Moderate weight gain does not influence bone metabolism in skeletally mature female rats

Ketogenic diet and β-hydroxybutyrate in osteoporosis: current progress and controversy

Diet has been proven to have significant impacts on the pathogenesis and treatment of osteoporosis. This review attempts to elucidate the current progress and controversy surrounding the ketogenic diet (KD) and β-hydroxybutyrate (BHB) in osteoporosis and offers a novel perspective on the prevention and treatment of osteoporosis. The ketogenic diet has been broadly used in the treatment of epilepsy, diabetes, obesity, and certain neoplasms by triggering ketone bodies, mainly BHB. However, in most osteoporosis-related clinical and preclinical studies, the ketogenic diet has demonstrated the detrimental effects of inhibiting bone accumulation and damaging bone microarchitecture. In contrast, BHB is thought to ameliorate osteoporosis by promoting osteoblastogenesis and inhibiting osteoclastogenesis. The main purpose of this review is to summarize the current research progress and hope that more basic and clinical experiments will focus on the similarities and differences between ketogenic diet (KD) and BHB in osteoporosis.

Higher weight in partially leptin-resistant db/+ mice is associated with positive effects on bone

Absence of leptin confers metabolic dysfunction resulting in morbid obesity. Bone growth and maturation are also impaired. Partial leptin resistance is more common than leptin deficiency and, when induced by feeding mice a high fat diet, often has a negative effect on bone. Here, we used a genetic model to investigate the skeletal effects of partial and total leptin resistance in mice. This was accomplished by comparing the skeletal phenotypes of 17-week-old female C57Bl6/J wild-type (WT) mice, partial leptin receptor-deficient (db/+) mice and leptin receptor-deficient (db/db) mice (n = 7-8/group), all fed a standard diet. Compared to WT mice, db/db mice were dramatically heavier and hyperleptinemic. These mice were also hypogonadal, hyperglycemic, osteopenic and had lower serum levels of bone turnover markers, osteocalcin and C-terminal telopeptide of type I collagen (CTX). Compared to WT mice, db/+ mice were 14% heavier, had 149% more abdominal white adipose tissue, and were mildly hyperglycemic. db/+ mice did not differ from WT mice in uterine weight or serum levels of markers of bone turnover, although there was a trend for lower osteocalcin. At the bone microarchitectural level, db/+ mice differed from WT mice in having more massive femurs and a trend (P = 0.072) for larger vertebrae. These findings suggest that db/+ mice fed a normal mouse diet compensate for partial leptin resistance by increasing white adipose tissue mass which results in higher leptin levels. Our findings suggest that db/+ mice are a useful diet-independent model for studying the effects of partial leptin resistance on the skeleton.

Diet composition influences the effect of high fat diets on bone in growing male mice

The effect of diet-induced obesity on bone in rodents is variable, with bone mass increases, decreases, and no impact reported. The goal of this study was to evaluate whether the composition of obesogenic diet may influence bone independent of its effect on body weight. As proof-of-principle, we used a mouse model to compare the skeletal effects of a commonly used high fat 'Western' diet and a modified high fat diet. The modified high fat diet included ground English walnut and was isocaloric for macronutrients, but differed in fatty acid composition and contained nutrients (e.g. polyphenols) not present in the standard 'Western' diet. Eight-week-old mice were randomized into 1 of 3 dietary treatments (n = 8/group): (1) low fat control diet (LF; 10 % kcal fat); (2) high fat 'Western' diet (HF; 46 % kcal fat as soybean oil and lard); or (3) modified high fat diet supplemented with ground walnuts (HF + walnut; 46 % kcal fat as soybean oil, lard, and walnut) and maintained on their respective diets for 9 weeks. Bone response in femur was then evaluated using dual energy x-ray absorptiometry, microcomputed tomography, and histomorphometry. Consumption of both obesogenic diets resulted in increased weight gain but differed in impact on bone and bone marrow adiposity in distal femur metaphysis. Mice consuming the high fat 'Western' diet exhibited a tendency for lower cancellous bone volume fraction and connectivity density, and had lower osteoblast-lined bone perimeter (an index of bone formation) and higher bone marrow adiposity than low fat controls. Mice fed the modified high fat diet did not differ from mice fed control (low fat) diet in cancellous bone microarchitecture, or osteoblast-lined bone perimeter, and exhibited lower bone marrow adiposity compared to mice fed the 'Western' diet. This proof-of-principal study demonstrates that two obesogenic diets, similar in macronutrient distribution and induction of weight gain, can have different effects on cancellous bone in distal femur metaphysis. Because the composition of the diets used to induce obesity in rodents does not recapitulate a common human diet, our finding challenges the translatability of rodent studies evaluating the impact of diet-induced obesity on bone.

Caloric Restriction and Hypothalamic Leptin Gene Therapy Have Differential Effects on Energy Partitioning in Adult Female Rats

Dieting is a common but often ineffective long-term strategy for preventing weight gain. Similar to humans, adult rats exhibit progressive weight gain. The adipokine leptin regulates appetite and energy expenditure but hyperleptinemia is associated with leptin resistance. Here, we compared the effects of increasing leptin levels in the hypothalamus using gene therapy with conventional caloric restriction on weight gain, food consumption, serum leptin and adiponectin levels, white adipose tissue, marrow adipose tissue, and bone in nine-month-old female Sprague-Dawley rats. Rats (n = 16) were implanted with a cannula in the 3rd ventricle of the hypothalamus and injected with a recombinant adeno-associated virus, encoding the rat gene for leptin (rAAV-Lep), and maintained on standard rat chow for 18 weeks. A second group (n = 15) was calorically-restricted to match the weight of the rAAV-Lep group. Both approaches prevented weight gain, and no differences in bone were detected. However, calorically-restricted rats consumed 15% less food and had lower brown adipose tissue Ucp-1 mRNA expression than rAAV-Lep rats. Additionally, calorically-restricted rats had higher abdominal white adipose tissue mass, higher serum leptin and adiponectin levels, and higher marrow adiposity. Caloric restriction and hypothalamic leptin gene therapy, while equally effective in preventing weight gain, differ in their effects on energy intake, energy expenditure, adipokine levels, and body composition.

Changes in bone mass associated with obesity and weight loss in humans: Applicability of animal models

The implications of obesity and weight loss for human bone health are not well understood. Although the bone changes associated with weight loss are similar in humans and rodents, that is not the case for obesity. In humans, obesity is generally associated with increased bone mass, an outcome which is exacerbated by advanced age and menopause. In rodents, by contrast, bone mass decreases in proportion to severity and duration of obesity, and is influenced by sex, age and mechanical load. Despite these discrepancies, rodents are frequently used to model the situation in humans. In this review, we summarise the existing knowledge of the effects of obesity and weight loss on bone mass in humans and rodents, focusing on the translatability of findings from animal models. We then describe how animal models should be used to broaden the understanding of the relationship between obesity, weight loss, and skeletal health in humans. Specifically, we highlight the aspects of study design that should be considered to optimise translatability of the rodent models of obesity and weight loss. Notably, the sex, age, and nutritional status of the animals should ideally match those of interest in humans. With these caveats in mind, and depending on the research question asked, our review underscores that animal models can provide valuable information for obesity and weight-management research.

Thirty days of spaceflight does not alter murine calvariae structure despite increased Sost expression

Previously our laboratory documented increases in calvaria bone volume and thickness in mice exposed to 15 days of spaceflight aboard the NASA Shuttle mission STS-131. However, the tissues were not processed for gene expression studies to determine what bone formation pathways might contribute to these structural adaptations. Therefore, this study was designed to investigate both the structural and molecular changes in mice calvariae after a longer duration of spaceflight. The primary purpose was to determine the calvaria bone volume and thickness of mice exposed to 30 days of spaceflight using micro-computed tomography for comparison with our previous findings. Because sclerostin, the secreted glycoprotein of the Sost gene, is a potent inhibitor of bone formation, our second aim was to quantify Sost mRNA expression using quantitative PCR. Calvariae were obtained from six mice aboard the Russian 30-day Bion-M1 biosatellite and seven ground controls. In mice exposed to 30 days of spaceflight, calvaria bone structure was not significantly different from that of their controls (bone volume was about 5% lower in spaceflight mice, p = 0.534). However, Sost mRNA expression was 16-fold (16.4 ± 0.4, p < 0.001) greater in the spaceflight group than that in the ground control group. Therefore, bone formation may have been suppressed in mice exposed to 30 days of spaceflight. Genetic responsiveness (e.g. sex or strain of animals) or in-flight environmental conditions other than microgravity (e.g. pCO₂ levels) may have elicited different bone adaptations in STS-131 and Bion-M1 mice. Although structural results were not significant, this study provides biochemical evidence that calvaria mechanotransduction pathways may be altered during spaceflight, which could reflect vascular and interstitial fluid adaptations in non-weight bearing bones. Future studies are warranted to elucidate the processes that mediate these effects and the factors responsible for discordant calvaria bone adaptations between STS-131 and Bion-M1 mice.

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Previously, 15 days of spaceflight augmented bone volume in mice calvariae.

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In this study, calvaria bone structure was not altered after 30 days of spaceflight.

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Sost mRNA expression was higher in murine calvariae after 30 days of spaceflight.

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Longer duration, or other spaceflight factors, may negate short-term calvarial growth.

Influence of body weight on bone mass, architecture and turnover

Weight-dependent loading of the skeleton plays an important role in establishing and maintaining bone mass and strength. This review focuses on mechanical signaling induced by body weight as an essential mechanism for maintaining bone health. In addition, the skeletal effects of deviation from normal weight are discussed. The magnitude of mechanical strain experienced by bone during normal activities is remarkably similar among vertebrates, regardless of size, supporting the existence of a conserved regulatory mechanism, or mechanostat, that senses mechanical strain. The mechanostat functions as an adaptive mechanism to optimize bone mass and architecture based on prevailing mechanical strain. Changes in weight, due to altered mass, weightlessness (spaceflight), and hypergravity (modeled by centrifugation), induce an adaptive skeletal response. However, the precise mechanisms governing the skeletal response are incompletely understood. Furthermore, establishing whether the adaptive response maintains the mechanical competence of the skeleton has proven difficult, necessitating the development of surrogate measures of bone quality. The mechanostat is influenced by regulatory inputs to facilitate non-mechanical functions of the skeleton, such as mineral homeostasis, as well as hormones and energy/nutrient availability that support bone metabolism. Although the skeleton is very capable of adapting to changes in weight, the mechanostat has limits. At the limits, extreme deviations from normal weight and body composition are associated with impaired optimization of bone strength to prevailing body size.

Fructose consumption does not worsen bone deficits resulting from high-fat feeding in young male rats

Dietary-induced obesity (DIO) resulting from high-fat (HF) or high-sugar diets produces a host of deleterious metabolic consequences including adverse bone development. We compared the effects of feeding standard rodent chow (Control), a 30% moderately HF (starch-based/sugar-free) diet, or a combined 30%/40% HF/high-fructose (HF/F) diet for 12weeks on cancellous/cortical bone development in male Sprague-Dawley rats aged 8weeks. Both HF feeding regimens reduced the lean/fat mass ratio, elevated circulating leptin, and reduced serum total antioxidant capacity (tAOC) when compared with Controls. Distal femur cancellous bone mineral density (BMD) was 23-34% lower in both HF groups (p<0.001) and was characterized by lower cancellous bone volume (BV/TV, p<0.01), lower trabecular number (Tb.N, p<0.001), and increased trabecular separation versus Controls (p<0.001). Cancellous BMD, BV/TV, and Tb.N were negatively associated with leptin and positively associated with tAOC at the distal femur. Similar cancellous bone deficits were observed at the proximal tibia, along with increased bone marrow adipocyte density (p<0.05), which was negatively associated with BV/TV and Tb.N. HF/F animals also exhibited lower osteoblast surface and reduced circulating osteocalcin (p<0.05). Cortical thickness (p<0.01) and tissue mineral density (p<0.05) were higher in both HF-fed groups versus Controls, while whole bone biomechanical characteristics were not different among groups. These results demonstrate that "westernized" HF diets worsen cancellous, but not cortical, bone parameters in skeletally-immature male rats and that fructose incorporation into HF diets does not exacerbate bone loss. In addition, they suggest that leptin and/or oxidative stress may influence DIO-induced alterations in adolescent bone development.

Alcohol: A Simple Nutrient with Complex Actions on Bone in the Adult Skeleton

BACKGROUND

Alcohol is an important nonessential component of diet, but the overall impact of drinking on bone health, especially at moderate levels, is not well understood. Bone health is important because fractures greatly reduce quality of life and are a major cause of morbidity and mortality in the elderly. Regular alcohol consumption is most common following skeletal maturity, emphasizing the importance of understanding the skeletal consequences of drinking in adults.

METHODS

This review focuses on describing the complex effects of alcohol on the adult skeleton. Studies assessing the effects of alcohol on bone in adult humans as well as skeletally mature animal models published since the year 2000 are emphasized.

RESULTS

Light to moderate alcohol consumption is generally reported to be beneficial, resulting in higher bone mineral density (BMD) and reduced age-related bone loss, whereas heavy alcohol consumption is generally associated with decreased BMD, impaired bone quality, and increased fracture risk. Bone remodeling is the principal mechanism for maintaining a healthy skeleton in adults and dysfunction in bone remodeling can lead to bone loss and/or decreased bone quality. Light to moderate alcohol may exert beneficial effects in older individuals by slowing the rate of bone remodeling, but the impact of light to moderate alcohol on bone remodeling in younger individuals is less certain. The specific effects of alcohol on bone remodeling in heavy drinkers are even less certain because the effects are often obscured by unhealthy lifestyle choices, alcohol-associated disease, and altered endocrine signaling.

CONCLUSIONS

Although there have been advances in understanding the complex actions of alcohol on bone, much remains to be determined. Limited evidence implicates age, skeletal site evaluated, duration, and pattern of drinking as important variables. Few studies systematically evaluating the impact of these factors have been conducted and should be made a priority for future research. In addition, studies performed in skeletally mature animals have potential to reveal mechanistic insights into the precise actions of alcohol and associated comorbidity factors on bone remodeling.

Hypothalamic leptin gene therapy reduces body weight without accelerating age-related bone loss

Excessive weight gain in adults is associated with a variety of negative health outcomes. Unfortunately, dieting, exercise, and pharmacological interventions have had limited long-term success in weight control and can result in detrimental side effects, including accelerating age-related cancellous bone loss. We investigated the efficacy of using hypothalamic leptin gene therapy as an alternative method for reducing weight in skeletally-mature (9 months old) female rats and determined the impact of leptin-induced weight loss on bone mass, density, and microarchitecture, and serum biomarkers of bone turnover (CTx and osteocalcin). Rats were implanted with cannulae in the 3rd ventricle of the hypothalamus and injected with either recombinant adeno-associated virus encoding the gene for rat leptin (rAAV-Leptin, n=7) or a control vector encoding green fluorescent protein (rAAV-GFP, n=10) and sacrificed 18 weeks later. A baseline control group (n=7) was sacrificed at vector administration. rAAV-Leptin-treated rats lost weight (-4±2%) while rAAV-GFP-treated rats gained weight (14±2%) during the study. At study termination, rAAV-Leptin-treated rats weighed 17% less than rAAV-GFP-treated rats and had lower abdominal white adipose tissue weight (-80%), serum leptin (-77%), and serum IGF1 (-34%). Cancellous bone volume fraction in distal femur metaphysis and epiphysis, and in lumbar vertebra tended to be lower (P<0.1) in rAAV-GFP-treated rats (13.5 months old) compared to baseline control rats (9 months old). Significant differences in cancellous bone or biomarkers of bone turnover were not detected between rAAV-Leptin and rAAV-GFP rats. In summary, rAAV-Leptin-treated rats maintained a lower body weight compared to baseline and rAAV-GFP-treated rats with minimal effects on bone mass, density, microarchitecture, or biochemical markers of bone turnover.

Paradoxical effects of partial leptin deficiency on bone in growing female mice

Morbidly obese, leptin-deficient ob/ob mice display low bone mass, mild osteoclast-rich osteopetrosis, and increased bone marrow adiposity. While partial leptin deficiency results in increased weight, the skeletal manifestations of partial leptin deficiency are less well defined. We therefore analyzed femora and lumbar vertebrae in growing (7-week-old) female C57BL/6 wildtype (WT) mice, partial leptin-deficient ob/+ mice, and leptin-deficient ob/ob mice. The bones were evaluated by dual energy absorptiometry, microcomputed tomography and histomorphometry. As expected, ob/+ mice were heavier, had more white adipose tissue, and lower serum leptin than WT mice, but were lighter and had less white adipose tissue than ob/ob mice. With a few exceptions, cancellous bone architecture, cell (osteoblast, osteoclast, and adipocyte), and dynamic measurements did not differ between WT and ob/+ mice. In contrast, compared to WT and ob/+ mice, ob/ob mice had lower cancellous bone volume fraction, and higher bone marrow adiposity in the femur metaphysis, and higher cancellous bone volume fraction in lumbar vertebra. Paradoxically, ob/+ mice had greater femoral bone volume than either WT or ob/ob mice. There was a positive correlation between body weight and femur volume in all three genotypes. However, the positive effect of weight on bone occurred with lower body weight in leptin-producing mice. The paradoxical differences in bone size among WT, ob/+, and ob/ob mice may be explained if leptin, in addition to stimulating bone growth and cancellous bone turnover, acts to lower the set-point at which increased body weight leads to a commensurate increase in bone size.

Systemic leptin administration in supraphysiological doses maintains bone mineral density and mechanical strength despite significant weight loss

The effects of leptin on bone are controversial. Although in vitro studies have shown that leptin stimulates osteoblast differentiation and mineralization and inhibits osteoclastogenesis, some rodent studies have shown that leptin administered centrally might result in decreased bone formation. In the present study we have investigated the skeletal effects of supraphysiological concentrations of leptin administered sc to rats. Female Fischer rats were given leptin 100 μg/d, 200 μg/d, or saline by continuous infusion for 9 wk. Bone mineral density (BMD) was measured by dual energy x-ray absorptiometry, bone microarchitecture was analyzed by micro-computed tomography, and biomechanical properties were tested by three-point bending experiments. At the end of the study, the body weight was significantly lower in rats receiving leptin compared with controls (-10.8% and -12.0% in low- and high-dose leptin groups, respectively). The high-dose leptin group also significantly lost weight compared with baseline. The plasma leptin concentration was 14- and 33-fold increased in the low- and high-dose groups, respectively. No significant differences in femoral BMD were observed. Whole-body BMD was significantly lower in the low-dose leptin group, whereas there was no difference between the high-dose leptin group and the control. Mechanical strength and microarchitecture were similar in the high-dose and the control group. The low-dose group, however, had decreased cortical volume in the femoral metaphysis, lowered bone strength, and altered moment of inertia. In conclusion, leptin given at very high doses maintains BMD, microarchitecture, and mechanical strength in female rats, despite a significant decrease in body weight.

Understanding leptin-dependent regulation of skeletal homeostasis

Despite growing evidence for adipose tissue regulation of bone mass, the role of the adipokine leptin in bone remodeling remains controversial. The majority of in vitro studies suggest leptin enhances osteoblastic proliferation and differentiation while inhibiting adipogenic differentiation from marrow stromal cells. Alternatively, some evidence demonstrates either no effect or a pro-apoptotic action of leptin on stromal cells. Similarly, in vivo work has demonstrated both positive and negative effects of leptin on bone mass. Most of the literature supports the idea that leptin suppresses bone mass by acting in the brainstem to reduce serotonin-dependent sympathetic signaling from the ventromedial hypothalamus to bone. However, other studies have found partly or entirely contrasting actions of leptin. Recently one study found a significant effect of surgery alone with intracerebroventricular administration of leptin, a technique crucial for understanding centrally-mediated leptin regulation of bone. Thus, two mainstream hypotheses for the role of leptin on bone emerge: 1) direct regulation through increased osteoblast proliferation and differentiation and 2) indirect suppression of bone formation through a hypothalamic relay. At the present time, it remains unclear whether these effects are relevant in only extreme circumstances (i.e. models with complete deficiency) or play an important homeostatic role in the regulation of peak bone acquisition and skeletal remodeling. Ultimately, determining the actions of leptin on the skeleton will be critical for understanding how the obesity epidemic may be impacting the prevalence of osteoporosis.

Bone turnover markers correlate with implant fixation in a rat model using LPS-doped particles to induced implant loosening

Revision surgery for particle-induced implant loosening in total joint replacement is expected to increase dramatically over the next few decades. This study was designed to investigate if local tissue and serum markers of bone remodeling reflect implant fixation following administration of lipopolysaccharide (LPS)-doped polyethylene (PE) particles in a rat model. Twenty-four rats received bilateral implantation of intramedullary titanium rods in the distal femur, followed by weekly bilateral intra-articular injection of either LPS-doped PE particles (n = 12) or vehicle that contained no particles (n = 12) for 12 weeks. The group in which the particles were injected had increased serum C-terminal telopeptide of type I collagen (CTX-I), decreased serum osteocalcin (OC), increased peri-implant eroded surface, decreased peri-implant bone volume, and decreased mechanical pull-out strength compared to the controls. Implant fixation strength was positively correlated with peri-implant bone volume and serum OC and inversely correlated with serum CTX-I, while energy to yield was positively correlated with serum OC and inversely correlated with the number of tartrate-resistant acid phosphatase positive cells at the interface and the amount of peri-implant eroded surface. There was no effect on trabecular bone volume at a remote site. Thus, the particle-induced impaired fixation in this rat model was directly associated with local and serum markers of elevated bone resorption and depressed bone formation, supporting the rationale of exploring both anticatabolic and anabolic strategies to treat and prevent particle-related implant osteolysis and loosening, and indicating that serum markers may prove useful in tracking implant fixation.

Hypothalamic leptin gene therapy prevents weight gain without long-term detrimental effects on bone in growing and skeletally mature female rats

Hypothalamic leptin gene therapy normalizes the mosaic skeletal phenotype of leptin-deficient ob/ob mice. However, it is not clear whether increased hypothalamic leptin alters bone metabolism in animals already producing the hormone. The objective of this study was to evaluate the long duration effects of recombinant adeno-associated virus-rat leptin (rAAV-Lep) hypothalamic gene therapy on weight gain and bone metabolism in growing and skeletally mature leptin-replete female Sprague-Dawley rats. Rats were either unoperated or implanted with cannulas in the third ventricle of the hypothalamus and injected with either rAAV-Lep or rAAV-GFP (control vector encoding green fluorescent protein) and maintained on standard rat chow fed ad libitum for either 5 or 10 weeks (starting at 3 months of age) or 18 weeks (starting at 9 months of age). Tibias, femurs, or lumbar vertebrae were analyzed by micro-computed tomography and/or histomorphometry. In comparison with age-matched rAAV-GFP rats, rAAV-Lep rats maintained a lower body weight for the duration of studies. At 5 weeks after vector administration, rAAV-Lep rats had lower cancellous bone volume and bone marrow adiposity but higher osteoblast perimeter compared with nonoperated controls. However, these values did not differ between the two groups at 10 weeks after vector administration. Differences in cancellous bone volume and architecture were not detected between the rAAV-Lep and rAAV-GFP groups at either time point. Also, rAAV-Lep had no negative effects on bone in the 9-month-old skeletally mature rats at 18 weeks after vector administration. We hypothesize that the transient reductions in bone mass and bone marrow adiposity at 5 weeks after vector administration were due to hypothalamic surgery. We conclude that increased hypothalamic leptin, sufficient to prevent weight gain, has minimal specific effects (rAAV-Lep versus rAAV-GFP) on bone metabolism in normal female rats.

Low dose parathyroid hormone maintains normal bone formation in adult male rats during rapid weight loss

A persistent negative energy balance results in bone loss. It is not clear whether the bone loss associated with chronic negative energy balance can be prevented. The objective of this study was to assess the efficacy of intermittent low dose parathyroid hormone (PTH) treatment in maintaining normal bone formation during severe energy restriction. Six-month-old male Fisher 344 rats were divided into 4 treatment groups: (1) baseline, (2) ad libitum (ad lib)-fed control, (3) energy-restricted (to consume 40% ad lib caloric intake), or (4) energy-restricted+low dose (1 μg/kg/d) PTH. Severe energy restriction for 14 d decreased body weight and serum leptin levels. Compared to ad lib-fed controls, energy-restricted rats had lower cancellous bone formation, higher osteoclast perimeter/bone perimeter and higher bone marrow adiposity in the proximal tibial metaphysis. Also, the energy-restricted rats had a lower periosteal bone formation rate at the tibia-fibula synostosis. Administration of PTH to energy-restricted rats had no effect on weight loss or osteoclast perimeter/bone perimeter. In contrast, energy-restricted rats treated with PTH had higher rates of cancellous and cortical bone formation compared to energy-restricted rats, and did not differ from the ad lib-fed control animals. Furthermore, PTH treatment maintained normal bone marrow adiposity. In conclusion, rapid weight loss in adult male rats was accompanied by decreased bone formation and increased bone marrow adiposity and these changes were prevented by low dose PTH treatment. Taken together, the results suggest that the energy cost of bone formation in adult rats is low and PTH therapy is effective in preventing the reduced bone formation associated with rapid weight loss.