Dietary protein deficiency induces osteoporosis in aged male rats

Influence of high and low protein intakes on age-related bone loss in rats submitted to adequate or restricted energy conditions

Low energy and protein intake has been suggested to contribute to the increased incidence of osteoporosis in the elderly. The impact of dietary protein on bone health is still a matter of debate. Therefore, we examined the effect of the modulation of protein intake under adequate or deficient energy conditions on bone status in 16-month-old male rats. The animals were randomly allocated to six groups (n = 10/group). Control animals were fed a diet providing either a normal-protein content (13%, C-NP) or a high-protein content (26%) (C-HP). The other groups received a 40% protein/energy-restricted diet (PER-NP and PER-HP) or a normal protein/energy-restricted diet (ER-NP and ER-HP). After 5 months of the experiment, protein intake (13% or 26%) did not modulate calcium retention or bone status in those rats, although a low-grade metabolic acidosis was induced with the HP diet. Both restrictions (PER and ER) decreased femoral bone mineral density and fracture load. Plasma osteocalcin and urinary deoxypyridinoline levels were lowered, suggesting a decrease in bone turnover in the PER and ER groups. Circulating insulin-like growth factor-I levels were also lowered by dietary restrictions, together with calcium retention. Adequate protein intake in the ER condition did not elicit any bone-sparing effect compared to PER rats. In conclusion, both energy and protein deficiencies may contribute to age-related bone loss. This study highlights the importance of sustaining adequate energy and protein provision to preserve skeletal integrity in the elderly.

Food restriction and simulated microgravity: effects on bone and serum leptin

Leptin is responsible for linking energy metabolism to bone mass. Because astronauts are commonly in negative energy balance during spaceflight, this study was designed to assess individual and combined effects of food restriction and simulated microgravity on bone mass and serum leptin. Six-month-old male Sprague-Dawley rats were randomly assigned to four groups ( n = 12 each): two hindlimb-unloading (HU) groups fed 100% (HU100) and 70% (HU70) and two cage-activity control (CC) groups fed 100% (CC100) and 70% (CC70) of their baseline food requirement. After 28 days, CC100 rats gained body weight, whereas all other groups lost body weight; this loss was greater in HU70 than in CC70 and HU100 rats. Serum leptin decreased in CC70 and HU100 (−60% and −27%, respectively) and was not detectable in HU70 animals. Percent osteoid surface in CC70 and HU100 was lower than that of CC100 (7.80%, 8.60% vs. 10.70%, respectively), and this decrease was more pronounced in HU70 animals (4.38%). Mineral apposition rate of CC70, HU100, and HU70 rats was lower than that of CC100 (1.5, 1.6, and 1.5 vs. 2.1 μm/day, respectively). Bone formation rate of CC70, HU100, and HU70 rats was lower than that of CC100 (13.4, 13.1, and 12.2 vs. 40.8 mm3·mm−2·day−1, respectively). The change in bone formation rate was correlated with the change in serum leptin value over 28 days ( r2 = 0.69, P = 0.0007). We conclude that moderate caloric restriction may cause bone loss at susceptible bone sites to a similar degree as does the unloading effect of microgravity; serum leptin may be an important endocrine regulator contributing to this change in skeletal integrity.

Long-term intake of a high-protein diet with or without potassium citrate modulates acid-base metabolism, but not bone status, in male rats

High dietary protein intake generates endogenous acid production, which may adversely affect bone health. Alkaline potassium citrate (Kcit)(2) may contribute to the neutralization of the protein-induced metabolic acidosis. We investigated the impact of 2 levels of protein intake and Kcit supplementation on acid-base metabolism and bone status in rats. Two-month-old Wistar male rats were randomly assigned to 4 groups (n = 30 per group). Two groups received a normal-protein content (13%) (NP) or a high-protein (HP) content diet (26%) for 19 mo. The 2 other groups received identical diets supplemented with Kcit (3.60%) (NPKcit and HPKcit). Rats were pair-fed based on the ad libitum intake of the HP group. At 9, 16, and 21 mo of age, 10 rats of each group were killed. The HP diet induced a metabolic acidosis characterized by hypercalciuria, hypermagnesuria, and hypocitraturia at all ages. Kcit supplementation neutralized this effect, as evidenced by decreased urinary calcium and magnesium excretion by the HPKcit rats. Femoral bone mineral density, biomechanical properties, bone metabolism biomarkers (osteocalcin and deoxypyridinoline), and plasma insulin-like growth factor 1 levels were not affected by the different diets. Nevertheless, at 21 mo of age, calcium retention was reduced in the HP group. This study suggests that lifelong excess of dietary protein results in low-grade metabolic acidosis without affecting the skeleton, which may be protected by an adequate calcium supply.

Intrauterine programming of bone. Part 1: alteration of the osteogenic environment

Osteoporosis is believed to partly be programmed in utero. Rat dams were given a low protein diet during pregnancy and 135 offspring studied at different ages. Bone biochemistry showed altered characteristics. Altered in utero diet has consequences for later life.

INTRODUCTION

Epidemiological studies suggest skeletal growth is programmed during intrauterine and early postnatal life. We have investigated this in a rat model of maternal protein insufficiency.

METHODS

Dams received either 18% w/w (control) or 9% w/w (low protein) diet during pregnancy, and the offspring were studied at selected time points (4, 8, 12, 16, 20, 47 weeks).

RESULTS

Alkaline phosphatase activity in controls reached peak levels from 8 to 20 weeks of age. In contrast, restricted diet offspring were at peak levels from 4 weeks of age. Peak levels were similar in both groups. Serum IGF-1 levels were lower in female restricted diet offspring at 4 weeks of age, and serum osteocalcin was significantly higher at 4 weeks of age in male and female offspring from mothers fed the restricted diet, whereas serum 25-OH vitamin D was significantly lower in restricted diet males at 8, 12, and 20 weeks of age.

CONCLUSIONS

These data indicate that a low protein diet in utero affected the osteogenic environment in the offspring with effects that persist into late adulthood. These results indicate the key role of the nutritional environment in early development on programming of skeletal development with implicit consequences in later life.

Intrauterine programming of bone. Part 2: alteration of skeletal structure

Osteoporosis is believed to be partly programmed in utero. Rat dams were given a low protein diet during pregnancy, and offspring were studied at different ages. Old aged rats showed site-specific strength differences. In utero nutrition has consequences in later life.

INTRODUCTION

Epidemiological studies suggest skeletal growth is programmed during intrauterine and early postnatal life. We hypothesize that age-related decrease in bone mass has, in part, a fetal origin and investigated this using a rat model of maternal protein insufficiency.

METHODS

Dams received either 18% w/w (control) or w/w 9% (low protein) diet during pregnancy, and the offspring were studied at selected time points (4, 8, 12, 16, 20, 47, 75 weeks).

RESULTS

Using micro-CT, we found that at 75 weeks of age female offspring from mothers fed a restricted protein diet during pregnancy had femoral heads with thinner, less dense trabeculae, femoral necks with closer packed trabeculae, vertebrae with thicker, denser trabeculae and midshaft tibiae with denser cortical bone. Mechanical testing showed the femoral heads and midshaft tibiae to be structurally weaker, whereas the femoral necks and vertebrae were structurally stronger.

CONCLUSIONS

Offspring from mothers fed a restricted protein diet during pregnancy displayed significant differences in bone structure and density at various sites. These differences result in altered bone characteristics indicative of significantly altered bone turnover. These results further support the need to understand the key role of the nutritional environment in early development on programming of skeletal development and consequences in later life.

Impact of energy and casein or whey protein intake on bone status in a rat model of age-related bone loss

In the elderly, nutritional deficiencies, such as low energy and protein intake, are suggested to increase the risk of osteoporotic fractures. Modulation of the amount and quality of protein intake under energy deficient conditions represents an interesting strategy to prevent aged-related bone loss. We investigated the effect of a 5-month dietary restriction on bone status in 16-month-old male rats. Rats were randomised into six groups (n 10 per group). Control animals were fed a normal diet containing either casein (N-C) or whey protein (N-WP). The other groups received a 40 % protein and energy-restricted diet with casein or whey protein (PER-C and PER-WP) or a normal protein and energy-restricted diet (ER-C and ER-WP). Both restrictions (PER and ER) induced a decrease in femoral bone mineral density (BMD), consistent with impaired biomechanical properties and a reduced cortical area at the diaphysis. Plasma osteocalcin and urinary deoxypyridinoline levels suggested a decrease in bone turnover in the PER and ER groups. Interestingly, circulating insulin-like growth factor 1 (IGF-1) levels were also lowered. Overall, normal protein intake did not elicit any bone sparing effect in energy-deficient rats. Regarding protein quality, neither casein nor WP appeared to significantly prevent the BMD decrease. This study confirms that nutritional deficiencies may contribute to osteopenia through decreased IGF-1 levels. Moreover, it seems that impaired bone status could not be significantly prevented by modulating the amount and quality of dietary proteins.

Defective implant osseointegration under protein undernutrition: prevention by PTH or pamidronate

Protein deficiency is associated with impaired titanium osseointegration. We studied whether systemic treatment with PTH or pamidronate could influence the resistance to pull-out of titanium rods implanted into rats proximal tibia under normal and isocaloric low protein intake. PTH or pamidronate prevented the deleterious effects of protein undernutrition on bone microarchitecture close to the implant and on mechanical fixation. PTH even significantly improved implant osseointegration.

INTRODUCTION

Protein deficiency is highly prevalent among elderly patients hospitalized in orthopedic wards. Reduced protein intake impairs titanium osseointegration in rats. Whether stimulator of bone formation or inhibitor of bone resorption could improve implant osseointegration under protein deprivation is not known. We studied the effects of systemic treatment with PTH or pamidronate on the resistance to pull-out of titanium rods implanted into rats proximal tibia under normal and isocaloric low protein intake.

MATERIALS AND METHODS

We measured the resistance to pull-out 1-mm-diameter titanium rods implanted into the proximal tibias of 49 adult female rats receiving a normal or an isocaloric low protein diet. After 2 wk on either diet, the implants were inserted, and the rats received PTH(1-34), pamidronate or saline vehicle for 8 wk. The tibias were removed for microCT morphometry, followed by the evaluation of pull-out strength.

RESULTS

Pull-out strength was lower in rats fed an isocaloric low protein diet compared with rats fed a normal protein intake (-29%). PTH and pamidronate significantly increased pull-out strength in animals fed a normal or a low protein diet, the effect of PTH being of higher magnitude. The PTH- or pamidronate-mediated increase in pull-out strength was associated with significant increases of relative bone volume, bone-to-implant contact, and trabecular thickness, whereas trabecular spacing was reduced, in the vicinity of the implants.

CONCLUSIONS

We confirmed that isocaloric low protein intake impairs titanium implant osseointegration. PTH or pamidronate prevented the deleterious effects of protein undernutrition and even significantly improved the implant osseointegration. These results indicate that systemic administration of PTH or pamidronate could be considered for preventing uncemented arthroplasty loosening in protein undernourished patients.

Quantitative Microcomputed Tomography Assessment of Intratrabecular, Intertrabecular, and Cortical Bone Architecture in a Rat Model of Severe Renal Osteodystrophy

OBJECTIVE

To determine the effects of renal osteodystrophy (ROD) on bone microarchitecture in growing rats.

METHODS

A total of 24 rats underwent 5/6 nephrectomy (NX) and were fed a high-phosphorus diet to induce ROD; another 6 underwent sham NX. In vitro microcomputed tomography images (GEMS, London, Ontario, Canada) were obtained in the femoral metaphysis and midshaft.

RESULTS

Trabecular and cortical bone volume/total volume (BV/TV) were significantly lower in NX specimens because of pores within the trabeculae and along the endosteal surface. Topological analysis using component labeling in 3-dimensions verified that trabecular pores connected to the marrow space. After the trabecular pores were filled using a morphological filter, trabecular thickness was significantly increased in NX. In contrast, cortical thickness was significantly decreased in NX compared with controls; however, after filling the endocortical pores, thickness did not differ.

CONCLUSIONS

The ROD resulted in decreased cortical and trabecular BV/TV, increased porosity, and increased trabecular thickness. Advanced image processing algorithms demonstrated the effects of cortical and trabecular porosity on BV/TV and structure in ROD.

High dietary phosphate intake reduces bone strength in the growing rat skeleton

Nutrition influences peak bone mass development in early adulthood. The effect of high dietary phosphate intake on the growing skeleton of 1-month-old male rats (n = 30) was assessed in an 8-week intervention. High dietary phosphate intake increased bone remodeling and impaired bone material properties, diminishing bone mechanical strength.

INTRODUCTION

High dietary phosphate intake is typical in the Western diet. Abundant phosphate intake enhances parathyroid secretion and bone metabolism. To study the influence of high dietary phosphate intake on growing bone homeostasis and structure, we submitted growing rats to experimental diets that varied in their phosphate content.

MATERIALS AND METHODS

One-month-old intact male rats (n = 30) were fed a control diet (Ca:P 1:1) or an experimental diet of either Ca:P 1:2 or Ca:P 1:3 for 8 weeks. At the beginning and the end of the study period, the right femurs were measured using DXA. Double labeling with tetracycline injection was performed 12 and 2 days before death. After death, hind legs were cut loose. Left femurs were processed for histomorphometry. Right femurs were measured with pQCT. Mechanical testing was performed on the right femoral neck and tibial shaft. Six right tibias were analyzed with microCT. Serum PTH, calcium, and phosphate contents were analyzed.

RESULTS

High-phosphate intake impaired growth of the animal, limited bone longitudinal growth, and restricted femur BMC and BMD build-up. Osteoclast number, osteoblast perimeter, and mineral apposition rate were increased, and trabecular area and width were decreased. Phosphate decreased femur midshaft total bone BMD, cortical bone BMD, and mean cortical thickness. High-phosphate diet reduced femoral neck and tibial shaft ultimate strength and tibia stiffness and toughness. In addition, serum PTH increased.

CONCLUSIONS

High dietary phosphate intake reduced growth, skeletal material, and structural properties and decreased bone strength in growing male rats. Adequate calcium could not overcome this.

Dietary protein: an essential nutrient for bone health

Nutrition plays a major role in the development and maintenance of bone structures resistant to usual mechanical loadings. In addition to calcium in the presence of an adequate vitamin D supply, proteins represent a key nutrient for bone health, and thereby in the prevention of osteoporosis. In sharp opposition to experimental and clinical evidence, it has been alleged that proteins, particularly those from animal sources, might be deleterious for bone health by inducing chronic metabolic acidosis which in turn would be responsible for increased calciuria and accelerated mineral dissolution. This claim is based on an hypothesis that artificially assembles various notions, including in vitro observations on the physical-chemical property of apatite crystal, short term human studies on the calciuric response to increased protein intakes, as well as retrospective inter-ethnic comparisons on the prevalence of hip fractures. The main purpose of this review is to analyze the evidence that refutes a relation of causality between the elements of this putative patho-physiological "cascade" that purports that animal proteins are causally associated with an increased incidence of osteoporotic fractures. In contrast, many experimental and clinical published data concur to indicate that low protein intake negatively affects bone health. Thus, selective deficiency in dietary proteins causes marked deterioration in bone mass, micro architecture and strength, the hallmark of osteoporosis. In the elderly, low protein intakes are often observed in patients with hip fracture. In these patients intervention study after orthopedic management demonstrates that protein supplementation as given in the form of casein, attenuates post-fracture bone loss, increases muscles strength, reduces medical complications and hospital stay. In agreement with both experimental and clinical intervention studies, large prospective epidemiologic observations indicate that relatively high protein intakes, including those from animal sources are associated with increased bone mineral mass and reduced incidence of osteoporotic fractures. As to the increased calciuria that can be observed in response to an augmentation in either animal or vegetal proteins it can be explained by a stimulation of the intestinal calcium absorption. Dietary proteins also enhance IGF-1, a factor that exerts positive activity on skeletal development and bone formation. Consequently, dietary proteins are as essential as calcium and vitamin D for bone health and osteoporosis prevention. Furthermore, there is no consistent evidence for superiority of vegetal over animal proteins on calcium metabolism, bone loss prevention and risk reduction of fragility fractures.

Low protein intake is associated with impaired titanium implant osseointegration

Low protein intake is highly prevalent among orthopaedic elderly patients. We studied the effects of an isocaloric low protein diet on the resistance to pull-out of titanium rods implanted into rats proximal tibia. Isocaloric low protein intake impairs titanium implant osseointegration, with a decreased strength needed to completely loose the implant and altered bone microarchitecture in its vicinity.

INTRODUCTION

Low protein intake is highly prevalent among elderly patients in orthopaedic wards and could retard fracture healing. It was previously shown that reduced protein intake decreases bone strength. Whether dietary protein intake could influence titanium implant osseointegration is unknown. We studied the effects of an isocaloric low protein diet on the resistance to pull-out of titanium rods implanted into rats proximal tibia.

MATERIALS AND METHODS

Forty-eight 11-month-old female rats were fed isocaloric diets containing 2.5% (low protein) or 15% (normal protein) casein from 2 weeks before the implantation of a 1-mm-diameter cylindrical titanium rod in the proximal metaphysis of each tibia. Four, 6, and 8 weeks after implantation, the tibias were removed for microCT histomorphometry to quantify bone-to-implant contact and bone trabecular microarchitecture around the implant. Resistance to implant pull-out was tested by recording the maximal force necessary to completely loosen the implant.

RESULTS

Pull-out strength was significantly lower in rats fed an isocaloric low protein diet by 6 and 8 weeks after implantation (-43%, p < 0.001 and -42%, p < 0.001, respectively) compared with rats fed a normal protein diet. Bone-to-implant contact was significantly lower in the low protein group 8 weeks after implantation (p < 0.05). Bone-to-implant contact and pull-out strength were correlated (r2= 0.57, p < 0.0001). BV/TV around the implant was 19.9 +/- 2.2% (SE) versus 31.8 +/- 3.3% (p < 0.05) at 6 weeks and 20.1 +/- 1.9% versus 29.8 +/- 3.2% (p < 0.05) at 8 weeks after implantation in the low protein and normal protein intake groups, respectively. Trabecular thickness was 96.2 +/- 3.7 versus 113.0 +/- 3.6 microm (p < 0.01) at 6 weeks and 101.4 +/- 2.7 versus 116.2 +/- 3.3 microm (p < 0.01) at 8 weeks in the corresponding groups. In a structure model index analysis, there was a significant shift to a more rod-like pattern in the low protein diet groups. All these changes were associated with lower plasma IGF-I levels.

CONCLUSIONS

Isocaloric low protein intake impairs titanium implant osseointegration, with decreased strength needed to completely loosen the implant and altered bone microarchitecture in the vicinity of the implant.

Diet and exercise during growth have site-specific skeletal effects: a co-twin control study

Exercise and improved nutrition offer safe, low-cost and widely applicable approaches to potentially reduce the burden of fractures. We conducted a cross-sectional study of 30 monozygotic and 26 dizygotic male twin pairs, aged 7-20 years to test the following hypotheses: (1) Associations between bone mass and dimensions and exercise are greater than between bone mass and dimensions and protein or calcium intakes; (2) exercise or nutrient intake are associated with appendicular bone mass before puberty and axial bone mass during and after puberty. Total body and posteroanterior (PA) lumbar spine bone mineral content (BMC) and mid-femoral shaft dimensions were measured using dual energy X-ray absorptometry (DEXA). Relationships between within-pair differences in nutrient intake (determined by weighed-food diaries) or exercise duration (determined by questionnaire) and within-pair differences in BMC and bone dimensions were tested using linear regression analysis. In multivariate analyses, within-pair differences in exercise duration were associated with within-pair differences in total body, leg and spine BMC, and cortical thickness. Every-hour-per-week difference in exercise was associated with a 31-g (1.2%) difference in total body BMC, a 10-g (1.4%) difference in leg BMC, a 0.5-g difference in spine BMC and a 0.1-mm difference in cortical thickness ( p <0.01- p <0.1). A 1-g difference in protein intake was associated with a 0.8-g (0.4%) difference in arm BMC ( p <0.05). These relationships were present in peri-pubertal and post-pubertal pairs but not in pre-pubertal pairs. Exercise during growth appears to have greater skeletal benefits than variations in protein or calcium intakes, with the site-specific effects evident in more mature twins.

Effect of Zinc supplementation on ethanol-mediated bone alterations

Ethanol consumption leads to bone alterations, mainly osteoporosis. Ethanol itself may directly alter bone synthesis, but other factors, such as accompanying protein malnutrition--frequently observed in alcoholics, chronic alcoholic myopathy with muscle atrophy, alcohol induced hypogonadism or hypercortisolism, or liver damage, may all contribute to altered bone metabolism. Some data suggest that zinc may exert beneficial effects on bone growth. Based on these facts, we analyzed the relative and combined effects of ethanol, protein malnutrition and treatment with zinc, 227 mg/l in the form of zinc sulphate, on bone histology, biochemical markers of bone formation (osteocalcin) and resorption (urinary hydroxyproline excretion), and hormones involved in bone homeostasis (insulin growth factor 1 (IGF-1), vitamin D, parathormone (PTH), free testosterone and corticosterone), as well as the association between these parameters and muscle fiber area and liver fibrosis, in eight groups of adult Sprague Dawley rats fed following the Lieber de Carli model during 5 weeks. Ethanol showed an independent effect on TBV (F=14.5, p<0.001), causing it to decrease, whereas a low protein diet caused a reduction in osteoid area (F=8.9, p<0.001). Treatment with zinc increased osteoid area (F=11.2, p<0.001) and serum vitamin D levels (F=3.74, p=0.057). Both ethanol (F=45, p<0.001) and low protein diet (F=46.8, p<0.01) decreased serum osteocalcin levels. Ethanol was the only factor independently related with serum IGF-1 (F=130.24, p<0.001), and also showed a synergistic interaction with protein deficiency (p=0.027). In contrast, no change was observed in hydroxyproline excretion and serum PTH levels. No correlation was found between TBM and muscle atrophy, liver fibrosis, corticosterone, or free testosterone levels, but a significant relationship was observed between type II-b muscle fiber area and osteoid area (rho=0.34, p<0.01). Osteoporosis is, therefore, present in alcohol treated rats. Both alcohol and protein deficiency lead to reduced bone formation. Muscle atrophy is related to osteoid area, suggesting a role for chronic alcoholic myopathy in decreased bone mass. Treatment with zinc increases osteoid area, but has no effect on TBV.

Pamidronate prevents bone loss and decreased bone strength in adult female and male rats fed an isocaloric low-protein diet

Isocaloric dietary protein deficiency is associated with decreased BMD and bone strength as well as depressed somatotroph and gonadotroph axis. Inhibition of increased bone resorption by the bisphosphonate pamidronate in rats fed an isocaloric low-protein diet fully prevents bone loss and alteration of bone strength.

INTRODUCTION

Isocaloric dietary protein deficiency is associated with decreased BMD and bone strength as well as depressed somatotroph and gonadotroph axis. This negative bone balance is the consequence of increased bone resorption and decreased bone formation. Whether inhibition of bone resorption could prevent low-protein diet-induced bone loss and alteration of biomechanics is not known.

MATERIALS AND METHODS

The effect of the bisphosphonate pamidronate was studied in 5.5-month-old female or 6-month-old male rats pair-fed a control (15% casein) or an isocaloric low-protein (2.5% casein) diet for 19 and 26 weeks, respectively. Pamidronate (0.6 mg/kg) was given subcutaneously 5 days/month for 4 months in female rats or for 5 months in male rats. BMD, microarchitecture, and bone strength were measured at the level of the proximal and midshaft tibia. Urinary deoxypyridinoline excretion, serum osteocalcin, and IGF-I were also measured.

RESULTS

The increase in bone resorption in female rats (+100%) and in male rats (+33%) fed a low-protein diet was prevented by pamidronate treatment. The reduced osteocalcin levels observed in rats fed a low-protein diet were further decreased in both female (-34%) and male (-30%) rats treated with pamidronate. The bone turnover decrease induced by pamidronate prevented bone strength reduction, trabecular bone loss, microarchitecture, and BMD alterations induced by the isocaloric low-protein diet. Similar effects were observed at the level of the midshaft tibia. Significant decrease of plasma IGF-I was observed in rats fed a low-protein diet independently of the pamidronate treatment.

CONCLUSION

In conclusion, inhibition of increased bone resorption in rats fed an isocaloric low-protein diet fully prevents bone loss and alteration of bone strength.

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.

Interleukin 10-deficient mice develop osteopenia, decreased bone formation, and mechanical fragility of long bones

BACKGROUND & AIMS

Bone loss is a common complication of human inflammatory bowel disease (IBD), but its mechanisms are not understood completely. We investigated bone metabolism in interleukin-10-deficient ( IL-10-/- ) mice, an animal model with IBD features.

METHODS

IL-10-/- male mice (8- and 12-weeks-old) and their age-matched wild-type counterparts (C57BL/6J) were studied. Bone mass of the femur was determined by ashing. Tibial cancellous and cortical bone mass and formation was measured by static and dynamic histomorphometry. Biomechanical strength of the femur was tested. Primary bone marrow stromal cell cultures were used to assess osteoblast generation. Serum levels of 25-OH vitamin D 3, insulin-like growth factor-1 (IGF-1), parathyroid hormone, osteocalcin, and deoxy-pyridinoline cross-links were measured. The presence of colitis was determined histologically, and by IL-12 and interferon-gamma (IFN-gamma) secretion from cultured colonic explants.

RESULTS

Eight- and 12-week-old IL-10-/- mice developed osteopenia of both cancellous and cortical bone, evidenced by lower femoral ash weight, cancellous bone area and surface, trabecular number, and decreased cortical bone area and width. Osteopenia was associated with mechanical fragility, manifested by decreased stiffness and mechanical load at fracture, and was caused by suppressed bone formation, indicated by decreased cancellous double-labeled surface, mineralizing surface, serum osteocalcin level, and mineralized nodule number in bone marrow stromal cell cultures. IL-10-/- mice with colitis had significantly less bone mass compared with IL-10-/- mice without colitis.

CONCLUSIONS

IL-10-/- mice develop the hallmarks of osteoporosis, that is, reduced bone mass, increased mechanical fragility, and suppressed bone formation. The presence of colitis is an important contributor to osteoporosis in IL-10-/- mice.

Dietary protein and bone health

The effects of dietary protein on bone health are paradoxical and need to be considered in context of the age, health status and usual diet of the population. Over the last 80 years numerous studies have demonstrated that a high protein intake increases urinary Ca excretion and that on average 1 mg Ca is lost in urine for every 1 g rise in dietary protein. This relationship is primarily attributable to metabolism of S amino acids present in animal and some vegetable proteins, resulting in a greater acid load and buffering response by the skeleton. However, many of these early studies that demonstrated the calciuric effects of protein were limited by low subject numbers, methodological errors and the use of high doses of purified forms of protein. Furthermore, the cross-cultural and population studies that showed a positive association between animal-protein intake and hip fracture risk did not consider other lifestyle or dietary factors that may protect or increase the risk of fracture. The effects of protein on bone appear to be biphasic and may also depend on intake of Ca- and alkali-rich foods, such as fruit and vegetables. At low protein intakes insulin-like growth factor production is reduced, which in turn has a negative effect on Ca and phosphate metabolism, bone formation and muscle cell synthesis. Although growth and skeletal development is impaired at very low protein intakes, it is not known whether variations in protein quality affect the achievement of optimal peak bone mass in adolescents and young adults. Prospective studies in the elderly in the USA have shown that the greatest bone losses occur in elderly men and women with an average protein intake of 16-50 g/d. Although a low protein intake may be indicative of a generally poorer diet and state of health, there is a need to evaluate whether there is a lower threshold for protein intake in the elderly in Europe that may result in increased bone loss and risk of osteoporotic fracture.

Noninvasive estimation of bone mass in ancient vertebrae

Histomorphometry is useful in the assessment of trabecular bone mass (TBM), and thus, in the estimation of the prevalence and intensity of osteopenia in ancient population groups. However, it is a destructive method. It is therefore necessary to explore the accuracy of nondestructive approaches, such as radiography, bone mineral density (BMD) assessed by double-energy X-ray absorptiometry (DEXA), bone density (BD), or optical density (OD) in the diagnosis of osteopenia. We selected 51 vertebrae out of a total sample composed of 333 T12, L1, and L2 vertebrae belonging to adult pre-Hispanic inhabitants from El Hierro. These vertebrae underwent histomorphometrical analysis, a fine-grained film radiography with assessment of trabecular pattern following standard methods, OD, DEXA-assessed BMD, and BD. The presence of biconcave vertebrae and wedge-shaped vertebrae was also assessed by measuring anterior height (a), posterior height (p), and height at the middle point of the vertebral body (m), and further calculating the indices 2m/(a + p) ("spine score") and a/p. Significant correlations were observed between TBM and BMD (r=0.43), TBM and BD (r=0.49), TBM and OD (r=0.52), BMD and OD (r=0.51), and BMD and BD (r=0.36), but not between TBM and the indices 2m/(a + p) and a/p. In the stepwise multiple correlation analysis between TBM and BMD, BD, and OD, OD entered into first place and BD into second place, whereas BMD became displaced; the multiple correlation coefficient was 0.63, with a standard error of 3.78. A BMD greater than 0.60 g/cm2, or a bone density greater than 0.60 g/cm3, excluded osteopenia (TBM <15%) with a specificity greater than 90%, whereas a BMD value less than 0.35 g/cm2, a BD less than 0.35 g/cm3, or optical density >1.6 excluded a normal bone mass (TBM >20%) with a specificity greater than 90%. Based on radiographic criteria on the total sample, we also conclude that the overall prevalence of vertebral fractures in the adult pre-Hispanic population of El Hierro of any age is 7.5%.

Running Inhibits Osteoporosis Induced by Protein-deficient (PD) Food Intake

Running at 0.7 km/h for 10 min every day inhibited development of osteoporosis caused by protein deficient (PD) food intake. Urine alkaline phosphatase (ALP), a marker of bone formation osteoporosis, was not elevated in rats fed PD, when the osteoporosis was inhibited by running. Estrogen supplementation increased bone-breaking energy (BBE), but did not increase bone mineral density (BMD), and did not decrease urinary ALP levels.

Maternal protein deficiency affects mesenchymal stem cell activity in the developing offspring

Epidemiological studies suggest that environmental influences such as maternal nutrition, programme skeletal growth during intrauterine and early postnatal life. However, the mechanism whereby the skeletal growth trajectory is modified remains unclear. We have addressed this using a rat model of maternal protein insufficiency to investigate the cellular mechanisms involved in the programming of bone development. The aims of this study were to determine whether colony formation (colony forming unit-fibroblastic, CFU-F), proliferation, and differentiation of bone marrow stromal cells from offspring of female rats maintained on normal (18% casein) or low (9% casein) protein was altered and, whether their responses to growth hormone (GH), 1,25(OH)(2)D3, and IGF-1 differed. Dams were fed an 18% casein (control) diet or 9% casein (low protein) diet from conception until the end of pregnancy. Offspring were then fed a normal protein diet until harvest at 8, 12, and 16 weeks after birth. At 8 weeks, total CFU-F and alkaline phosphatase-positive CFU-F were significantly (P < 0.01) reduced in the low protein group compared to controls. At 12 weeks, no significant differences were observed in colony formation. Modulation of osteoblast proliferation and differentiation by IGF-1 and GH was observed (P < 0.01) in the control group at 8 weeks and the low protein group at 12 weeks. Alkaline phosphatase specific activity was significantly decreased at 8 weeks (P < 0.001) in the low protein group. At 12 and 16 weeks this was reversed, with significantly increased specific activity in the low protein group. These results suggest that normal proliferation and differentiation of mesenchymal stem cells were delayed by maternal protein restriction during early life. Furthermore, these results suggest that, with skeletal maturity, "catch-up" or a physiological shift in bone cell activity was present in the low protein group. These alterations in mesenchymal stem cell function by the early environment may represent an important candidate mechanism for the programming of osteoporosis and associated consequences in later life.

UK Food Standards Agency alpha-linolenic acid workshop report

The UK Food Standards Agency convened a group of expert scientists to review current research investigating whether n-3 polyunsaturated fatty acids (PUFA) from plant oils (alpha-linolenic acid; ALA) were as beneficial to cardiovascular health as the n-3 PUFA from the marine oils, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The workshop also aimed to establish priorities for future research. Dietary intake of ALA has been associated with a beneficial effect on CHD; however, the results from studies investigating the effects of ALA supplementation on CHD risk factors have proved equivocal. The studies presented as part of the present workshop suggested little, if any, benefit of ALA, relative to linoleic acid, on risk factors for cardiovascular disease; the effects observed with fish-oil supplementation were not replicated by ALA supplementation. There is a need, therefore, to first prove the efficacy of ALA supplementation on cardiovascular disease, before further investigating effects on cardiovascular risk factors. The workshop considered that a beneficial effect of ALA on the secondary prevention of CHD still needed to be established, and there was no reason to look further at existing CHD risk factors in relation to ALA supplementation. The workshop also highlighted the possibility of feeding livestock ALA-rich oils to provide a means of increasing the dietary intake in human consumers of EPA and DHA.

Dietary essential amino acid supplements increase bone strength by influencing bone mass and bone microarchitecture in ovariectomized adult rats fed an isocaloric low-protein diet

This study was designed to investigate whether the administration of dietary essential amino acid supplements in adult rats made osteoporotic by estrogen deficiency and reduced protein intake could reverse the deleterious effects caused by these maneuvers. This animal model was selected to mimic the situation observed in elderly women in whom estrogen deficiency and/or low-protein intake (but also calcium and vitamin D deficiency) are known to contribute to the pathogenesis of osteoporosis. Six-month-old rats were ovariectomized (OVX) and fed an isocaloric 2.5% casein diet for 10 weeks or sham-operated (SHAM) and fed an isocaloric 15% casein diet. The animals fed the 2.5% casein diet were given isocaloric supplements of essential amino acids in similar relative proportion to that of casein at doses of 2.5% or 5% of total diet for an additional 16 weeks. Vertebrae, femur, and tibia bone mineral density (BMD); ultimate strength; and microtomographic histomorphometry were evaluated before and after dietary essential amino acid supplements. Essential amino acid supplements increased vertebrae, femur, and tibia bone strength in OVX rats fed a low-protein diet. The mechanical changes induced by this dietary isocaloric supplement were associated with the prevention of a further BMD decrease or even with some increases and changes in microarchitecture such as from a rod to a plate trabecular spacial configuration and increased cortical thickness. Higher insulin-like growth factor (IGF) I levels, as well as greater bone formation and reduced bone resorption as assessed by biochemical markers of bone remodeling, were found in rats receiving essential amino acid supplements. In conclusion, dietary essential amino acid supplements increased bone strength through modifications of BMD, trabecular architecture, and cortical thickness possibly by an IGF-I-mediated process.

Double-energy X-ray absorptiometry in the diagnosis of osteopenia in ancient skeletal remains

Bone mineral density (BMD) assessed by double-energy X-ray absorptiometry (DEXA) accurately estimates the bone mass in living individuals, and is thus the method usually employed in the diagnosis and follow-up of osteopenia. It is preferred, in clinical settings, to the more invasive and destructive histomorphometrical assessment of trabecular bone mass in undecalcified bone samples. This study was performed in order to examine the value of DEXA-assessed BMD at the proximal end of the right tibia, either alone or in combination with the cortico-medullary index at the midshaft point of the right tibia (CMI), in the diagnosis of osteopenia in a prehistoric sample composed of 95 pre-Hispanic individuals from Gran Canaria. Age at death could be estimated in 34 cases. Diagnosis of osteopenia was performed by histomorphometrical assessment of trabecular bone mass (TBM) in an undecalcified bone section of a small portion of the proximal epiphysis of the right tibia. A high prevalence of osteopenia was found among the population of Gran Canaria. Both TBM and BMD were significantly lower in the older individuals than in younger ones, and BMD was also significantly lower in female individuals. BMD was moderately correlated with TBM (r = +0.51); the correlation was higher if CMI was included (multiple r = +0.615). BMD values lower than 0.7 g/cm2 showed a high specificity (>93%) at excluding normal TBM values. These methods were prospectively applied in a further sample of 21 right tibiae from Gran Canaria, Tenerife, and El Hierro. The results were similar to those obtained in the larger sample. Thus, DEXA-assessed BMD combined with CMI (noninvasive procedures) may be useful in detecting osteopenia in ancient populations.

Protein intake and bone disorders in the elderly

Malnutrition, most notably protein deficiency, contributes to the occurrence of osteoporotic fractures not only by decreasing bone mass but also by altering muscle function. Furthermore, malnutrition is associated with increased morbidity in patients with osteoporotic fractures. The somatomedin system (IGF-1) may be directly involved in the pathogenesis of osteoporotic hip fractures and their complications in elderly patients. A low IGF-1 level is a risk factor for hip fracture. In subjects with appropriate intakes of vitamin D and calcium, giving protein supplements to correct an inadequate spontaneous protein intake increases circulating IGF-1 levels, improves clinical outcomes after hip fracture, and prevents bone mineral density loss at the proximal femur. Supplemental protein also significantly reduces the length of inpatient rehabilitation care. These data emphasize the importance of adequate nutrient intake in the prevention and treatment of osteoporotic fractures.