Bone resorption is decreased postprandially by intestinal factors and glucagon-like peptide-2 is a possible candidate

Gut hormones and bone homeostasis: potential therapeutic implications

Bone resorption follows a circadian rhythm, with a marked reduction in circulating markers of resorption (such as carboxy-terminal telopeptide region of collagen type I in serum) in the postprandial period. Several gut hormones, including glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide 1 (GLP1) and GLP2, have been linked to this effect in humans and rodent models. These hormones are secreted from enteroendocrine cells in the gastrointestinal tract in response to a variety of stimuli and effect a wide range of physiological processes within and outside the gut. Single GLP1, dual GLP1-GIP or GLP1-glucagon and triple GLP1-GIP-glucagon receptor agonists have been developed for the treatment of type 2 diabetes mellitus and obesity. In addition, single GIP, GLP1 and GLP2 analogues have been investigated in preclinical studies as novel therapeutics to improve bone strength in bone fragility disorders. Dual GIP-GLP2 analogues have been developed that show therapeutic promise for bone fragility in preclinical studies and seem to exert considerable activity at the bone material level. This Review summarizes the evidence of the action of gut hormones on bone homeostasis and physiology.

Gut-Derived Peptide Hormone Analogues and Potential Treatment of Bone Disorders in Obesity and Diabetes Mellitus

Obesity and diabetes mellitus are prevalent metabolic disorders that have a detrimental impact on overall health. In this regard, there is now a clear link between these metabolic disorders and compromised bone health. Interestingly, both obesity and diabetes lead to elevated risk of bone fracture which is independent of effects on bone mineral density (BMD). In this regard, gastrointestinal (GIT)-derived peptide hormones and their related long-acting analogues, some of which are already clinically approved for diabetes and/or obesity, also seem to possess positive effects on bone remodelling and microarchitecture to reduce bone fracture risk. Specifically, the incretin peptides, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), as well as glucagon-like peptide-2 (GLP-2), exert key direct and/or indirect benefits on bone metabolism. This review aims to provide an initial appraisal of the relationship between obesity, diabetes and bone, with a focus on the positive impact of these GIT-derived peptide hormones for bone health in obesity/diabetes. Brief discussion of related peptides such as parathyroid hormone, leptin, calcitonin and growth hormone is also included. Taken together, drugs engineered to promote GIP, GLP-1 and GLP-2 receptor signalling may have potential to offer therapeutic promise for improving bone health in obesity and diabetes.

Signs of a Glucose- and Insulin-Independent Gut-Bone Axis and Aberrant Bone Homeostasis in Type 1 Diabetes

CONTEXT

Gut hormones seem to play an important role in postprandial bone turnover, which also may be affected by postprandial plasma glucose excursions and insulin secretion.

OBJECTIVE

To investigate the effect of an oral glucose tolerance test (OGTT) and an isoglycemic intravenous glucose infusion (IIGI) on bone resorption and formation markers in individuals with type 1 diabetes and healthy controls.

METHODS

This observational case-control study, conducted at the Center for Clinical Metabolic Research, Gentofte Hospital, Hellerup, Denmark, included 9 individuals with C-peptide negative type 1 diabetes and 8 healthy controls matched for gender, age, and body mass index. Subjects underwent an OGTT and a subsequent IIGI. We analyzed changes in bone resorption assessed by measurements of carboxy-terminal type I collagen crosslinks (CTX) and in bone formation as assessed by procollagen type I N-terminal propeptide (PINP) concentrations.

RESULTS

Baseline CTX and PINP levels were similar in the 2 groups. Both groups exhibited significantly greater suppression of CTX during OGTT than IIGI. PINP levels were unaffected by OGTT and IIGI, respectively, in healthy controls. Participants with type 1 diabetes displayed impaired suppression of CTX-assessed bone resorption and inappropriate suppression of PINP-assessed bone formation during OGTT.

CONCLUSION

Our data suggest the existence of a gut-bone axis reducing bone resorption in response to oral glucose independently of plasma glucose excursions and insulin secretion. Subjects with type 1 diabetes showed impaired suppression of bone resorption and reduced bone formation during OGTT, which may allude to the reduced bone mineral density and increased fracture risk characterizing these individuals.

Minimally invasive biomarkers in human and non-human primate evolutionary biology: Tools for understanding variation and adaptation

BACKGROUND

The use of minimally invasive biomarkers (MIBs - physiological biomarkers obtained from minimally invasive sample types) has expanded rapidly in science and medicine over the past several decades. The MIB approach is a methodological strength in the field of human and non-human primate evolutionary biology (HEB). Among humans and our closest relatives, MIBs provide unique opportunities to document phenotypic variation and to operationalize evolutionary hypotheses.

AIMS

This paper overviews the use of MIBs in HEB. Our objectives are to (1) highlight key research topics which successfully implement MIBs, (2) identify promising yet under-investigated areas of MIB application, and (3) discuss current challenges in MIB research, with suggestions for advancing the field.

DISCUSSION AND CONCLUSIONS

A range of MIBs are used to investigate focal topics in HEB, including energetics and life history variation/evolution, developmental plasticity, and social status and dominance relationships. Nonetheless, we identify gaps in existing MIB research on traits such as physical growth and gut function that are central to the field. Several challenges remain for HEB research using MIBs, including the need for additional biomarkers and methods of assessment, robust validations, and approaches that are standardized across labs and research groups. Importantly, researchers must provide better support for adaptation and fitness effects in hypothesis testing (e.g., by obtaining complementary measures of energy expenditure, demonstrating redundancy of function, and performing lifetime/longitudinal analyses). We point to continued progress in the use of MIBs in HEB to better understand the past, present, and future of humans and our closest primate relatives.

Reduced postprandial bone resorption and greater rise in GLP-1 in overweight and obese individuals after an α-glucosidase inhibitor: a double-blinded randomized crossover trial

When taken with a meal, α-glucosidase inhibitors (α-GI) reduce the rise in postprandial glucose and increase glucagon-like peptide-1 (GLP-1), and this may lower bone turnover. In this study, a salacinol-type α-GI increased GLP-1 and markedly reduced postprandial bone resorption compared to placebo, suggesting it could have implications for bone health.

INTRODUCTION

Animal and clinical trials indicate that α-glucosidase inhibitors attenuate postprandial glycemic indices and increase secretion of GLP-1. In addition, GLP-1 acts on bone by inhibiting resorption. The goal in this study was to determine if a salacinol α-GI alters postprandial bone turnover and can be explained by changes in serum GLP-1.

METHODS

In this double-blind, placebo-controlled crossover study, healthy overweight/obese adults (body mass index 29.0 ± 3.8 kg/m²; 21-59 years; n = 21) received a fixed breakfast and, in random order, were administered Salacia chinensis (SC; 500 mg) or placebo. A fasting blood sample was taken before and at regular intervals for 3 h after the meal. Serum was measured for bone turnover markers, C-terminal telopeptide of type I collagen (CTX) and osteocalcin, and for glycemic indices and gut peptides.

RESULTS

Compared to placebo, SC attenuated the bone resorption marker, CTX, at 60, 90, and 120 min (p < 0.05) after the meal, and decreased osteocalcin, at 180 min (p < 0.05). As expected, SC attenuated the postprandial rise in glucose compared with placebo, whereas GLP-1 was increased at 60 min (p < 0.05) with SC. Serum GLP-1 explained 41% of the variance for change in postprandial CTX (p < 0.05).

CONCLUSION

This study indicates that attenuating postprandial glycemic indices, with an α-GI, markedly decreases postprandial bone resorption and can be explained by the rise in GLP-1. Future studies should determine whether longer term α-GI use benefits bone health.

Analysis of Body Perception, Preworkout Meal Habits and Bone Resorption in Child Gymnasts

The beneficial effects of physical activity on body image perception and bone are debated among artistic gymnasts. Gymnasts seem to be at greater risk of developing body dissatisfaction, eating disorders and osteoporosis due to inadequate nutrition and attention to the appearance of the body. The objective of this work was to investigate the association between the artistic gymnast and a more favorable body image compared to their sedentary peers and if a preworkout high-carbohydrate meal (HCM; 300 kcal, 88% carbohydrates, 9% protein, 3% fat) or high-protein meal (HPM; 300 kcal, 55% carbohydrates, 31% protein, 13% fat) is able to attenuate bone resorption in young rhythmic gymnasts. Twenty-eight preadolescent female gymnasts were examined. Self-esteem tests were used to analyze body image perception. Preworkout eating habits were examined by short food frequency questions (FFQ) validated for children. The biomarker of the bone resorption C-terminal telopeptide region of collagen type 1 (CTX) was measured in the urine (fasting, postmeal and postworkout). Gymnasts reported higher satisfaction with their body appearance compared to sedentary peers. Of the gymnasts, 30% did not have a preworkout meal regularly, and the timing of the consumption was variable. Bone resorption was decreased by the HCM, consumed 90 min before the training, with respect to the HPM. The study suggests that playing artistic gymnastics is associated with a positive body self-perception in a child. The variability in preworkout meal frequency and timing need attention to prevent inadequate eating habits in light of the ability of the HCM to reduce acute bone resorption.

Preserved postprandial suppression of bone turnover markers, despite increased fasting levels, in postmenopausal women

CONTEXT

Menopause leads to an increased bone turnover associated with a high risk of fractures. Bone turnover is inhibited by meal intake, to some extent mediated by gut hormones, and interventions based on these endocrine changes may have potential in future prevention of osteoporosis.

OBJECTIVE

To investigate whether postmenopausal women exhibit postprandial suppression of bone turnover markers to the same extent as premenopausal women, despite higher fasting levels. Furthermore, to assess whether menopausal differences in bone turnover markers are related to postmenopausal changes in plasma gut hormone levels.

METHODS

A cross-sectional study of 21 premenopausal, 9 perimenopausal, and 24 postmenopausal women between 45 and 60 years of age. Serum/plasma levels of bone turnover markers and gut hormones were investigated during a 120 min oral glucose tolerance test. Bone turnover markers included N-terminal propeptide of type-I procollagen (PINP, bone formation marker) and carboxyterminal collagen I crosslinks (CTX-I, bone resorption marker). Gut hormone secretion was evaluated from responses of glucagon-like peptide-1 (GLP-1), glucagon-like peptide-2 (GLP-2) and glucose-dependent insulinotropic polypeptide (GIP).

RESULTS

Fasting levels of s-CTX-I were increased in peri- and postmenopausal women compared to premenopausal women (p = 0.001). Despite higher fasting levels, the relative postprandial s-CTX-I suppression was comparable across menopausal status (p = 0.14). Fasting levels of s-PINP were also increased in postmenopausal women compared to premenopausal women (p < 0.001) with comparable and modest s-PINP suppression over menopause (p = 0.13). Postprandial plasma GLP-1 (p = 0.006) and GLP-2 (p = 0.01) were significantly increased in postmenopausal women compared to premenopausal women while GIP responses were slightly increased in the perimenopausal group (p = 0.02) but comparable between pre- and postmenopausal women. None of the postprandial gut hormone increases predicted postprandial bone turnover suppression in these women.

CONCLUSIONS

Glucose-induced suppression of bone turnover markers is preserved in postmenopausal women, despite significantly higher fasting values, indicating that CTX-I lowering treatments based on these postprandial mechanisms might be a feasible strategy to prevent postmenopausal osteoporosis.

Consumption of nutrients and insulin resistance suppress markers of bone turnover in subjects with abdominal obesity

OBJECTIVE

Abdominal obesity and type 2 diabetes are associated with insulin resistance and low bone turnover along with an increased fracture risk. The mode of action is poorly understood. The bone resorption marker, C-terminal telopeptide type 1 collagen (CTX), and to a lesser extent, the bone formation marker, Procollagen type 1 N-terminal propeptide (P1NP) appear to be inhibited by food consumption. The link between food consumption, insulin resistance and bone turnover remains to be clarified. Primarily we aimed to compare the postprandial CTX, P1NP and PTH responses by two frequently applied methods in assessing metabolic health; oral glucose tolerance test (OGTT) and mixed meal tolerance test. Secondly, we explored the effect of insulin resistance on bone marker responses.

METHODS

We enrolled 64 subjects with abdominal obesity. Following 10 h of fasting, subjects initially underwent a standard OGTT (300 kcal) and approximately one week later a mixed meal tolerance test (1130 kcal). Circulating CTX, P1NP and PTH were assessed on both days at time = 0, after 30 min and after 90 min for comparison of the two interventions. We analyzed glucose and insulin levels for the assessment of insulin resistance. Additionally, we measured plasma calcium levels along with the gut hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like-peptide 2 (GLP-2) in an attempt to identify possible mediators of the postprandial bone response.

RESULTS

CTX, P1NP and PTH were suppressed by OGTT and the mixed meal; the latter induced a more pronounced suppression after 90 min. Calcium levels were similar between OGTT and meal. GIP and GLP-2 levels increased after both interventions, although only the meal induced a sustained increase after 90 min. Fasting P1NP was inversely associated with insulin resistance. The meal-induced suppression of P1NP (but not CTX or PTH) was inversely associated with level of insulin resistance.

CONCLUSION

The acute postprandial suppression of bone turnover markers is extended after ingestion of a mixed meal compared to an OGTT. The response appears to be independent of gender and prompted by a reduction in PTH. The study additionally indicates a possible link between the development of insulin resistance and low bone turnover - which may be of key essence in the development of the fragile bone structure and increased fracture risk demonstrated in subjects with abdominal obesity and T2D.

Nutrient Intake Prior to Exercise Is Necessary for Increased Osteogenic Marker Response in Diabetic Postmenopausal Women

Type 2 diabetes increases bone fracture risk in postmenopausal women. Usual treatment with anti-resorptive bisphosphonate drugs has some undesirable side effects, which justified our interest in the osteogenic potential of nutrition and exercise. Since meal eating reduces bone resorption, downhill locomotion increases mechanical stress, and brief osteogenic responsiveness to mechanical stress is followed by several hours of refractoriness, we designed a study where 40-min of mechanical stress was manipulated by treadmill walking uphill or downhill. Exercise preceded or followed two daily meals by one hour, and the meals and exercise bouts were 7 hours apart. Fifteen subjects each performed two of five trials: No exercise (SED), uphill exercise before (UBM) or after meals (UAM), and downhill exercise before (DBM) or after meals (DAM). Relative to SED trial, osteogenic response, defined as the ratio of osteogenic C-terminal propeptide of type I collagen (CICP) over bone-resorptive C-terminal telopeptide of type-I collagen (CTX) markers, increased in exercise-after-meal trials, but not in exercise-before-meal trials. CICP/CTX response rose significantly after the first exercise-after-meal bout in DAM, and after the second one in UAM, due to a greater CICP rise, and not a decline in CTX. Post-meal exercise, but not the pre-meal exercise, also significantly lowered serum insulin response and homeostatic model (HOMA-IR) assessment of insulin resistance.

Glucagon like peptide 2 has a positive impact on osteoporosis in ovariectomized rats

AIMS

In this study, we evaluate the effects of glucagon-like peptide 2 (GLP-2) on bone microarchitecture, bone turnover markers (BTMs) and inflammation markers in ovariectomized (OVX) rats.

MATERIAL AND METHODS

In total, 31 Sprague-Dawley rats were divided into the following three groups: sham (control sham-operated with vehicle, n = 7), OV (OVX with vehicle, n = 12), and GLP-2 (OVX with GLP-2, n = 12). Intervention began at the 12th week after surgery and lasted for 4 weeks. The dosage of the GLP-2 was 160 μg/kg/d through subcutaneous injections, and normal saline was used as the vehicle agent. After 4 weeks of treatment, serum BTM and inflammation marker levels were measured by ELISA, and femora samples were analyzed by qRT-PCR, micro-CT, histology and histomorphometry.

KEY FINDINGS

After 4 weeks of treatment, serum TRAcP-5b and RANKL levels as well as the CTX-1/P1NP ratio in the GLP-2 group decreased, and ALP activity, P1NP level, and OPG/RANKL ratio increased significantly; qRT-PCR analysis showed that mRNA levels of RANKL decreased, and Runx2, ALP, and Col-1 levels as well as the OPG/RANKL ratio increased significantly in the GLP-2 group compared with the OV group. In bone histology analysis, GLP-2 significantly decreased the AV/MV, Oc.N and Oc.S but increased the Ob.N, BFR and MAR. Analysis with μ-CT showed that the BMD, BV/TV, Tb.N and Conn.D increased significantly in the GLP-2 group compared with the OV group. The levels of serum inflammation markers TNF-α, IL-1β and IL-6 decreased, and TGF-β levels increased in the GLP-2 group compared with the OV group.

SIGNIFICANCE

GLP-2 may have a positive impact on osteoporosis by promoting bone formation, inhibiting bone resorption and decreasing circulatory inflammation in ovariectomized rats.

Gut Hormones and Their Effect on Bone Metabolism. Potential Drug Therapies in Future Osteoporosis Treatment

Bone homeostasis displays a circadian rhythm with increased resorption during the night time as compared to day time, a difference that seems-at least partly-to be caused by food intake during the day. Thus, ingestion of a meal results in a decrease in bone resorption, but people suffering from short bowel syndrome lack this response. Gut hormones, released in response to a meal, contribute to this link between the gut and bone metabolism. The responsible hormones appear to include glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), known as incretin hormones due to their role in regulating glucose homeostasis by enhancing insulin release in response to food intake. They interact with their cognate receptors (GIPR and GLP-1R), which are both members of the class B G protein-coupled receptors (GPCRs), and already recognized as targets for treatment of metabolic diseases, such as type 2 diabetes mellitus (T2DM) and obesity. Glucagon-like peptide-2 (GLP-2), secreted concomitantly with GLP-1, acting via another class B receptor (GLP-2R), is also part of this gut-bone axis. Several studies, including human studies, have indicated that these three hormones inhibit bone resorption and, moreover, that GIP increases bone formation. Another hormone, peptide YY (PYY), is also secreted from the enteroendocrine L-cells (together with GLP-1 and GLP-2), and acts mainly via interaction with the class A GPCR NPY-R2. PYY is best known for its effect on appetite regulation, but recent studies have also shown an effect of PYY on bone metabolism. The aim of this review is to summarize the current knowledge of the actions of GIP, GLP-1, GLP-2, and PYY on bone metabolism, and to discuss future therapies targeting these receptors for the treatment of osteoporosis.

Risk factors for loss of bone mineral density after curative esophagectomy

Micronutrient and fat malabsorption and altered enteroendocrine signaling occur after esophagectomy for cancer; however, the impact of malnutrition on bone health in this cohort has not been previously investigated. In this study, the prevalence of osteoporosis increased after curative surgery, associated with disease-specific, treatment-related, and population risk factors.

PURPOSE

Improved oncologic outcomes in esophageal cancer (EC) have resulted in increased survivorship and a focus on long-term quality of life. Malnutrition and micronutrient malabsorption are common among patients with EC, but the effect on bone metabolism is not known. The aim of this study was to characterize changes in bone mineral density (BMD) following curative esophagectomy.

METHODS

Consecutive disease-free patients who underwent esophagectomy with gastric conduit for pathologically node-negative disease from 2000 to 2014 were included. BMD was assessed at vertebral levels T12-L5 by computed tomography using a simple trabecular region-of-interest attenuation technique, and serum markers of nutritional status and bone metabolism were examined. Independent risk factors for osteoporosis were identified by multivariable logistic regression.

RESULTS

Seventy-five consecutive patients were studied. Osteoporosis was present in 25% at diagnosis. BMD declined at 1 and 2 years postoperatively (144.3 ± 45.8 versus 128.6 ± 46.2 and 122.7 ± 43.5 Hounsfield Units (HU), P < 0.0001), with increased osteoporosis prevalence to 38% and 44% (P = 0.049), respectively. No significant postoperative change in vitamin D, calcium, or phosphate was observed, but alkaline phosphatase increased significantly (P < 0.001). While female sex (P = 0.004) and ASA grade (P = 0.043) were independently associated with osteoporosis at diagnosis, age (P = 0.050), female sex (P = 0.023), smoking (P = 0.024), and pathologic T stage (P = 0.023) were independently predictive of osteoporosis at 1 year postoperatively.

CONCLUSIONS

Osteoporosis is prevalent among disease-free patients post-esophagectomy for EC, associated with disease-specific, treatment-related, and population risk factors. Strategies which minimize BMD decline should be considered to avoid fragility fractures in this cohort.

Increased Body Weight and Fat Mass After Subchronic GIP Receptor Antagonist, but Not GLP-2 Receptor Antagonist, Administration in Rats

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) are hormones secreted from the enteroendocrine cells after a meal. They exert their actions through activation of G protein-coupled receptors (R), the GIPR and GLP-2R, respectively. Both have been reported to influence metabolism. The purpose of the study was to investigate the role of the hormones in the regulation of lipid and bone homeostasis by subchronic treatment with novel GIPR and GLP-2R antagonists. Rats were injected once daily with vehicle, GIPR, or GLP-2R antagonists for 3 weeks. Body weight, food intake, body composition, plasma lipoprotein lipase (LPL), adipokines, triglycerides and the marker of bone resorption carboxy-terminal collagen crosslinks (CTX), were examined. In rats, subchronic treatment with GIPR antagonist, rat GIP (3-30)NH₂, did not modify food intake and bone resorption, but significantly increased body weight, body fat mass, triglycerides, LPL, and leptin levels compared with vehicle treated rats. Subchronic (Pro3)GIP (a partial GIPR agonist), GLP-2(11-33), and GLP-2(3-33) (GLP-2R antagonists) treatment did not affect any parameter. The present results would be consistent with a role for GIP, but not GLP-2, in the maintenance of lipid homeostasis in rats, while neither GIPR nor GLP-2R antagonism appeared to influence bone resorption in rats.

The effect of meals on bone turnover - a systematic review with focus on diabetic bone disease

INTRODUCTION

Type 2 diabetes is associated with an increased risk of bone fractures. Bone mineral density (BMD) is increased and bone turnover is low in type 2 diabetes and the increased BMD does not explain the increased fracture risk. However, the low bone turnover may lead to insufficient bone renewal with unrepaired micro-cracks and thus increase fracture risk. Ingestion of food acutely decreases bone resorption markers and the macronutrient composition of meals and meal frequency may influence bone metabolism adversely in subjects with unhealthy eating patterns, e.g., patients with type 2 diabetes.

AREAS COVERED

The treatment strategy of bone disease in type 2 diabetics is covered in this review. The current management of diabetic bone disease consists of anti-osteoporotic treatment. However, anti-resorptives may further reduce an already low bone turnover with uncertain effects. Furthermore, the acute and long-term effects of meal ingestion, weight loss alone and in combination with exercise as well as the possible underlying mechanisms are covered in this systematic review.

EXPERT COMMENTARY

Current management of diabetic bone disease is based on principles of anti-osteoporotic treatment in non-diabetic subjects. However, studies are urged to investigate whether anti-resorptives are equally beneficial in type 2 diabetes as in non-diabetic individuals.

The interrelation between FGF23 and glucose metabolism in humans

AIMS

Different studies point to a link between glucose metabolism and Fibroblast Growth Factor 23 (FGF23), an osteocyte-derived phosphaturic hormone. We aimed to investigate in humans the effect of (I) a glucose load and (II) a hyperinsulinemic-euglycemic clamp on FGF23 concentrations and conversely (III) the effect of a diet-induced increase in FGF23 concentration on glucose and insulin concentrations.

METHODS

Plasma cFGF23 concentrations were measured during: I. an oral glucose tolerance test in eight adults with impaired glucose tolerance and vitamin D deficiency and II. a hyperinsulinemic-euglycemic clamp in nine healthy adults. III. Serum glucose and insulin concentrations were measured in nine healthy adults receiving a single-day phosphate-enriched or -restricted diet.

RESULTS

I. A glucose load decreased FGF23 and phosphate concentrations. II. The hyperinsulinemic-euglycemic clamp decreased phosphate concentrations, but did not affect FGF23 concentrations. III. Fasting insulin and glucose concentrations remained unchanged after a diet-induced increase in FGF23 concentration.

CONCLUSIONS

An oral glucose load in vitamin D deficient patients with impaired glucose metabolism decreased FGF23 concentrations, which cannot be attributed to changes in insulin concentration. Thus, bone may react rapidly after glucose loading by alternating FGF23 secretion. A diet-induced increase in FGF23 concentrations did not affect fasting glucose or insulin levels.

Osteocalcin levels on oral glucose load in women being investigated for polycystic ovary syndrome

OBJECTIVE

Osteocalcin (OC) might play a hormone-like role in energy metabolism and the regulatory circuit between the pancreas and osteoblasts. Effects of a 75-g oral glucose tolerance test (OGTT) on total OC, undercarboxylated (ucOC), and carboxylated osteocalcin (cOC) in insulin-resistant (IR) and noninsulin-resistant (nIR) premenopausal women was evaluated, and the relationships of changes in OC, ucOC, and cOC with area under the curve (AUC) insulin and the Matsuda index were examined.

METHODS

In this cross-sectional study, 105 premenopausal women underwent OGTT; 18 were IR (homeostatic model assessment of insulin resistance [HOMA-IR] > 2.6; (2 with type 2 diabetes, 2 with impaired glucose tolerance), and 87 were nIR (3 with impaired glucose tolerance). Changes in total OC, ucOC, and cOC were evaluated 60 and 120 minutes after glucose loading.

RESULTS

At baseline, IR subjects had significantly lower levels of total OC, cOC, and ucOC. In nIR women, total OC decreased by 19% from 18.0 ng/mL (14.5-24.7) at baseline to 14.6 ng/mL (10.9-17.8) after 120 minutes, ucOC decreased by 22% from 3.2 ng/mL (2.1-4.5) to 2.5 ng/mL (1.7-3.5), and cOC decreased by 26% from 14.9 ng/mL (12.1-20.4) to 11.1 ng/mL (9.0-14.5) (P < .001, respectively). No significant decreases were noted in IR subjects. The declines in OC and cOC predicted AUCinsulin (ΔOC: β = 0.301, P = .001; ΔcOC: β = 0.315, P < .001) and the Matsuda index (ΔOC: β = -0.235, P = .003; ΔcOC: β = -0.245, P = .002).

CONCLUSIONS

Glucose intake lowers levels of OC, ucOC, and cOC in nIR women, the extent of which predicts IR and insulin sensitivity in premenopausal women. OC parameters seem suppressed in IR women. There might be a differential osteoblast response to oral glucose in IR and nIR women, with OC reflecting this finding.

The role of enteric hormone GLP-2 in the response of bone markers to a mixed meal in postmenopausal women with type 2 diabetes mellitus

BACKGROUND

Type 2 diabetes mellitus (T2D) is a complex disease associated with several chronic complications, including bone fragility and high fracture risk due to mechanisms not yet fully understood. The influence of the gastrointestinal tract and its hormones on bone remodeling has been demonstrated in healthy individuals. Glucagon-like peptide 2 (GLP-2), an enteric hormone secreted in response to nutrient intake, has been implicated as a mediator of nutrient effects on bone remodeling. This study aimed to analyze the dynamics of bone resorption marker C-terminal telopeptide of type I collagen (CTX), bone formation marker osteocalcin, and GLP-2 in response to a mixed meal in diabetic postmenopausal women.

METHODS

Forty-three postmenopausal women with osteopenia or osteoporosis (20 controls - group CO - and 23 diabetic - group T2D) were subjected to a standard mixed meal tolerance test, with determination of serum CTX, plasma osteocalcin and serum GLP-2 concentrations at baseline and 30, 60, 120 and 180 minutes after the meal.

RESULTS

T2D women had higher body mass index as well as higher femoral neck and total hip bone mineral density. At baseline, luteinizing hormone, follicle-stimulating hormone, osteocalcin and CTX levels were lower in group T2D. In response to the mixed meal, CTX and osteocalcin levels decreased and GLP-2 levels increased in both groups. The expected CTX suppression in response to the mixed meal was lower in group T2D.

CONCLUSIONS

Bone turnover markers were significantly reduced in T2D women at baseline. Confirming the role of nutrient intake as a stimulating factor, GLP-2 increased in response to the mixed meal in both groups. Importantly, CTX variation in response to the mixed meal was reduced in T2D women, suggesting abnormal response of bone remodeling to nutrient intake in T2D.

Effect of fasting versus feeding on the bone metabolic response to running

Individuals often perform exercise in the fasted state, but the effects on bone metabolism are not currently known. We compared the effect of an overnight fast with feeding a mixed meal on the bone metabolic response to treadmill running. Ten, physically-active males aged 28 ± 4y (mean ±SD) completed two, counterbalanced, 8d trials. After 3d on a standardised diet, participants performed 60 min of treadmill running at 65% VO(2max) on Day 4 following an overnight fast (FAST) or a standardised breakfast (FED). Blood samples were collected at baseline, before and during exercise, for 3h after exercise, and on four consecutive follow-up days (FU1-FU4). Plasma/serum were analysed for the c-terminal telopeptide region of collagen type 1 (β-CTX), n-terminal propeptides of procollagen type 1 (P1NP), osteocalcin (OC), bone alkaline phosphatase (bone ALP), parathyroid hormone (PTH), albumin-adjusted calcium, phosphate, osteoprotegerin (OPG), cortisol, leptin and ghrelin. Only the β-CTX response was significantly affected by feeding. Pre-exercise concentrations decreased more in FED compared with FAST (47% vs 26%, P<0.001) but increased during exercise in both groups and were not significantly different from baseline at 1h post-exercise. At 3h post-exercise, concentrations were decreased (33%, P<0.001) from baseline in FAST and significantly lower (P<0.001) than in FED. P1NP and PTH increased, and OC decreased during exercise. Bone markers were not significantly different from baseline on FU1-FU4. Fasting had only a minor effect on the bone metabolic response to subsequent acute, endurance exercise, reducing the duration of the increase in β-CTX during early recovery, but having no effect on changes in bone formation markers. The reduced duration of the β-CTX response with fasting was not fully explained by changes in PTH, OPG, leptin or ghrelin.

Bariatric surgery and bone disease: from clinical perspective to molecular insights

The use of bariatric surgery for the treatment of morbid obesity has increased annually for the last decade. Although many studies have demonstrated the efficacy and durability of bariatric surgery for weight loss, there are limited data regarding long-term side effects of these procedures. Recently, there has been an increased focus on the impact of bariatric surgery on bone metabolism. Bariatric surgery utilizes one or more of three mechanisms of action resulting in sustained weight loss. These include restriction (gastric banding, vertical banded gastroplasty and sleeve gastrectomy), malabsorption surgery with or without associated restriction (Roux en Y gastric bypass, duodenal switch, biliopancreatic diversion and jejunoileal bypass) and changes in gut-derived hormones that control energy metabolism also referred to as neuro-hormonal control of energy metabolism (Roux en Y gastric bypass, duodenal switch, biliopancreatic diversion, jejunoileal bypass, surgical procedures as above and gastric sleeve). Weight reduction has been associated with increased bone resorption but the mechanisms behind this have not yet been fully elucidated. Each of the mechanisms of action of bariatric surgery (restriction, malabsorption, neuro-hormonal control of energy metabolism) may uniquely affect bone resorption. In this paper we will review the current state of knowledge regarding the relationship between bariatric surgery and bone metabolism with emphasis on possible mechanisms of action such as malnutrition, hormonal interactions and mechanical unloading of the skeleton. Further, we suggest a future research agenda.

Effects of vildagliptin on postprandial markers of bone resorption and calcium homeostasis in recently diagnosed, well-controlled type 2 diabetes patients

BACKGROUND

Bone metabolism is a dynamic process that is influenced by food ingestion. Endogenous incretins have been shown to be important regulators of bone turnover. The aim of the present study was to assess whether a dipeptidylpeptidase (DPP)-4 inhibitor affects markers of bone resorption and calcium homeostasis.

METHODS

The present study was a single-center, double blind, randomized clinical trail. Fifty-nine drug-naïve patients with type 2 diabetes (T2D) were randomized to either 1 year treatment with the DPP-4 inhibitor vildagliptin (100 mg, once daily; n = 29) or placebo (n = 30). Patients received a standardized breakfast after measurement of serum concentrations of cross-linked C-terminal telopeptide (s-CTx), a bone resorption marker influenced by food intake, before and after 50 weeks treatment.

RESULTS

Vildagliptin did not change postprandial s-CTx concentrations compared with pretreatment levels (between-group ratio 1.15 ± 0.17; P = 0.320). Fasting serum alkaline phosphatase, calcium, and phosphate were also unaffected y 1 year treatment with vildagliptin.

CONCLUSIONS

Treatment with vildagliptin for 1 year was not associated with changes in markers of bone resorption and calcium homeostasis in drug-naïve patients with T2D and mild hyperglycemia.

The effect of oral glucose tolerance test on serum osteocalcin and bone turnover markers in young adults

Osteocalcin (OC) is an osteoblast-derived protein implicated in the regulation of glucose tolerance and energy metabolism. This endocrine function has been suggested to be exerted via its undercarboxylated form, which has been shown to induce expression of adiponectin, insulin, and islet cell proliferation in mice. Furthermore, insulin has recently been shown to regulate the biological activity of OC in bone. Our aim was to explore the association between glucose and bone metabolism by evaluating the effect of a standard 75 g oral glucose tolerance test (OGTT) on serum OC, carboxylated OC (cOC) and bone-turnover markers (BTMs) C terminal telopeptide (βCTX-I) and N terminal propeptide (PINP) of type I collagen and tartrate-resistant acid phosphatase 5b (TRACP5b). Serum samples collected at 0 and at 120 min were analyzed in a cohort of normoglycemic young adults (n = 23, mean age 23.6 years). During OGTT a significant decrease was observed in all BTMs (P < 0.001 for all variables). The median decreases from 0 to 120 min for OC, cOC, βCTX-I, PINP, and TRACP5b were -32.1% (-37.9 to -19.6), -34.4% (-39.8 to -22.2), -61.4% (-68.5 to -53.0), -26.8% (-33.2 to -19.2), and -44.5% (-48.3 to -40.2), respectively. A strong association between the changes in OC and cOC was observed (r = 0.83, P < 0.001). The decrease in PINP was associated with changes in OC, whereas the changes in βCTX-I and TRACP5b were not associated with decreases in OC or cOC. The observed OGTT-induced changes in bone-derived proteins were partially independent of each other and potentially mediated by different mechanisms.

The physiology of incretins

The discovery of incretins − glucagon-like peptide (GLP)-1 and glucose-dependent insulinotrop peptide (GIP) −, clarification of their physiological properties as well as therapeutic application of incretin-based blood glucose lowering drugs opened new perspectives in the medical management of type 2 diabetes. New results of basic research investigations led to revaluation of the role of GIP in metabolic processes and a more established use of GLP-1 action. The article overviews the most relevant data of production and effects of incretins, as well as future possibilities of their therapeutic use. Orv. Hetil., 2011, 152, 1922–1930.

[Abnormalities of bone metabolism in bariatric surgery]

Obesity is an ever-increasing disease in our environment, and a major risk factor for the development of other chronic diseases that increase morbidity and mortality. Bariatric surgery is an effective treatment for obesity, since it not only allows a significant and sustained loss of weight, but also an important reduction of its co-morbidities. However, this treatment is not free of complications, since along with weight loss, frequent nutritional and metabolic deficiencies have been described. These complications are particularly frequent when mixed surgical procedures with a restrictive and malabsortive component such as Roux-en-Y gastric bypass and biliopancreatic diversion are performed. The nutritional deficiencies observed include, to a greater or lesser extent, malabsorption of fat and liposoluble vitamins, as well as other micronutrients such as calcium. As a result, disorders of bone mineral metabolism with skeletal manifestations that include bone mass reduction, increased bone turnover and defective bone mineralization may develop.

Postprandial bone turnover is independent of calories above 250 kcal

Background

The mechanisms causing bone turnover after food intake have not yet been elucidated. Several gut hormones are secreted in the postprandial phase, proportional to meal calorie content, and possibly one or more of these could influence bone turnover. The aim of this study was to investigate bone turnover in proportion to graded-calorie and fixed calcium containing meals.

Methods

A group of healthy volunteers were given six meals with calories varying from 250 to 3000 kcal on different occasions. All the meals contained 500 mg of calcium. C-telopeptide type I collagen (CTX) was measured before and 180 min after each meal.

Results

All meals significantly reduced CTX between 35.8 ± 5.6% and 44.8 ± 3.8%. No significant difference in CTX was however apparent for the different calorie containing meals. Observed differences suggest a trend to greater CTX suppression with lower protein and higher fat content of meals.

Conclusion

Changes in CTX are not proportional to calorie contents when the meals contain 500 mg of calcium. Further studies should now determine whether patients with increased bone resorption would benefit from multiple small meals to slow down the rate of bone loss.

Reduced amylin levels are associated with low bone mineral density in women with anorexia nervosa

CONTEXT

Anorexia nervosa, characterized by extreme low body weight due to reduced nutrient intake, is associated with severe bone loss. Peptide hormones, including amylin, GIP, and GLP2, are released immediately after nutrient intake and may be involved in the regulation of bone turnover.

OBJECTIVE

To investigate fasting levels of amylin, GIP, and GLP2 and their relationships with bone mineral density (BMD) in women with anorexia nervosa compared to healthy controls.

DESIGN

Cross-sectional.

SETTING

Clinical Research Center.

STUDY PARTICIPANTS

15 women with anorexia nervosa and 16 healthy controls.

INTERVENTION

None.

MAIN OUTCOME MEASURES

Fasting serum amylin, GIP, and GLP2, and BMD.

RESULTS

Women with anorexia nervosa had significantly lower fasting serum amylin and GIP levels than healthy controls. Fasting serum GLP2 levels were not significantly different between groups. Fasting amylin levels were positively associated with BMD and Z-score at the PA spine, total hip, and femoral neck. Fasting amylin levels were also positively associated with weight and percent fat; after controlling for these variables, amylin was still a significant predictor of BMD and Z-score at the femoral neck and of Z-score at the total hip. In the anorexia nervosa group, there was a trend toward an inverse association between amylin and C-terminal telopeptide (CTX) levels (R=-0.47, p=0.08). GIP and GLP2 levels did not predict BMD at any site.

CONCLUSION

Decreased secretion of amylin may be a mechanism through which reduced nutrient intake adversely affects BMD in anorexia nervosa.

The effect of glucagon-like peptide-2 on arterial blood flow and cardiac parameters

BACKGROUND

Glucagon-like peptide-2 (GLP-2) is known to increase mesenteric blood flow. The aim of the study was to evaluate the effect of GLP-2 on blood flow in different vascular sites, and dynamic changes in cardiac parameters.

METHODS

10 healthy volunteers were given 450 nmol subcutaneous (SC) GLP-2 or isotonic saline (5 subjects) in a single blinded manner. During the following 90 min, blood flow in the superior mesenteric artery (SMA), celiac artery (CA), renal artery (RA), common carotid artery (CCA) was measured using Doppler ultrasound (US), and cardiovascular variables were measured by impedance cardiography and finger plethysmography. Plasma GLP-2 was measured at times 0, 30 and 60 min.

RESULTS

Compared to the placebo group, GLP-2 elicited a 27% decrease in the resistance index (RI) and a 269.4% increase in Time Averaged Maximal Velocity (TAMV) in the SMA (P<0.01). CA, RA and CCA: There were no significant changes in RI or TAMV in the GLP-2 or placebo group, and no change in CA diameter. Cardiac parameters: GLP-2 increased cardiac output (CO), stroke volume (SV) and heart rate (HR) compared to baseline (respectively: 15.3, 4.81 and 8.2% (P<0.001, P<0.01 and P<0.01)). The CO, SV and HR changes were not significantly different from the placebo group. Mean plasma GLP-2 serum levels in the placebo group at times 0, 30 and 60 min were 22.8, 23.4 and 23.2 pmol/l. In the GLP-2 group 20.3, 1273 and 1725 pmol/l.

CONCLUSION

SC GLP-2 increased SMA blood flow, as previously shown, but elicited no changes in other vascular sites. CO and HR increased significantly, presumably due to the increased mesenteric blood flow.

Gastrointestinal peptides and bone health

PURPOSE OF REVIEW

To outline recent developments in research surrounding gastrointestinal peptides and their role in skeletal regulation.

RECENT FINDINGS

Bone remodeling is influenced by many regulatory systems, which interact to ensure that the complex demands upon mineralized tissue are met without undue compromise. These include local actions such as mechanical factors, but are dominated by systemic endocrine factors. Although the involvement of hypothalamo-pituitary actions on bone homeostasis is well defined, growing evidence suggests that peripheral tissues and the circulating factors they produce represent an important regulatory axis in bone. Given the critical role of diet in mineral homeostasis, the gastrointestinal tract is a rich source of circulating factors capable of regulating bone homeostasis. After a review of manuscripts on known mechanisms and effects of gastrointestinal peptide on bone, these were summarized.

SUMMARY

Although clearly an exciting and emergent field of research, more studies are required to define the specific actions of gastrointestinal regulator in bone, in particular, the relative contribution of systemic and local effects, to aid interpretation of their potential impact on human health and disease. Nonetheless, this exciting research will further our understanding on bone physiology and provide novel approaches to therapy in a wide range of skeletal conditions.

Four-month treatment with GLP-2 significantly increases hip BMD: a randomized, placebo-controlled, dose-ranging study in postmenopausal women with low BMD

We have previously shown that repeated dosing of glucagon-like peptide-2 (GLP-2) at 10 p.m. in postmenopausal women for 14 days results in a dose-dependent decrease in the nocturnal bone resorption, as assessed by s-CTX. In contrast, bone formation, as assessed by serum osteocalcin, appeared to be unaffected by treatment with exogenous GLP-2, at least over 14 days. The present study extends the observation period to four months. The study was a double-blind placebo-controlled dose-ranging trial comparing three different doses of GLP-2 (0.4 mg, 1.6 mg and 3.2 mg GLP-2, administered nightly) against a saline control injection. We examined safety and tolerability, and the effects on biochemical markers of bone turnover and the effect on bone mineral density. Injection of 0.4 mg, 1.6 mg and 3.2 mg GLP-2 resulted in similar reduction in the nocturnal rise of s-CTX, at Treatment Day 120 the mean difference to placebo was approximately -150%*h at AUC(0-10H) (P<0.01). Osteocalcin levels were unaffected in the 10-hour period after injection indicating that injections of 0.4 mg, 1.6 mg and 3.2 mg GLP-2 do not exert any acute stimulatory or inhibitory effect on bone formation. Treatment with GLP-2 resulted in a significant dose-dependent increase in total hip BMD over the course of the study that for the 3.2 mg GLP-2 group reached 1.1% (P=0.007) from baseline. The overall rates of adverse events in the 4 treatment groups were similar and there were no signs of tachyphylaxis or antibodies against GLP-2. The results indicate that GLP-2 produces a substantial decrease in bone resorption without suppression of bone formation thereby changing the bone remodeling balance in favor of bone formation, particularly at the hip.

Effects of treatment with glucagon-like peptide-2 on bone resorption in colectomized patients with distal ileostomy or jejunostomy and short-bowel syndrome

OBJECTIVE

The gut hormone GLP-2 (glucagon-like peptide-2) seems to be involved in the circadian pattern of bone resorption, whereas parathyroid hormone (PTH) is an established key hormone in bone turnover. Endogenous GLP-2 secretion is lacking in colectomized patients with short-bowel syndrome (SBS) and they have reduced bone mineral density (BMD). The aim of the study was to investigate the anti-resorptive effect (assessed by s-CTX) of 14 days of GLP-2 treatment in these patients and to determine whether 56 days of treatment would improve BMD. PTH secretion in response to GLP-2 was also investigated in colectomized SBS patients and colectomized controls (with ileostomy).

MATERIAL AND METHODS

Eight SBS patients and 13 patients with ileostomy were treated with subcutaneous injections of 1600 microg GLP-2 at bedtime for 56 and 14 consecutive days, respectively. BMD was determined at days 1 and 56 in SBS patients. On days 1 and 14, measurements of CTX, P1NP and PTH were taken 4 h after the GLP-2 injection.

RESULTS

Patients with ileostomy showed a significant reduction in bone resorption after GLP-2 injections at days 1 and 14. In contrast, there was no change in s-CTX after 1 and 14 days in the SBS patients, and after 56 days of GLP-2 treatment there was no improvement in BMD. A significant reduction in PTH secretion in response to GLP-2 was observed only in patients with ileostomy.

CONCLUSIONS

The decreased bone resorption in response to GLP-2 injections cannot be elicited in SBS patients and therefore precludes treatment of their osteopenia with GLP-2. The anti-resorptive response to GLP-2 seems to require an intact small intestine and may involve suppression of PTH secretion.

Reduction in bone resorption by exogenous glucagon-like peptide-2 administration requires an intact gastrointestinal tract

OBJECTIVE

Biochemical markers for bone resorption (s-CTX) are reduced by food intake, whereas markers for bone formation seem to be unaffected by meal status. Glucagon-like peptide-2 (GLP-2) is a peptide secreted from endocrine L cells in the intestinal mucosa in relation to food-intake. Subcutaneous GLP-2 treatment has been shown to reduce bone resorption in postmenopausal women. The objective of this study was to investigate the ability of exogenous GLP-2 to reduce bone resorption in patients with jejunostomy or ileostomy and to elucidate whether an intact gastrointestinal tract and the ability to secrete GLP-2 are required for meal-induced inhibition of bone resorption.

MATERIAL AND METHODS

Fifteen control subjects, 13 colectomized patients with an ileostomy and 12 colectomized patients with a jejunostomy (remnant small bowel 89 +/- 53 cm) were given: a) a subcutaneous injection of 1600 microg GLP-2, b) placebo and c) 3.8 MJ of a breakfast meal. Blood was sampled for measurements of s-CTX, s-osteocalcin and GLP-2 for 4 h after each intervention.

RESULTS

After the GLP-2 injection, only control subjects showed a significant reduction in s-CTX (24% +/- 13%, p = 0.05, 120 min) compared with baseline values. Patients with an ileostomy had a preserved endogenous postprandial GLP-2 secretion, which was absent in patients with a jejunostomy. Consumption of a meal reduced s-CTX in all groups but significantly less so in the jejunostomy group.

CONCLUSIONS

Reductions in bone resorption by exogenous GLP-2 require an intact gastrointestinal tract. The decreased meal-induced inhibition of bone resorption in the jejunostomy patients, who lack a GLP-2 response, supports the view that GLP-2 plays a role in postprandial reduction in bone resorption.

The amino- and carboxyterminal cross-linked telopeptides of collagen type I, NTX-I and CTX-I: a comparative review

Bone diseases such as osteoporosis or bone metastases are a continuously growing problem in the ageing populations across the world. In recent years, great efforts have been made to develop specific and sensitive biochemical markers of bone turnover that could help in the assessment and monitoring of bone turnover. The amino- and carboxyterminal cross-linked telopeptides of type I collagen (NTX-I and CTX-I, respectively) are two widely used bone resorption markers that attracted great attention due to their relatively high sensitivity and specificity for the degradation of type I collagen, and their rapid adaptation to automated analyzers. However, the clinical performance of both markers differs significantly depending on the clinical situation. These differences have caused considerable confusion and uncertainty. If used correctly, both markers have great potential to improve the management of many bone diseases. We here review the biochemistry, analytical background and clinical performance of NTX-I and CTX-I, as documented in the accessible literature until March 2008.