effect of deficient osteocyte-derived IGF-I expression on bone repletion is unexpected since previous studies show that these Igf1 osteocyte cKO mice exhibited impaired developmental growth and displayed complete resistance to bone anabolic effects of loading. These studies suggest that there is a dichotomy between the mechanisms necessary for anabolic responses to mechanical loading and the regulatory hormonal and anabolic skeletal repletion following low dietary calcium challenge. In conclusion, to our knowledge this study has demonstrated for the first time that osteocyte-derived IGF-I, which is essential for anabolic bone response to mechanical loading, is not a key regulatory factor for bone repletion after a low calcium challenge.

Anorexia nervosa and bone metabolism

Anorexia nervosa (AN) is a psychiatric disorder characterized by self-induced starvation with a lifetime prevalence of 2.2% in women. The most common medical co-morbidity in women with AN is bone loss, with over 85% of women having bone mineral density values more than one standard deviation below an age comparable mean. The low bone mass in AN is due to multiple hormonal adaptations to under nutrition, including hypothalamic amenorrhea and growth hormone resistance. Importantly, this low bone mass is also associated with a seven-fold increased risk of fracture. Therefore, strategies to effectively prevent bone loss and increase bone mass are critical. We will review hormonal adaptations that contribute to bone loss in this population as well as promising new therapies that may increase bone mass and reduce fracture risk in AN.

Determinants of GH resistance in malnutrition

States of undernutrition are characterized by GH resistance. Decreased total energy intake, as well as isolated protein-calorie malnutrition and isolated nutrient deficiencies, result in elevated GH levels and low levels of IGF1. We review various states of malnutrition and a disease state characterized by chronic undernutrition - anorexia nervosa - and discuss possible mechanisms contributing to the state of GH resistance, including fibroblast growth factor 21 and Sirtuin 1. We conclude by examining the hypothesis that GH resistance is an adaptive response to states of undernutrition, in order to maintain euglycemia and preserve energy.

Bone metabolism in anorexia nervosa

Anorexia nervosa (AN), a psychiatric disorder predominantly affecting young women, is characterized by self-imposed, chronic nutritional deprivation and distorted body image. AN is associated with a number of medical comorbidities including low bone mass. The low bone mass in AN is due to an uncoupling of bone formation and bone resorption, which is the result of hormonal adaptations aimed at decreasing energy expenditure during periods of low energy intake. Importantly, the low bone mass in AN is associated with a significant risk of fractures and therefore treatments to prevent bone loss are critical. In this review, we discuss the hormonal determinants of low bone mass in AN and treatments that have been investigated in this population.

Role of Osteocyte-derived Insulin-Like Growth Factor I in Developmental Growth, Modeling, Remodeling, and Regeneration of the Bone

The osteocyte has long been considered to be the primary mechanosensory cell in the bone. Recent evidence has emerged that the osteocyte is also a key regulator of various bone and mineral metabolism and that its regulatory effects are in part mediated through locally produced osteocyte-derived factors, such as sclerostin, receptor activator of nuclear factor-kappa B ligand (RANKL), and fibroblast growth factor (FGF)-23. Osteocytes secrete large amounts of insulin-like growth factor (IGF)-I in bone. Although IGF-I produced locally by other bone cells, such as osteoblasts and chondrocytes, has been shown to play important regulatory roles in bone turnover and developmental bone growth, the functional role of osteocyte-derived IGF-I in the bone and mineral metabolism has not been investigated and remains unclear. However, results of recent studies in osteocyte Igf1 conditional knockout transgenic mice have suggested potential regulatory roles of osteocyte-derived IGF-I in various aspects of bone and mineral metabolism. In this review, evidence supporting a regulatory role for osteocyte-derived IGF-I in the osteogenic response to mechanical loading, the developmental bone growth, the bone response to dietary calcium depletion and repletion, and in fracture repair is discussed. A potential coordinated regulatory relationship between the effect of osteocyte-derived IGF-I on bone size and the internal organ size is also proposed.

Effect of GH/IGF-1 on Bone Metabolism and Osteoporsosis

Background. Growth hormone (GH) and insulin-like growth factor (IGF-1) are fundamental in skeletal growth during puberty and bone health throughout life. GH increases tissue formation by acting directly and indirectly on target cells; IGF-1 is a critical mediator of bone growth. Clinical studies reporting the use of GH and IGF-1 in osteoporosis and fracture healing are outlined. Methods. A Pubmed search revealed 39 clinical studies reporting the effects of GH and IGF-1 administration on bone metabolism in osteopenic and osteoporotic human subjects and on bone healing in operated patients with normal GH secretion. Eighteen clinical studies considered the effect with GH treatment, fourteen studies reported the clinical effects with IGF-1 administration, and seven related to the GH/IGF-1 effect on bone healing. Results. Both GH and IGF-1 administration significantly increased bone resorption and bone formation in the most studies. GH/IGF-1 administration in patients with hip or tibial fractures resulted in increased bone healing, rapid clinical improvements. Some conflicting results were evidenced. Conclusions. GH and IGF-1 therapy has a significant anabolic effect. GH administration for the treatment of osteoporosis and bone fractures may greatly improve clinical outcome. GH interacts with sex steroids in the anabolic process. GH resistance process is considered.

IGF-I increases markers of osteoblastic activity and reduces bone resorption via osteoprotegerin and RANK-ligand

Background

Bone is one of the major target tissues for Insulin-like Growth Factor I (IGF-I). Low doses of IGF-I were able to improve liver-associated osteopenia. In the present work, a model of partial IGF-I deficiency was used in order to provide insight into the mechanisms of the beneficial actions of IGF-I replacement therapy in bone.

Methods

Several proteins involved in osteoblastic/osteocyte and osteoclastic differentiation and activity were studied in the three experimental groups: control (CO) group (wild type mice, Igf^+/+, n = 10), heterozygous Igf^+/- group with partial IGF-I deficiency (Hz, n = 10), and heterozygous Igf^+/- mice treated with IGF-I for 10 days (Hz + IGF-I, n = 10).

Results

Data in this paper confirm that the simple partial IGF-I deficiency is responsible for osteopenia, determined by densitometry and histopathology. These findings are associated with a reduced gene expression of osteoprotegerin, sclerostin, calcitonin receptor (CTR), insulin-like growth factor binding protein 5 and RUNX2. IGF-I replacement therapy normalized CTR gene expression and reduced markers of osteoclastic activity.

Conclusions

Low doses of IGF-I constituted a real replacement therapy that normalized IGF-I serum levels improving the expression of most of these proteins closely involved in bone-forming, and reducing bone resorption by mechanisms related to osteoprotegerin, RANKL and PTH receptor.

Function of matrix IGF-1 in coupling bone resorption and formation

Balancing bone resorption and formation is the quintessential component for the prevention of osteoporosis. Signals that determine the recruitment, replication, differentiation, function, and apoptosis of osteoblasts and osteoclasts direct bone remodeling and determine whether bone tissue is gained, lost, or balanced. Therefore, understanding the signaling pathways involved in the coupling process will help develop further targets for osteoporosis therapy, by blocking bone resorption or enhancing bone formation in a space- and time-dependent manner. Insulin-like growth factor type 1 (IGF-1) has long been known to play a role in bone strength. It is one of the most abundant substances in the bone matrix, circulates systemically and is secreted locally, and has a direct relationship with bone mineral density. Recent data has helped further our understanding of the direct role of IGF-1 signaling in coupling bone remodeling which will be discussed in this review. The bone marrow microenvironment plays a critical role in the fate of mesenchymal stem cells and hematopoietic stem cells and thus how IGF-1 interacts with other factors in the microenvironment are equally important. While previous clinical trials with IGF-1 administration have been unsuccessful at enhancing bone formation, advances in basic science studies have provided insight into further mechanisms that should be considered for future trials. Additional basic science studies dissecting the regulation and the function of matrix IGF-1 in modeling and remodeling will continue to provide further insight for future directions for anabolic therapies for osteoporosis.

Bone metabolism in anorexia nervosa: molecular pathways and current treatment modalities

Eating disorders are associated with a multitude of metabolic abnormalities which are known to adversely affect bone metabolism and structure. We aimed to comprehensively review the literature on the effects of eating disorders, particularly anorexia nervosa (AN), on bone metabolism, bone mineral density (BMD), and fracture incidence. Furthermore, we aimed to highlight the risk factors and potential management strategies for patients with eating disorders and low BMD. We searched the MEDLINE/OVID (1950-July 2011) and EMBASE (1980-July 2011) databases, focussing on in vitro and in vivo studies of the effects of eating disorders on bone metabolism, bone mineral density, and fracture incidence. Low levels of estrogen, testosterone, dehydroepiandrosterone, insulin-like growth factor-1 (IGF-1), and leptin, and high levels of cortisol, ghrelin, and peptide YY (PYY) are thought to contribute to the 'uncoupling' of bone turnover in patients with active AN, leading to increased bone resorption in comparison to bone formation. Over time, this results in a high prevalence and profound degree of site-specific BMD loss in women with AN, thereby increasing fracture risk. Weight recovery and increasing BMI positively correlate with levels of IGF-1 and leptin, normalisation in the levels of cortisol, as well as markers of bone formation and resorption in both adolescent and adult patients with AN. The only treatments which have shown promise in reversing the BMD loss associated with AN include: physiologic dose transdermal and oral estrogen, recombinant human IGF-1 alone or in combination with the oral contraceptive pill, and bisphosphonate therapy.

Neuroendocrine dysregulation and the growth hormone-IGF-1 axis in anorexia nervosa

Anorexia nervosa is a common psychiatric disorder characterized by extreme, self-induced starvation and is associated with a number of medical complications, including significant loss of bone mass. Disruption of the hypothalamic-pituitary axis has been demonstrated in anorexia nervosa and contributes to both loss of established bone mass in adults and failure to accrue normal bone mass in adolescents. Anorexia nervosa is associated with the development of a state of acquired growth hormone (GH) resistance, characterized by low IGF-1 and elevated GH levels, which may be mediated in part by FGF-21. Administration of supraphysiologic recombinant human GH does not result in an increase in markers of bone formation. However, treatment with recombinant human IGF-1, in combination with an oral contraceptive, increases markers of bone formation as well as bone mineral density, and may be a novel way to treat the bone loss associated with anorexia nervosa.

Obstacles in the optimization of bone health outcomes in the female athlete triad

Maintaining low body weight for the sake of performance and aesthetic purposes is a common feature among young girls and women who exercise on a regular basis, including elite, college and high-school athletes, members of fitness centres, and recreational exercisers. High energy expenditure without adequate compensation in energy intake leads to an energy deficiency, which may ultimately affect reproductive function and bone health. The combination of low energy availability, menstrual disturbances and low bone mineral density is referred to as the 'female athlete triad'. Not all athletes seek medical assistance in response to the absence of menstruation for 3 or more months as some believe that long-term amenorrhoea is not harmful. Indeed, many women may not seek medical attention until they sustain a stress fracture. This review investigates current issues, controversies and strategies in the clinical management of bone health concerns related to the female athlete triad. Current recommendations focus on either increasing energy intake or decreasing energy expenditure, as this approach remains the most efficient strategy to prevent further bone health complications. However, convincing the athlete to increase energy availability can be extremely challenging. Oral contraceptive therapy seems to be a common strategy chosen by many physicians to address bone health issues in young women with amenorrhoea, although there is little evidence that this strategy improves bone mineral density in this population. Assessment of bone health itself is difficult due to the limitations of dual-energy X-ray absorptiometry (DXA) to estimate bone strength. Understanding how bone strength is affected by low energy availability, weight gain and resumption of menses requires further investigations using 3-dimensional bone imaging techniques in order to improve the clinical management of the female athlete triad.

Areal and volumetric bone mineral density and geometry at two levels of protein intake during caloric restriction: a randomized, controlled trial

Weight reduction induces bone loss by several factors, and the effect of higher protein (HP) intake during caloric restriction on bone mineral density (BMD) is not known. Previous study designs examining the longer-term effects of HP diets have not controlled for total calcium intake between groups and have not examined the relationship between bone and endocrine changes. In this randomized, controlled study, we examined how BMD (areal and volumetric), turnover markers, and hormones [insulin-like growth factor 1 (IGF-1), IGF-binding protein 3 (IGFBP-3), 25-hydroxyvitamin D, parathyroid hormone (PTH), and estradiol] respond to caloric restriction during a 1-year trial using two levels of protein intake. Forty-seven postmenopausal women (58.0 ± 4.4 years; body mass index of 32.1 ± 4.6 kg/m(2) ) completed the 1-year weight-loss trial and were on a higher (HP, 24%, n = 26) or normal protein (NP, 18%, n = 21) and fat intake (28%) with controlled calcium intake of 1.2 g/d. After 1 year, subjects lost 7.0% ± 4.5% of body weight, and protein intake was 86 and 60 g/d in the HP and NP groups, respectively. HP compared with NP diet attenuated loss of BMD at the ultradistal radius, lumbar spine, and total hip and trabecular volumetric BMD and bone mineral content of the tibia. This is consistent with the higher final values of IGF-1 and IGFBP-3 and lower bone-resorption marker (deoxypyridinoline) in the HP group than in the NP group (p < .05). These data show that a higher dietary protein during weight reduction increases serum IGF-1 and attenuates total and trabecular bone loss at certain sites in postmenopausal women.

Effect of nutrition on plasma C-type natriuretic peptide forms in adult sheep: evidence for enhanced C-type natriuretic peptide degradation during caloric restriction

Previous studies in lambs and children show that the plasma concentration of amino terminal pro-C-type natriuretic peptide (NTproCNP), a stable product of proCNP, is strongly correlated with skeletal growth and markers of bone formation. Consistent with these findings, CNP expression is sensitive to nutritional status and is reduced by caloric restriction (CR) in both the fetus and the postnatal lamb. However, the effect of nutritional status on CNP in the adult, once linear growth is complete, is unknown. Hypothesizing that reduced CNP synthesis during CR is contingent on the presence of active growth plates, we studied the effect of CR ( 25% of maintenance) or loading (CL, 200% of maintenance) on CNP forms and alkaline phosphatase (ALP) in adult ewes and compared the findings to responses in a control group (C) fed a maintenance diet of 10.6 MJ of metabolizable energy. Live body weight was reduced (17%) in the CR group and increased (10%) in the CL group after 16 days of intervention. Plasma CNP concentration and ALP both fell in CR sheep and were significantly lower than C (P < .05 for both), returning toward basal levels 1 week after refeeding. In contrast, plasma NTproCNP did not differ (CR vs C). There were no significant changes in CNP forms and ALP in CL sheep compared with C. Fall in plasma CNP but not in NTproCNP in CR adult sheep suggests that CNP degradation (not synthesis) is altered, and contrasts with previous findings in growing lambs where CR reduces both CNP forms.

Evaluation and management of osteoporosis and fragility fractures in the elderly

Osteoporosis is characterized by low bone mass and microarchitectural deterioration that leads to increased bone fragility and fracture. The medical, psychosocial and economic burden that fragility fractures have on individuals and society is staggering. As the geriatric segment of the population continues to expand, so to will the magnitude of this epidemic. There are multiple mechanisms influencing bone quality and bone loss with age. Fragility fracture is a composite of multiple intrinsic and extrinsic factors related to the individual and their environment. Fall prevention remains the cornerstone of management in this problem. The FRAX® fracture risk assessment program, which estimates the 10-year probability of a major osteoporotic fracture, is an exciting new tool in assessing risk. Novel therapeutics, including zoledronic acid, strontium and teriparatide, are now available to complement proven osteoporosis treatments and more effectively decrease fracture risk in vulnerable individuals. Agents in Phase III trials, including denosumab and lasofoxifene, will probably increase the armamentarium of tools clinicians can use to combat the growing problem of osteoporosis and its complications.

Effects of rhIGF-1 administration on surrogate markers of bone turnover in adolescents with anorexia nervosa

BACKGROUND

Adolescents with anorexia nervosa (AN) have low bone density and low levels of surrogate markers of bone formation. Low bone density is a consequence of hormonal alterations that include hypogonadism and decreases in IGF-1, a bone trophic factor. Although IGF-1 is key to pubertal bone accretion, and effects have been demonstrated in adults, there are no data regarding the effect of recombinant human (rh) IGF-1 administration in adolescents with AN.

OBJECTIVES

We hypothesized that rhIGF-1 would cause an increase in PINP, a bone formation marker, in girls with AN, without any effect on CTX, a bone resorption marker.

SUBJECTS AND METHODS

RhIGF-1 was administered at a dose of 30-40 mcg/k twice daily to 10 consecutive girls with AN 12-18 years old for 7-9 days. Ten age-matched girls with AN were followed without rhIGF-1 for a similar period. IGF-1, PINP and CTX levels were measured.

RESULTS

RhIGF-1 administration caused an increase in IGF-1 from day-1 to day-4/5 (p<0.0001) and day-1 to day-8/9 (p<0.0001). Simultaneously, PINP increased from day-1 to day-4/5 (p=0.004) and day-1 to day-8/9 (p=0.004), with a smaller increase from day-4/5 to day-8/9 (p=0.048). CTX levels did not change with rhIGF-1 administration. No changes occurred in IGF-1 or PINP levels in girls not receiving rhIGF-1; however, CTX levels increased significantly (p=0.01). Percent change in PINP was significantly higher (p=0.02) and percent change in CTX was significantly lower (p=0.006) in girls who received rhIGF-1 compared to those who did not receive any intervention. RhIGF-1 was well tolerated without hypoglycemia.

CONCLUSION

Short-term administration of rhIGF-1 causes an increase in a surrogate bone formation markers in girls with AN without significant side effects.

Effects of taurine supplementation on bone mineral density in ovariectomized rats fed calcium deficient diet

Taurine supplementation has been shown to have a beneficial effect on femur bone mineral content in ovariectomized rats. It therefore seemed desirable to find out whether the beneficial effect of taurine on ovariectomized rats fed calcium deficient diet could also be reproduced. Forty female Sprague-Dawley rats were divided into two groups. One group was OVX and the other group received sham operation (SHAM), and received either control diet or a taurine supplemented diet for 6 weeks. All rats were fed on calcium deficient diet (AIN-93: 50% level of calcium) and deionized water. Bone mineral density (BMD) and bone mineral content (BMC) were measured in spine and femur. The serum and urine concentrations of calcium and phosphorus were determined. Bone formation was measured by serum osteocalcin and alkaline phosphatase (ALP) concentrations. Bone resorption rate was measured by deoxypyridinoline (DPD) crosslinks immunoassay and corrected for creatinine. Urinary calcium and phosphorus excretion, osteocalcin in blood and cross link value were not significantly different among the groups. Within the OVX group, the taurine supplemented group had not higher femur bone mineral content than the control group. This study established the need for a study on the taurine effect on bone with different calcium levels.

Relationship among insulinlike growth factor I concentrations, bone mineral density, and biochemical markers of bone turnover in postmenopausal women: a population-based study

OBJECTIVE

: To assess the association among serum insulinlike growth factor I (IGF-I) concentrations, bone mineral density (BMD), and biochemical markers of bone turnover in a large group of postmenopausal women from the general population.

DESIGN

: As an extension of a larger epidemiological study, the Iranian Multicentral Osteoporosis Study, a total of 406 healthy postmenopausal women (age, 59.0 +/- 7.6 years) were randomly selected from 13 clusters in Bushehr Port. IGF-I, serum CrossLaps, degradation products of C-terminal telopeptides of type I collagen, and osteocalcin were measured by highly specific enzyme-linked immunosorbent assay. BMD was determined for the lumbar spine (L2-4) and proximal femur using dual-energy x-ray absorptiometry.

RESULTS

: The mean (+/- SD) serum IGF-I concentration for all postmenopausal women was 183.35 +/- 65.60 ng/mL. In age-adjusted analyses, there was no correlation between IGF-I and BMD at the lumbar spine and femoral neck. Compared with women in the lowest quartile of IGF-I, women in the highest quartile had a significantly greater means of osteocalcin (P = 0.04) and alkaline phosphatase (P = 0.01). Analysis by quartiles of IGF-I did not reveal an association with serum CrossLaps.

CONCLUSIONS

: Circulating IGF-I is associated with biochemical markers of bone formation, but there is no relationship among IGF-I, degradation products of C-terminal telopeptides of type I collagen, and BMD in postmenopausal women. Clearly more work will be needed before serum IGF-I can be used in clinical practice as a risk predictor for postmenopause-associated loss of bone mass.

Mineral density and biomechanical properties of bone tissue from male Arctic foxes (Vulpes lagopus) exposed to organochlorine contaminants and emaciation

We investigated the impact from dietary OC (organochlorine) exposure and restricted feeding (emaciation) on bone mineral density (BMD; g hydroxy-apatite cm(-2)) in femoral, vertebrate, skull and baculum osteoid tissue from farmed Arctic blue foxes (Vulpes lagopus). For femur, also biomechanical properties during bending (displacement [mm], load [N], energy absorption [J] and stiffness [N/mm]) were measured. Sixteen foxes (EXP) were fed a wet food containing 7.7% OC-polluted minke whale (Balaenoptera acutorostrata) blubber in two periods of body fat deposition (Aug-Dec) and two periods of body fat mobilisation (Jan-July) in which the food contained less energy and only 2% blubber. SigmaOC food concentration in the food containing 7.7% whale blubber was 309 ng/g wet mass. This corresponded to a SigmaOC exposure of ca. 17 microg/kg body mass/d and a responding SigmaOC residue in subcutaneous adipose tissue of ca. 1700 ng/g live mass in the 8 EXP fat foxes euthanized after 16 months. A control group (CON) composed of 15 foxes were fed equal daily caloric amounts of clean pork (Sus scrofa) fat. After 16 months, 8 EXP and 7 CON foxes were euthanized (mean body mass=9.25 kg) while the remaining 8 EXP and 8 CON foxes were given restricted food rations for 6 months resulting in a body weight reduction (mean body mass=5.46 kg). The results showed that only BMD(skull) vs. BMD(vertebrae) were significantly correlated (R=0.68; p=0.03; n=10) probably due to a similar composition of trabecular and cortical osteoid tissue. No difference in any of the BMD measurements or femoral biomechanical properties was found between EXP and CON foxes although BMD baculum was 1.6-folds lower in the EXP group. However, lean summer foxes had significantly lower femoral biomechanical properties measured as displacement (mm), energy absorption (J) and time (s) biomechanical properties than fat winter foxes (all p<0.004). This indicates lower stiffness and softer bones from fasting which is in agreement with previous studies. Further, it should be kept in mind when studying bone tissues in Arctic mammals also in order to avoid confounding effects from body condition.

Strategies to reverse bone loss in women with functional hypothalamic amenorrhea: a systematic review of the literature

Functional hypothalamic amenorrhea (FHA) impairs the attainment of peak bone mass and as such can increase the risk of fractures later in life. To document available treatment strategies, we conducted a systematic review of the literature. We report that hormonal therapies have limited effectiveness in increasing bone mass, whereas increased caloric intake resulting in weight gain and/or resumption of menses is an essential strategy for restoring bone mass in women with FHA.

INTRODUCTION

Women with functional hypothalamic amenorrhea (FHA) may not achieve peak bone mass (PBM), which increases the risk of stress fractures, and may increase the risk of osteoporotic fractures in later life.

METHODS

To identify effective treatment strategies for women with FHA, we conducted a systematic review of the literature. We included randomized controlled trials (RCTs), cross-sectional studies, and case studies that reported on the effects of pharmacological and non-pharmacological interventions on bone mineral density (BMD) or bone turnover in women with FHA.

RESULTS

Most published studies (n=26) were designed to treat the hormonal abnormalities observed in women with FHA (such as low estrogen, leptin, insulin-like growth factor-1, and DHEA); however none of these treatments demonstrated consistent improvements in BMD. Therapies containing an estrogen given for 8-24 months resulted in variable improvements (1.0-19.0%) in BMD, but failed to restore bone mass to that of age-matched controls. Three studies reported on the use of bisphosphonates (3-12 months) in anorexic women, which appear to have limited effectiveness to improve BMD compared to nutritional treatments. Another three investigations showed no improvements in BMD after androgen therapy (DHEA and testosterone) in anorexic women. In contrast, reports (n=9) describing an increase in caloric intake that results in weight gain and/or the resumption of menses reported a 1.1-16.9% increase in BMD concomitant with an improvement in bone formation and reduction in bone resorption markers.

CONCLUSIONS

Our literature review indicates that the most successful, and indeed essential strategy for improving BMD in women with FHA is to increase caloric intake such that body mass is increased and there is a resumption of menses. Further long-term studies to determine the persistence of this effect and to determine the effects of this and other strategies on fracture risk are needed.

The sequential expression profiles of growth factors from osteoprogenitors [correction of osteroprogenitors] to osteoblasts in vitro

In this study, we delineate the sequential expression of selected growth factors associated with bone formation in vitro. Mineralization, osteocalcin, and alkaline phosphatase (ALP-2) were measured to monitor the differentiation and maturation of osteoprogenitor cells collected from C57BL mice. Bone-related growth factors, including transforming growth factor beta (TGF-beta), fibroblast growth factor 2 (FGF-2), platelet-derived growth factor (PDGF), insulinlike growth factor (IGF)-1, vascular endothelial growth factor (VEGF), bone morphogenetic protein (BMP)-2, and BMP-7, were selected. Enzyme-linked immunosorbent assay (ELISA) and reverse transcriptase polymerase chain reaction (RT-PCR) were used to measure growth factors at the protein and messenger ribonucleic acid (mRNA) level, respectively. The results found that ALP-2 expression increased progressively over time, whereas mineralization and osteocalcin did not become evident until culture day 14. VEGF and IGF-1 were upregulated early during proliferation. PDGF and TGF-beta mRNA expression was bimodal. FGF-2 and BMP-2 mRNAs were expressed only later in differentiation. FGF-2 mRNA signal levels were highest at day 14 and remained prominent through day 28 of culture. BMP-2 showed a similar profile as FGF-2. BMP-7 was not detectable using RT-PCR or ELISA. Strong correlations existed for the expression patterns between several early-response growth factors (VEGF, TGF-beta, and IGF-1) and were also evident for several late-response growth factors (BMP-2, PDGF, and FGF-2). Differential expression for grouped sets of growth factors occurs during the temporal acquisition of bone-specific markers as osteoprogenitor cell maturation proceeds in vitro.

Relationships between serum adipokines, insulin levels, and bone density in girls with anorexia nervosa

BACKGROUND

Adolescents with anorexia nervosa (AN) have low bone mineral density (BMD). Adipokines and insulin play an important role in bone metabolism in healthy individuals. However, their association with bone metabolism in AN is unknown.

OBJECTIVE

The aim of the study was to determine whether adipokines and insulin are independently associated with measures of BMD in adolescents with AN and controls.

DESIGN/METHODS

Levels of adiponectin and insulin, fasting and after oral glucose, were evaluated in 17 AN patients and 19 controls (age, 12-18 yr), in whom hormonal parameters [GH, IGF-I, cortisol, estradiol, leptin, ghrelin, and peptide YY (PYY)] had been previously determined. Body composition, bone mineral content, and BMD at the lumbar spine, hip, femoral neck, and total body were assessed by dual energy x-ray absorptiometry. Two bone formation and bone resorption markers were examined.

SETTING

The study was conducted at a General Clinical Research Center.

RESULTS

Adiponectin differed between AN subjects and controls after controlling for fat mass and decreased in both after oral glucose (P = 0.02 and 0.07). On regression modeling, independent associations were observed of: 1) body mass index and adiponectin with lumbar spine bone mineral apparent density Z-scores (r(2) = 0.45); 2) lean mass, PYY, and ghrelin with hip Z-scores (r(2) = 0.55); 3) adiponectin and lean mass with femoral neck-bone mineral apparent density Z-scores (r(2) = 0.34); and 4) lean mass, PYY, GH, and ghrelin with total body-bone mineral content/height Z-scores (r(2) = 0.64), for the combined group. Adiponectin was also independently associated with BMD, and insulin was associated with bone turnover markers in the groups considered separately.

CONCLUSIONS

Adiponectin contributes significantly to the variability of bone density, and insulin contributes to bone turnover markers in adolescent girls.

Response of plasma CNP forms to acute anabolic and catabolic interventions in growing lambs

Using a novel marker of C-type natriuretic peptide (CNP) synthesis [amino-terminal pro-CNP (NT-proCNP)], we have recently shown that plasma NT-proCNP is strongly correlated with skeletal growth and markers of bone formation and is reversibly reduced by glucocorticoids. The effects on CNP of other catabolic or anabolic factors, known to affect skeletal growth, are unknown. Accordingly, we have studied the response of plasma CNP forms to acute catabolic (caloric restriction) and anabolic [growth hormone (GH) stimulation] interventions in lambs and related the findings to circulating IGF-I levels, growth velocity, and markers of bone formation. Lambs fed a reduced caloric intake (25% of normal) for 6 days exhibited reduced live weight, plasma urea, and IGF-I (P < 0.001 for all) compared with control lambs. Basal levels of NT-proCNP (40.1 +/- 0.9 pmol/l) fell promptly to a nadir (28.1 +/- 0.8 pmol/l, P < 0.001) on day 6, returning rapidly to basal levels upon refeeding. Although plasma alkaline phosphatase (ALP) fell (P < 0.001), reductions in metacarpal growth velocity were not significant within the 12-day period of study. In contrast to caloric restriction, long-acting bovine recombinant GH (2.5 mg/kg on days 0 and 6), as expected, increased plasma IGF-I more than twofold above control for 12 days (P < 0.001). Growth velocity did not differ during the 30 days of observation, and, consistent with unchanged growth velocity, plasma NT-proCNP and ALP were also unaffected. In conclusion, CNP synthesis and markers of bone formation are acutely sensitive to catabolism but unaffected by doses of GH that fail to stimulate skeletal growth.

Anorexia nervosa and osteoporosis

Anorexia nervosa (AN), a condition of severe undernutrition, is associated with low bone mineral density (BMD) in adults and adolescents. Whereas adult women with AN have an uncoupling of bone turnover markers with increased bone resorption and decreased bone formation markers, adolescents with AN have decreased bone turnover overall. Possible contributors to low BMD in AN include hypoestrogenism and hypoandrogenism, undernutrition with decreased lean body mass, and hypercortisolemia. IGF-I, a known bone trophic factor, is reduced despite elevated growth hormone (GH) levels, leading to an acquired GH resistant state. Elevated ghrelin and peptide YY levels may also contribute to impaired bone metabolism. Weight recovery is associated with recovery of BMD but this is often partial, and long-term and sustained weight recovery may be necessary before significant improvements are observed. Anti-resorptive therapies have been studied in AN with conflicting results. Oral estrogen does not increase BMD or prevent bone loss in AN. The combination of bone anabolic and anti-resorptive therapy (rhIGF-I with oral estrogen), however, did result in a significant increase in BMD in a study of adult women with AN. A better understanding of the pathophysiology of low BMD in AN, and development of effective therapeutic strategies is critical. This is particularly so for adolescents, who are in the process of accruing peak bone mass, and in whom a failure to attain peak bone mass may occur in AN in addition to loss of established bone.

Relationship between insulin-like growth factor I, dehydroepiandrosterone sulfate and proresorptive cytokines and bone density in cystic fibrosis

INTRODUCTION

Patients with cystic fibrosis (CF) are known to be at risk for early osteoporosis, and the mechanisms that mediate bone loss are still being delineated. The aim of the present investigation was to investigate if a correlation exists in these patients between skeletal measurements by dual-energy x-ray absorptiometry (DXA) and two anabolic factors, dehydroepiandrosterone (DHEA) and insulin-like growth factor I (IGF-I), and proresorptive factors such as the cytokines interleukin-1beta, tumor necrosis factor alpha, and interleukin-6.

METHODS

We studied 32 outpatients (18 females; mean age: 26.2+/-7.9 years) at a tertiary care medical center. The subjects had venous samples obtained, underwent anthropometric and bone mineral density (BMD) measurements, and completed a health survey. Serum IGF-I concentrations were below the age-adjusted mean in 78% of the participants, and DHEA sulfate (DHEAS) concentrations were low in 72%. Serum concentrations of all cytokines were on the low side of normal; nonetheless, there was a modest inverse correlation between IL-1beta and BMD at all sites.

RESULTS

In univariate analyses, IGF-I and DHEAS were significant correlates of BMD or bone mineral content. In final multivariate models controlling for anthropometric and other variables of relevance to bone density, only IGF-I was identified as a significant independent skeletal predictor. While alterations in DHEAS, IGF-I, and specific cytokines may contribute to skeletal deficits in patients with CF, of these factors a low IGF-I concentration appears to be most strongly correlated with BMD.

CONCLUSIONS

These findings may have therapeutic implications for enhancing bone density in these patients.

Insulin-like growth factor-I and parathyroid hormone: potential new therapeutic agents for the treatment of osteoporosis

Millions of men and women worldwide are afflicted by osteoporosis. As this number is projected to increase over the next half century, attempts to forestall the disease process are being tested. Conventional therapies centre on maintaining bone mineral density (BMD) by blocking bone resorption. However, there is a growing need for drugs that can stimulate new bone formation (anabolic agents), thereby reducing future fracture risk. Both parathyroid hormone (PTH) and insulin like growth factor-I (IGF-I) have been the subject of numerous animal and human studies to determine whether these peptides have promise for the treatment of low bone mass. Although the mechanisms of action for these agents are not well delineated, each protein can activate the osteoblast and thereby enhance bone mass. The effects of these drugs on the osteoclast are less well defined. At the current time, there is considerably more interest in PTH than IGF-I, for several reasons. Firstly, animal studies with PTH are more impressive in terms of histomorphometry and biomechanics than IGF-I; secondly, PTH is specific for the skeleton whereas IGF-I is ubiquitous; and thirdly, human studies with PTH have been relatively devoid of significant side-effects. Large scale Phase III trials are now underway in the US and Europe. It is anticipated that at least one of these peptides may prove to have a role in the prevention and treatment of osteoporosis.

Human bone collagen synthesis is a rapid, nutritionally modulated process

We developed a direct assay of human bone collagen synthesis using [13C] or [15N] proline and applied it to determine the effects of feeding in young healthy men. Surprisingly, postabsorptive bone collagen synthesis is not sluggish, being approximately 0.07%/h more rapid than that of muscle protein, and capable of being stimulated within 4 h of intravenous feeding by 66 +/- 13%.

INTRODUCTION

All current methods for estimation of bone collagen turnover are indirect, depending on the assay of collagen "markers." Our aim was to develop a direct method for human bone collagen synthesis to be used to study its physiology and pathology, and specifically, in the first instance, the effect of feeding.

MATERIALS AND METHODS

We applied, over 2 h, flooding doses of [13C] and [15N] proline to label iliac crest bone collagen in eight young healthy men. The rate of collagen synthesis was determined as the rate of labeling of collagen hydroxyproline (assayed by gas chromatography-combustion-isotope ratio mass spectrometry in collagen extracted by differential solubility) compared with plasma proline labeling (assayed by gas chromatography-mass spectrometry). We also determined (in a second group of eight young healthy men) the effect of intravenous nutrition (glucose, lipid emulsion, and amino acids (in the ratio of 55%:30%:15% energy, respectively).

RESULTS

Free bone proline labeling was 92 +/- 6% of that of plasma proline, supporting the flooding dose assumption. Human iliac crest bone collagen is heterogeneous, with NaCl-EDTA, 0.5 M acetic acid, pepsin-acetic acid, and hot water-extractable pools being responsible for approximately 1%, 3%, 8%, and 81% of content, respectively. The synthetic rates were 0.58 +/- 0.1, 0.24 +/- 0.05, 0.07 +/- 0.02, and 0.06 +/- 0.01%/h, respectively, giving an average rate of approximately 0.066%/h. [13C] and [15N] proline gave identical results. Intravenous nutrition caused the disappearance of proline label from the procollagen pool and its increased appearance in the less extractable pools, suggesting nutritional stimulation of collagen processing.

CONCLUSION

The results show (1) that iliac crest bone collagen synthesis is faster than generally assumed and of the same order as muscle protein turnover and (2) that feeding increases synthesis by approximately 66%. Given its ability to detect physiologically meaningful responses, the method should provide a new approach to studying the regulation of bone collagen turnover.

Endocrine control of body composition in infancy, childhood, and puberty

Body composition exhibits marked variations across the early human lifetime. The precise physiological mechanisms that drive such developmental adaptations are difficult to establish. This clinical challenge reflects an array of potentially confounding factors, such as marked intersubject differences in tissue compartments; the incremental nature of longitudinal intrasubject variations in body composition; technical limitations in quantitating the unobserved mass of mineral, fat, water, and muscle ad seriatim; and the multifold contributions of genetic, dietary, environmental, hormonal, nutritional, and behavioral signals to physical and sexual maturation. From an endocrine perspective (reviewed here), gonadal sex steroids and GH/IGF-I constitute prime determinants of evolving body composition. The present critical review examines hormonal regulation of body composition in infancy, childhood, and puberty.

Physiological aspects and clinical sequelae of energy deficiency and hypoestrogenism in exercising women

Amenorrhoea associated with reduced caloric intake and strenuous exercise leads to hypoestrogenism and is associated with clinical manifestations that include disordered eating, stress fractures, osteoporosis, and, as recently reported, a potential increase in the risk of premature cardiovascular disease. Disordered eating, menstrual irregularities and bone loss comprise the clinical condition known as the 'female athlete triad'. The aetiology of the triad is linked to a high prevalence of disordered eating and cognitive restraint. This results in inadequate caloric intake for high exercise energy expenditures that leads to energy deficiency. This in turn stimulates compensatory mechanisms, such as weight loss or energy conservation, subsequently causing a central suppression of reproductive function and concomitant hypoestrogenism. Because the aetiology of menstrual disturbances and the female athlete triad is causally linked to energy deficiency, there is no justification for fears that exercise itself is unhealthy for women. However, improved detection, monitoring, and treatment of all components of the triad in exercising women should be emphasized. This paper critically reviews the physiological aspects and clinical sequelae of energy deficiency and hypoestrogenism associated with the female athlete triad in exercising women.

Dose-response relationships between energy availability and bone turnover in young exercising women

To help refine nutritional guidelines for military servicewomen, we assessed bone turnover after manipulating the energy availability of 29 young women. Bone formation was impaired by less severe restrictions than that which increased bone resorption. Military servicewomen and others may need to improve their nutrition to avoid these effects.

INTRODUCTION

We determined the dose-response relationship between energy availability (defined as dietary energy intake minus exercise energy expenditure) and selected markers of bone turnover in 29 regularly menstruating, habitually sedentary, young women of normal body composition.

MATERIALS AND METHODS

For 5 days in the early follicular phase of two menstrual cycles separated by at least 2 months, subjects expended 15 kcal/kgLBM/day in supervised exercise at 70% of aerobic capacity and consumed controlled amounts of a clinical dietary product in balanced (45 kcal/kgLBM/day) and one of three restricted (either 10, 20, or 30 kcal/kgLBM/day) energy availability treatments in random order. Blood was sampled at 10-minute intervals, and urine was collected for 24 h. Samples were assayed for plasma osteocalcin (OC), serum type I procollagen carboxy-terminal propeptide (PICP), and urinary N-telopeptide (NTX).

RESULTS

NTX concentrations (p < 0.01) and indices of bone resorption/formation uncoupling (Z(NTX-OC) and Z(NTX-PICP); both p < 10(-4)) were increased by the 10 kcal/kgLBM/day treatment. OC and PICP concentrations were suppressed by all restricted energy availability treatments (all p < 0.05). PICP declined linearly (p < 10(-6)) with energy availability, whereas most of the suppression of OC occurred abruptly between 20 and 30 kcal/kgLBM/day (p < 0.05).

CONCLUSIONS

These dose-response relationships closely resembled those of particular reproductive and metabolic hormones found in the same experiment and reported previously: similar relationships were observed for NTX and estradiol; for PICP and insulin; and for OC, triiodothyronine (T3), and insulin-like growth factor (IGF)-I. The uncoupling of bone resorption and formation by severely restricted energy availability, if left to continue, may lead to irreversible reductions in BMD, and the suppression of bone formation by less severe restrictions may prevent young women from achieving their genetic potential for peak bone mass. More prolonged experiments are needed to determine the dose-response relationships between chronic restrictions of energy availability and bone turnover.

Effects of recombinant human insulin-like growth factor (IGF)-I and estrogen administration on IGF-I, IGF binding protein (IGFBP)-2, and IGFBP-3 in anorexia nervosa: a randomized-controlled study

Administration of recombinant human (rh) IGF-I has been shown to have positive effects on bone density in anorexia nervosa, but the effects of rhIGF-I and estrogen on IGF binding protein (IGFBP)-2 and IGFBP-3 in anorexia nervosa are not known. Sixty-five osteopenic women with anorexia nervosa were randomized to rhIGF-I (30 micro g/kg sc twice daily) alone (n = 15), daily ethinyl estradiol (Ovcon 35) with rhIGF-I (n = 15), estradiol and placebo (n = 15), or placebo (n = 14) for 9 months. Subjects were 25.6 +/- 0.8 yr of age, low weight (body mass index 16.6 +/- 0.2 kg/m(2)) and osteopenic (T scores -2.06 +/- 0.09 for spine and -1.76 +/- 0.13 for hip). IGFBP-3 correlated with total hip bone density (r = 0.47, P = 0.0002) and was a significant predictor of hip bone density (P = 0.010) independent of IGF-I and body mass index in a multivariate regression model. During therapy, IGFBP-2 increased by 48 +/- 19 ng/ml in response to rhIGF-I and decreased by -38 +/- 22 ng/ml in response to placebo (P = 0.011). IGFBP-3 decreased (-895 +/- 120 ng/ml) in response to rhIGF-I but showed a minimal change (-53 +/- 99 ng/ml) in response to placebo (P < 0.0001). In contrast, no significant effect of estrogen was seen on IGF-I, IGFBP-2 or IGFBP-3. Among patients receiving rhIGF-I, the change in IGFBP-2 was inversely associated with the change in total hip bone density (R = -0.47, P = 0.013). In conclusion, our data suggest that chronic rhIGF-I administration increases IGF-I and IGFBP-2 and decreases IGFBP-3 in women with anorexia nervosa. IGFBP-2 and IGFBP-3 may be important determinants of bone density in this population.

Mechanisms by which nutritional disorders cause reduced bone mass in adults

Nutritional disorders that cause bone loss in adults include disordered eating behaviors (female athlete triad and anorexia nervosa), gastrointestinal diseases (celiac sprue, inflammatory bowel disease, and other malabsorption syndromes), alcoholism, and hypervitaminosis A. These disorders exert their effects on bone through a number of mechanisms, including estrogen deficiency. Deficiencies of anabolic hormones may also be important, including insulin-like growth factor I (IGF-I), a nutritionally regulated bone trophic factor. In addition, low weight itself is a risk factor for bone loss and decreased bone formation. Reduced calcium and vitamin D availability, with resultant secondary hyperparathyroidism, is another important mechanism of bone loss in nutritional disorders. This review discusses nutritional causes of reduced bone mass in adults and how nutritional disorders exert deleterious effects on the skeleton.

Long-term effects of nutrient intervention on markers of bone remodeling and calciotropic hormones in late-postmenopausal women

BACKGROUND

Adequate intakes of calcium and vitamin D reduce bone loss and fracture risk in the elderly. Other nutrients also affect bone health, and adequate intakes may influence bone turnover and balance.

OBJECTIVE

We compared the long-term effects on bone turnover markers and calciotropic hormones of a multinutrient supplement, a calcium and vitamin D supplement, and dietary instruction aimed at increasing calcium intake through foods.

DESIGN

Ninety-nine healthy postmenopausal women participated in a 3-y, randomized trial, receiving either 1) supplemental calcium (1450 mg/d) and vitamin D [10 microg (400 IU)/d], 2) calcium, vitamin D, and other nutrients (multinutrient supplement), or 3) dietary instruction (dietary control group). Data are from 83 subjects who completed the trial.

RESULTS

Increases over baseline in calcium intakes and serum 25-hydroxyvitamin D concentrations were sustained over 3 y in all treatment groups. Circulating parathyroid hormone concentrations were reduced at year 1 in all treatment groups but trended toward baseline thereafter. Bone turnover markers followed a similar pattern, and none of the changes in biochemical concentrations differed significantly between groups.

CONCLUSIONS

All 3 interventions offer long-term feasibility for increasing calcium intake and serum 25-hydroxyvitamin D concentrations. The dietary addition of micronutrients implicated in skeletal physiology confers no obvious bone-sparing effect in healthy postmenopausal women beyond that of calcium and vitamin D alone. The attenuation over time in suppression of parathyroid hormone and bone turnover might help explain why nutrient intervention tends to have less of a bone-sparing effect than do skeletally active medications such as estrogen or bisphosphonates.

Effects of recombinant human IGF-I and oral contraceptive administration on bone density in anorexia nervosa

Over 90% of women with anorexia nervosa demonstrate osteopenia, and almost 40% demonstrate osteoporosis at one or more skeletal sites. In addition to estrogen deficiency causing an increase in bone resorption, nutritional effects on the GH-IGI-I axis may contribute to the severe bone loss in this population by decreasing bone formation. We tested the hypothesis that recombinant human IGF-I (rhIGF-I) would increase bone density in women with anorexia nervosa and furthermore assessed the effects of combined rhIGF-I and oral contraceptive administration (OCP) in this population. Sixty osteopenic women with Diagnosis and Statistical Manual of Mental Disorders IV Revised confirmed anorexia nervosa [age (25.2 +/- 0.7 yr, range 18-38 yr), body mass index (17.8 +/- 0.3 kg/m(2) ), spinal bone mineral density T score (-2.1 +/- 0.1 SD) were randomized to one of four treatment groups [rhIGF-I (30 microg/kg sc twice daily) and a daily oral contraceptive (Ovcon 35, 35 microg ethinyl estradiol and 0.4 mg norethindrone], rhIGF-I alone (30 microg/kg sc twice daily), oral contraceptive alone, or neither treatment for 9 months. All subjects received calcium 1500 mg/d and a standard multivitamin containing 400 IU of vitamin D. Administration of rhIGF-I was placebo controlled and blinded to subjects. The rhIGF-I was titrated to maintain IGF-I levels within the age-adjusted normal range for each patient and was well tolerated. The effects of rhIGF-I and OCP were analyzed simultaneously among all subjects in a factorial analysis and in an analysis of the four individual treatment groups. Anteroposterior spinal bone density increased significantly in response to rhIGF-I (1.1% +/- 0.5% vs. -0.6% +/- 0.8%, P = 0.05, all rhIGF-I vs. all placebo treated, respectively, by analysis of covariance). In contrast, OCP did not result in increased bone density (0.8% +/- 0.6% vs. -0.4% +/- 0.8%, P = 0.21, all OCP vs. all non-OCP treated, respectively, by analysis of covariance). However, bone density increased to the greatest extent in the combined treatment group (rhIGF-I and OCP), compared with control patients receiving no active therapy (1.8% +/- 0.8% vs. 0.3% +/- 0.6% vs. -0.2% +/- 0.8% vs. -1.0% +/- 1.3%, rhIGF-I and OCP vs. rhIGF-I alone vs. OCP alone vs. no active therapy, P < 0.05 for rhIGF-I and OCP vs. no active therapy). These data demonstrate that osteopenic women with anorexia nervosa treated with rhIGF-I showed more beneficial changes in bone density, compared with patients not treated with rhIGF-I. Antiresorptive therapy with OCP is not sufficient to improve bone density in undernourished patients, but such therapy may augment the effects of rhIGF-I in a combined treatment strategy. Further long-term studies are needed to investigate the effects of rhIGF-I and combined anabolic/antiresorptive strategies on bone in women with anorexia nervosa.