Renal acid, urinary cyclic AMP, and hydroxyproline excretion as affected by level of protein, sulfur amino acid, and phosphorus intake

Dietary Protein Intake above the Current RDA and Bone Health: A Systematic Review and Meta-Analysis

Dietary intake of protein is fundamental for optimal acquisition and maintenance of bone across all life stages; however, it has been hypothesized that intakes above the current recommended dietary allowance (RDA) might be beneficial for bone health. We utilized the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines when preparing and reporting this systematic review and meta-analysis. A literature search strategy through April 11, 2017, was developed for the following 3 databases: PubMed, Ovid Medline, and Agricola. Included studies were those randomized controlled trials and prospective cohort studies among healthy adults ages 18 and older that examined the relationships between varying doses of protein intake at or above the current U.S. RDA (0.8 g/kg/d or 10%-15% of total caloric intake) from any source on fracture, bone mineral density (BMD)/bone mineral content (BMC), and/or markers of bone turnover. Twenty-nine articles were included for data extraction (16 randomized controlled trials [RCTs] and 13 prospective cohort studies). Meta-analysis of the prospective cohort studies showed high vs low protein intakes resulted in a statistically significant 16% decrease in hip fractures (standardized mean difference [SMD] = 0.84, 95% confidence interval [CI], 0.73, 0.95; I² = 36.8%). Data from studies included in these analyses collectively lean toward the hypothesis that protein intake above the current RDA is beneficial to BMD at several sites. This systematic review supports that protein intakes above the current RDA may have some beneficial role in preventing hip fractures and BMD loss. There were no differences between animal or plant proteins, although data in this area were scarce. Larger, long-term, and more well-controlled clinical trials measuring fracture outcomes and BMD are needed to adequately assess whether protein intake above the current RDA is beneficial as a preventative measure and/or intervention strategy for osteoporosis. Key teaching points: • • Bone health is a multifactorial musculoskeletal issue, and optimal protein intakes are key in developing and maintaining bone throughout the life span. • • Dietary protein at levels above the current RDA may be beneficial in preventing hip fractures and BMD loss. • • Plant vs animal proteins do not seem to differ in their ability to prevent bone loss; however, data in this area are scarce. • • Larger, long-term RCTs using women not using hormone replacement therapy (HRT) are needed to adequately assess the magnitude of impact that protein intakes above the RDA have on preventing bone loss.

Dietary protein and bone health: a systematic review and meta-analysis from the National Osteoporosis Foundation

Background: Considerable attention has recently focused on dietary protein's role in the mature skeleton, prompted partly by an interest in nonpharmacologic approaches to maintain skeletal health in adult life.Objective: The aim was to conduct a systematic review and meta-analysis evaluating the effects of dietary protein intake alone and with calcium with or without vitamin D (Ca±D) on bone health measures in adults.Design: Searches across 5 databases were conducted through October 2016 including randomized controlled trials (RCTs) and prospective cohort studies examining 1) the effects of "high versus low" protein intake or 2) dietary protein's synergistic effect with Ca±D intake on bone health outcomes. Two investigators independently conducted abstract and full-text screenings, data extractions, and risk of bias (ROB) assessments. Strength of evidence was rated by group consensus. Random-effects meta-analyses for outcomes with ≥4 RCTs were performed.Results: Sixteen RCTs and 20 prospective cohort studies were included in the systematic review. Overall ROB was medium. Moderate evidence suggested that higher protein intake may have a protective effect on lumbar spine (LS) bone mineral density (BMD) compared with lower protein intake (net percentage change: 0.52%; 95% CI: 0.06%, 0.97%, I²: 0%; n = 5) but no effect on total hip (TH), femoral neck (FN), or total body BMD or bone biomarkers. Limited evidence did not support an effect of protein with Ca±D on LS BMD, TH BMD, or forearm fractures; there was insufficient evidence for FN BMD and overall fractures.Conclusions: Current evidence shows no adverse effects of higher protein intakes. Although there were positive trends on BMD at most bone sites, only the LS showed moderate evidence to support benefits of higher protein intake. Studies were heterogeneous, and confounding could not be excluded. High-quality, long-term studies are needed to clarify dietary protein's role in bone health. This trial was registered at www.crd.york.ac.uk as CRD42015017751.

Reducing the Dietary Acid Load: How a More Alkaline Diet Benefits Patients With Chronic Kidney Disease

It has been proposed that a low-protein diet will slow progression of chronic kidney disease although studies have not always supported this belief. The accepted practice is that 60% to 70% of protein comes from high biological value (HBV) protein, but this limits patient choice and patients struggle to follow the diet. When a diet with only 30% HBV protein was trialed, there was a significant increase in serum bicarbonate, and patients preferred the diet. The dietary advice given in predialysis clinics was changed. HBV protein was restricted to approximately 50% of total protein, bread and cereal foods were allowed freely, and fruits and vegetables (F&V) were encouraged. Patients who followed the diet have seen a slowing of progression and occasionally regression of their renal function. Both observations and scientific literature indicate that this is because of a reduction in the acid content of the diet. When foods are metabolized, most proteins produce acid, and most F&V produce alkali. A typical 21^st-century diet produces 50 to 100 mEq H⁺ per day which the kidney is challenged to excrete. Acid is excreted with phosphate and is limited to about 45 mEq H⁺ per day. With chronic kidney disease, this falls progressively to below 20 mEq H⁺ per day. Historically, ammonium excretion was believed to be excretion of acid (NH₃⁺ + H⁺ → NH₄⁺), but it is now understood to be a by-product in the neutralization of acid by glutamine. The remaining acid is neutralized or stored within the body. Bone and muscle are lost in order to neutralize the acid. Acid also accumulates within cells, and serum bicarbonate falls. The author postulates that reducing the acid load through a low-protein diet with greater use of vegetable proteins and increased F&V intake will slow progression or occasionally improve renal function while maintaining the nutritional status of the individual.

Effect of feeding graded levels of crude protein on nutrient utilization and feather growth in Lady Amherst's pheasants

In order to find out the optimum level of crude protein (CP) in the diet of captive Lady Amherst's pheasants (LAP) on molt, 18 male birds were randomly distributed into three groups of six each in an experiment based on completely randomized block design. The CP content of the diets of birds in groups I, II, and III was 13.4, 16.5, and 19.1%, respectively. Intake and apparent balance of nitrogen increased linearly (P < 0.001) as CP content of the diet increased. Intake and utilization of energy, calcium, and phosphorous were similar among groups. Body mass change and growth rate of feathers were significantly (P < 0.01) lower in group I as compared to groups II and III. There was a positive co-relationship between ME intake and change in body weight (R(2) = 0.89, F = 126.4, P < 0.001). Regression analysis indicates that LAP can maintain body mass when ME supply is 122.2 Kcal/kg BW(0.75)/d. Linear relationships between intake and apparent retention of N, Ca, and P as expressed on mg/kg BW(0.75)/d were all significant. Apparent nitrogen retention, and mean feather growth rate was lower in birds fed diet containing 13.4% CP. Feeding of the diets containing 16.5% CP resulted in improved retention of nitrogen, and mean feather growth rate. Further increase in dietary concentration of CP to 19.1% showed no further improvement. It was concluded that a diet containing 16.5% CP would be optimum for Lady Amherst's pheasants during molt.

Supplementing a low-protein diet with dibasic amino acids increases urinary calcium excretion in young women

Increasing dietary protein within a physiologic range stimulates intestinal calcium absorption, but it is not known if specific amino acids or dietary protein as a whole are responsible for this effect. Therefore, we selectively supplemented a low-protein (0.7 g/kg) diet with either the calcium-sensing receptor-activating amino acids (CaSR-AAAs) L-tryptophan, L-phenylalanine, and L-histidine, or the dibasic amino acids (DAAs) L-arginine and L-lysine, to achieve intakes comparable to the content of a high-protein diet (2.1 g/kg) and measured intestinal calcium absorption. Fourteen young women took part in a placebo-controlled, double-blind, crossover feeding trial in which each participant ingested a 6-d low-protein diet supplemented with CaSR-AAAs, DAAs, or methylcellulose capsules (control) after an 11-d adjustment period. All participants ingested all 3 diets in random order. Intestinal calcium absorption was measured between days 5 and 6 using dual-stable calcium isotopes ((42)Ca, (43)Ca, and (44)Ca). There was no difference in calcium absorption between the diet supplemented with CaSR-AAAs (22.9 ± 2.0%) and the control diet (22.3 ± 1.4%) (P = 0.64). However, calcium absorption tended to be greater during the DAA supplementation period (25.2 ± 1.4%) compared with the control diet period (22.3 ± 1.4%) (P < 0.10). Larger and longer clinical trials are needed to clarify the possible benefit of arginine and lysine on calcium absorption.

Dietary acid load: a novel nutritional target in chronic kidney disease?

Nonvolatile acid is produced from the metabolism of organic sulfur in dietary protein and the production of organic anions during the combustion of neutral foods. Organic anion salts that are found primarily in plant foods are directly absorbed in the gastrointestinal tract and yield bicarbonate. The difference between endogenously produced nonvolatile acid and absorbed alkali precursors yields the dietary acid load, technically known as the net endogenous acid production, and must be excreted by the kidney to maintain acid-base balance. Although typically 1 mEq/kg/day, dietary acid load is lower with greater intake of fruits and vegetables. In the setting of CKD, a high dietary acid load invokes adaptive mechanisms to increase acid excretion despite reduced nephron number, such as increased per nephron ammoniagenesis and augmented distal acid excretion mediated by the renin-angiotensin system and endothelin-1. These adaptations may promote kidney injury. Additionally, high dietary acid loads produce low-grade, subclinical acidosis that may result in bone and muscle loss. Early studies suggest that lowering the dietary acid load can improve subclinical acidosis, preserve bone and muscle, and slow the decline of glomerular filtration rate in animal models and humans. Studies focusing on hard clinical outcomes are needed.

Update on nutrients involved in maintaining healthy bone

Osteoporosis is a leading cause of morbidity and mortality in the elderly and influences quality of life, as well as life expectancy. Currently, there is a growing interest among the medical scientists in search of specific nutrients and/or bioactive compounds of natural origin for the prevention of disease and maintenance of bone health. Although calcium and vitamin D have been the primary focus of nutritional prevention of osteoporosis, a recent research has clarified the importance of several additional nutrients and food constituents. Based on this review of the literature, supplementation with vitamins B, C, K, and silicon could be recommended for proper maintenance of bone health, although further clinical studies are needed. The results of studies on long-chain polyunsaturated fatty acids, potassium, magnesium, copper, selenium, and strontium are not conclusive, although studies in vitro and in animal models are interesting and promising.

Causal assessment of dietary acid load and bone disease: a systematic review & meta-analysis applying Hill's epidemiologic criteria for causality

Background

Modern diets have been suggested to increase systemic acid load and net acid excretion. In response, alkaline diets and products are marketed to avoid or counteract this acid, help the body regulate its pH to prevent and cure disease. The objective of this systematic review was to evaluate causal relationships between dietary acid load and osteoporosis using Hill's criteria.

Methods

Systematic review and meta-analysis. We systematically searched published literature for randomized intervention trials, prospective cohort studies, and meta-analyses of the acid-ash or acid-base diet hypothesis with bone-related outcomes, in which the diet acid load was altered, or an alkaline diet or alkaline salts were provided, to healthy human adults. Cellular mechanism studies were also systematically examined.

Results

Fifty-five of 238 studies met the inclusion criteria: 22 randomized interventions, 2 meta-analyses, and 11 prospective observational studies of bone health outcomes including: urine calcium excretion, calcium balance or retention, changes of bone mineral density, or fractures, among healthy adults in which acid and/or alkaline intakes were manipulated or observed through foods or supplements; and 19 in vitro cell studies which examined the hypothesized mechanism. Urine calcium excretion rates were consistent with osteoporosis development; however calcium balance studies did not demonstrate loss of whole body calcium with higher net acid excretion. Several weaknesses regarding the acid-ash hypothesis were uncovered: No intervention studies provided direct evidence of osteoporosis progression (fragility fractures, or bone strength as measured using biopsy). The supporting prospective cohort studies were not controlled regarding important osteoporosis risk factors including: weight loss during follow-up, family history of osteoporosis, baseline bone mineral density, and estrogen status. No study revealed a biologic mechanism functioning at physiological pH. Finally, randomized studies did not provide evidence for an adverse role of phosphate, milk, and grain foods in osteoporosis.

Conclusions

A causal association between dietary acid load and osteoporotic bone disease is not supported by evidence and there is no evidence that an alkaline diet is protective of bone health.

Phosphate decreases urine calcium and increases calcium balance: a meta-analysis of the osteoporosis acid-ash diet hypothesis

Background

The acid-ash hypothesis posits that increased excretion of "acidic" ions derived from the diet, such as phosphate, contributes to net acidic ion excretion, urine calcium excretion, demineralization of bone, and osteoporosis. The public is advised by various media to follow an alkaline diet to lower their acidic ion intakes. The objectives of this meta-analysis were to quantify the contribution of phosphate to bone loss in healthy adult subjects; specifically, a) to assess the effect of supplemental dietary phosphate on urine calcium, calcium balance, and markers of bone metabolism; and to assess whether these affects are altered by the b) level of calcium intake, c) the degree of protonation of the phosphate.

Methods

Literature was identified through computerized searches regarding phosphate with surrogate and/or direct markers of bone health, and was assessed for methodological quality. Multiple linear regression analyses, weighted for sample size, were used to combine the study results. Tests of interaction included stratification by calcium intake and degree of protonation of the phosphate supplement.

Results

Twelve studies including 30 intervention arms manipulated 269 subjects' phosphate intakes. Three studies reported net acid excretion. All of the meta-analyses demonstrated significant decreases in urine calcium excretion in response to phosphate supplements whether the calcium intake was high or low, regardless of the degree of protonation of the phosphate supplement. None of the meta-analyses revealed lower calcium balance in response to increased phosphate intakes, whether the calcium intake was high or low, or the composition of the phosphate supplement.

Conclusion

All of the findings from this meta-analysis were contrary to the acid ash hypothesis. Higher phosphate intakes were associated with decreased urine calcium and increased calcium retention. This meta-analysis did not find evidence that phosphate intake contributes to demineralization of bone or to bone calcium excretion in the urine. Dietary advice that dairy products, meats, and grains are detrimental to bone health due to "acidic" phosphate content needs reassessment. There is no evidence that higher phosphate intakes are detrimental to bone health.

Soy proteins and isoflavones affect bone mineral density in older women: a randomized controlled trial

BACKGROUND

Soy foods contain several components (isoflavones and amino acids) that potentially affect bone. Few long-term, large clinical trials of soy as a means of improving bone mineral density (BMD) in late postmenopausal women have been conducted.

OBJECTIVE

Our goal was to evaluate the long-term effect of dietary soy protein and/or soy isoflavone consumption on skeletal health in late postmenopausal women.

DESIGN

We conducted a randomized, double-blind, placebo-controlled clinical trial in 131 healthy ambulatory women aged >60 y. Ninety-seven women completed the trial. After a 1-mo baseline period, subjects were randomly assigned into 1 of 4 intervention groups: soy protein (18 g) + isoflavone tablets (105 mg isoflavone aglycone equivalents), soy protein + placebo tablets, control protein + isoflavone tablets, and control protein + placebo tablets.

RESULTS

Consumption of protein powder and isoflavone pills did not differ between groups, and compliance with the study powder and pills was 80-90%. No significant differences in BMD were observed between groups from baseline to 1 y after the intervention or in BMD change between equol and non-equol producers. However, there were significant negative correlations between total dietary protein (per kg) and markers of bone turnover (P < 0.05).

CONCLUSIONS

Because soy protein and isoflavones (either alone or together) did not affect BMD, they should not be considered as effective interventions for preserving skeletal health in older women. The negative correlation between dietary protein and bone turnover suggests that increasing protein intakes may suppress skeletal turnover. This trial was registered at ClinicalTrials.gov as NCT00668447.

Dietary protein and calcium interact to influence calcium retention: a controlled feeding study

BACKGROUND

The effect of meat protein on calcium retention at different calcium intakes is unresolved.

OBJECTIVE

The objective was to test the effect of dietary protein on calcium retention at low and high intakes of calcium.

DESIGN

In a randomized controlled feeding study with a 2 x 2 factorial crossover design, healthy postmenopausal women (n = 27) consumed either approximately 675 or approximately 1510 mg Ca/d, with both low and high protein (providing 10% and 20% energy) for 7 wk each, separated by a 3-wk washout period. After 3 wk, the entire diet was extrinsically labeled with (47)Ca, and isotope retention was monitored by whole-body scintillation counting. Clinical markers of calcium and bone metabolism were measured.

RESULTS

High compared with low dietary protein significantly increased calcium retention from the low-calcium (29.5% compared with 26.0% absorbed) but not the high-calcium diet (18% absorbed). For the low-calcium diet, this effect nearly balanced a protein-related 0.5-mmol/d greater urinary calcium excretion. Protein-related calciuretic effects were independent of dietary calcium. Testing at 1, 2, 3, 5, and 7 wk showed no long-term adaptation in urinary acidity or urinary calcium excretion. High compared with low dietary protein decreased urinary deoxypyridinoline and increased serum insulin-like growth factor I without affecting parathyroid hormone, osteocalcin, bone-specific alkaline phosphatase, or tartrate-resistant acid phosphatase.

CONCLUSIONS

In healthy postmenopausal women, a moderate increase in dietary protein, from 10% to 20% of energy, slightly improved calcium absorption from a low-calcium diet, nearly compensating for a slight increase in urinary calcium excretion. Under practical dietary conditions, increased dietary protein from animal sources was not detrimental to calcium balance or short-term indicators of bone health.

Meta-analysis of the quantity of calcium excretion associated with the net acid excretion of the modern diet under the acid-ash diet hypothesis

BACKGROUND

The acid-ash diet hypothesis of osteoporosis suggests that acid from the modern diet causes a demineralization of the skeleton, and mobilized bone calcium is excreted. A systematic approach has not been used to summarize the findings of the numerous studies about the hypothesis.

OBJECTIVES

The purpose of this meta-analysis was to estimate the quantity of net acid excretion and calciuria associated with the modern diet, to assess the association between acid excretion and calcium excretion, and to assess the influence of urine preservatives on calcium measurement.

DESIGN

We systematically searched for trials of the acid-ash hypothesis and conducted a meta-analysis.

RESULTS

Twenty-five of 105 studies met the inclusion criteria. The estimated quantity of net acid excretion from the weighted average of the control diets from 11 studies was 47 mEq/d. The increase in urinary calcium with a change in renal net acid excretion depended on whether the urine was acidic or alkaline (P < 0.001). A significant linear relation was observed between net acid excretion and calcium excretion for both acidic and alkaline urine (P < 0.001). The estimated change in urine calcium associated with a change of 47 mEq of net acid excretion in acidic urine was 1.6 mmol/d (66 mg/d) of calcium.

CONCLUSION

Evidence suggests a linear association between changes in calcium excretion in response to experimental changes in net acid excretion. However, this finding is not evidence that the source of the excreted calcium is bone or that this calciuria contributes to the development of osteoporosis.

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.

The effect of high salt and high protein intake on calcium metabolism, bone composition and bone resorption in the rat

The effects of salt (NaCl) supplementation of rat diets (50 g/kg diet), with normal (200 g/kg) or high (500 g/kg) dietary casein content, were studied in 3-week-old male rats over a 3-week period. Weight gain was reduced by dietary salt but was unaffected by dietary casein. Salt-supplemented rats exhibited a two-and three-fold increase in urinary Mg and Ca excretion respectively, irrespective of dietary casein content. Dietary casein had no effect on urinary Ca or Mg. Salt reduced femoral mass but not femoral mass expressed relative to body weight, but neither variable was affected by dietary casein. Femoral Mg and P contents and concentrations were unaffected by dietary salt or casein. While femoral Ca concentration was unaffected by dietary salt, the Ca content was reduced by salt supplementation, irrespective of dietary casein content. Neither the content nor concentration of Ca in femora was affected by dietary casein. Urinary pyridinoline and deoxypyridinoline levels were increased by salt supplementation, irrespective of dietary casein content, but were unaffected by casein. Net Ca absorption was unaffected by dietary salt or casein. In conclusion, these results show that salt supplementation over the short-term increased the rate of bone resorption in rats. This was as a consequence of Na-induced calciuria. On the other hand, a high dietary protein intake had no effect on Ca metabolism, bone composition or bone resorption, nor did it augment the Na-induced calciuria or increased rate of bone resorption.

Lithiasis and Risk Factors

Nephrolithiasis is a worldwide disease with high clinical and economic costs. The increasing incidence in industrialized countries seems to be related to several risk factors, which are partly inherited and partly acquired. Although risk factors in urolithiasis are still under discussion, their identification would provide a notable gain for the patient in terms of stone episodes, and for the health service in terms of costs. This article presents an easy classification of risk factors based on clinical background.

Meat and soy protein affect calcium homeostasis in healthy women

We showed that increasing dietary protein from omnivorous sources increases intestinal calcium absorption and urinary calcium, whereas a low-protein diet decreases calcium absorption and lowers urinary calcium. To assess the effect of soy protein on this relation, we substituted soy for meat in high- and low-protein diets fed to healthy women. The study consisted of a 2-wk adjustment period followed by a 4-d experimental period in which 20 healthy women consumed, in random order, the following 4 diets: high-protein soy-based, low-protein soy-based, high-protein meat-based, low-protein meat-based. Measures of calcium homeostasis were evaluated at baseline and after 4 d of the experimental period. At 24 h, net acid excretion was higher during the high- compared with the low-protein intervention (P < 0.05), and during the meat compared with the soy intervention (P < 0.05). The high-protein diets increased 24-h urinary calcium (P < 0.001), but urinary calcium did not differ due to the type of protein. Serum concentrations of parathyroid hormone and calcitriol, and urinary nephrogenous cAMP were higher during the low- compared with the high-protein intervention and during the soy compared with the meat protein (P < 0.05). In a subset of subjects, intestinal calcium absorption tended to be lower (P = 0.1) when they consumed the soy diets rather than the meat diets. These data indicate that when soy protein is substituted for meat protein, there is an acute decline in dietary calcium bioavailability.

Impact of dietary habits on stone incidence

Changes in dietary habits and lifestyle are suggested to contribute markedly to the rise in the prevalence and incidence of urolithiasis during the past decades. Insufficient fluid intake and diets rich in animal protein are considered to be important determinants of stone formation. Overweight and associated dietary pattern additionally contribute to the increasing incidence and prevalence of stone disease. Reduction of overweight through extreme fasting or high-protein weight-loss diets, e.g. Atkins diet, also appear to affect stone formation. Although there is evidence that changes in dietary habits can reduce urinary risk factors and the risk of stone formation, further randomized controlled clinical trials are necessary to evaluate long-term effects of dietary interventions on stone disease.

The effect of soy protein and soy isoflavones on calcium metabolism in postmenopausal women: a randomized crossover study

BACKGROUND

Evidence suggests that soy isoflavones act as estrogen agonists and have beneficial skeletal effects, but the effects on calcium metabolism in humans are not known.

OBJECTIVE

This study tested whether soybean isoflavones, soy protein, or both alter calcium metabolism in postmenopausal women.

DESIGN

Calcium metabolism in 15 postmenopausal women was studied by using metabolic balance and kinetic modeling in a randomized, crossover design of three 1-mo controlled dietary interventions: soy protein isolate enriched with isoflavones (soy-plus diet), soy protein isolate devoid of isoflavones (soy-minus diet), and a casein-whey protein isolate (control diet).

RESULTS

There was no significant difference between the diets in net acid excretion (P = 0.12). Urinary calcium excretion was significantly (P < 0.01) less with consumption of either of the soy diets (soy-plus diet: 85 +/- 34 mg/d; soy-minus diet: 80 +/- 34 mg/d) than with consumption of the control diet (121 +/- 63 mg/d), but fractional calcium absorption was unaffected by treatment. Endogenous fecal calcium was significantly (P < 0.01) greater with consumption of the soy-minus diet than with consumption of the other diets. Total fecal calcium excretion, bone deposition and resorption, and calcium retention were not significantly affected by the dietary regimens.

CONCLUSIONS

The lower urinary calcium seen with the consumption of an isolated soy protein than with that of an isolated milk protein was not associated with improved calcium retention. This finding reinforces the importance of evaluating all aspects of calcium metabolism. Soy isoflavones did not significantly affect calcium metabolism.

The effect of a high-protein, high-sodium diet on calcium and bone metabolism in postmenopausal women and its interaction with vitamin D receptor genotype

The influence of a high-Na, high-protein (calciuric) diet on Ca and bone metabolism was investigated in postmenopausal women (aged 50-67 years) who were stratified by vitamin D receptor (VDR) genotype. In a crossover trial, twenty-four women were randomly assigned to a diet high in protein (90 g/d) and Na (180 mmol/d) or a diet adequate in protein (70 g/d) and low in Na (65 mmol/d) for 4 weeks, followed by crossover to the alternative dietary regimen for a further 4 weeks. Dietary Ca intake was maintained at usual intakes (about 20 mmol (800 mg)/d). Urinary Na, K, Ca, N and type I collagen cross-linked N-telopeptide (NTx; a marker of bone resorption), plasma parathyroid hormone (PTH), serum 25-hydroxycholecalciferol (25(OH)D3), 1,25-dihydroxycholecalciferol (1,25(OH)2D3), osteocalcin and bone-specific alkaline phosphatase (B-Alkphase) were measured in 24 h urine samples and fasting blood samples collected at the end of each dietary period. The calciuric diet significantly (P<0.05) increased mean urinary Na, N, K, Ca and NTx (by 19 %) compared with the basal diet, but had no effect on circulating 25(OH)D3, 1,25(OH)2D3, PTH, osteocalcin or B-Alkphase in the total group (n 24). There were no differences in serum markers or urinary minerals between the basal and calciuric diet in either VDR genotype groups. While the calciuric diet significantly increased urinary NTx (by 25.6 %, P<0.01) in the f+ VDR group (n 10; carrying one or more (f) Fok I alleles), it had no effect in the f- VDR group (n 14; not carrying any Fok I alleles). It is concluded that the Na- and protein-induced urinary Ca loss is compensated for by increased bone resorption and that this response may be influenced by VDR genotype.

Dietary protein, calcium metabolism, and skeletal homeostasis revisited

High dietary protein intakes are known to increase urinary calcium excretion and, if maintained, will result in sustained hypercalciuria. To date, the majority of calcium balance studies in humans have not detected an effect of dietary protein on intestinal calcium absorption or serum parathyroid hormone. Therefore, it is commonly concluded that the source of the excess urinary calcium is increased bone resorption. Recent studies from our laboratory indicate that alterations in dietary protein can, in fact, profoundly affect intestinal calcium absorption. In short-term dietary trials in healthy adults, we fixed calcium intake at 20 mmol/d while dietary protein was increased from 0.7 to 2.1 g/kg. Increasing dietary protein induced hypercalciuria in 20 women [from 3.4 +/- 0.3 ( +/- SE) during the low-protein to 5.4 +/- 0.4 mmol/d during the high-protein diet]. The increased dietary protein was accompanied by a significant increase in intestinal calcium absorption from 18.4 +/- 1.3% to 26.3 +/- 1.5% (as determined by dual stable isotopic methodology). Dietary protein intakes at and below 0.8 g/kg were associated with a probable reduction in intestinal calcium absorption sufficient to cause secondary hyperparathyroidism. The long-term consequences of these low-protein diet-induced changes in mineral metabolism are not known, but the diet could be detrimental to skeletal health. Of concern are several recent epidemiologic studies that demonstrate reduced bone density and increased rates of bone loss in individuals habitually consuming low-protein diets. Studies are needed to determine whether low protein intakes directly affect rates of bone resorption, bone formation, or both.

Controlled high meat diets do not affect calcium retention or indices of bone status in healthy postmenopausal women

Calcium balance is decreased by an increased intake of purified proteins, although the effects of common dietary sources of protein (like meat) on calcium economy remain controversial. We compared the effects of several weeks of controlled high and low meat diets on body calcium retention, using sensitive radiotracer and whole body scintillation counting methodology. Healthy postmenopausal women (n = 15) consumed diets with similar calcium content (approximately 600 mg), but either low or high in meat (12 vs. 20% of energy as protein) for 8 wk each, in a randomized crossover design. After 4 wk of equilibration of each diet, calcium retention was measured by extrinsically labeling the 2-d menu with (47)Ca, followed by whole body scintillation counting for 28 d. Urinary and blood indicators of bone metabolism were also determined for each diet. Calcium retention was not different during the high and low meat dietary periods (d 28, mean +/- pooled SD: 17.1 and 15.6%, +/-0.6%, respectively; P = 0.09). An initially higher renal acid excretion in subjects consuming the high meat compared with the low meat diet decreased significantly with time. The diets did not affect urinary calcium loss or indicators of bone metabolism. In conclusion, under controlled conditions, a high meat compared with a low meat diet for 8 wk did not affect calcium retention or biomarkers of bone metabolism in healthy postmenopausal women. Calcium retention is not reduced when subjects consume a high protein diet from common dietary sources such as meat.

Low protein intake: the impact on calcium and bone homeostasis in humans

Increasing dietary protein results in an increase in urinary calcium. Despite over 80 y of research, the source of the additional urinary calcium remains unclear. Because most calcium balance studies found little effect of dietary protein on intestinal calcium absorption, it was assumed that the skeleton was the source of the calcium. The hypothesis was that the high endogenous acid load generated by a protein-rich diet would increase bone resorption and skeletal fracture. However, there are no definitive nutrition intervention studies that show a detrimental effect of a high protein diet on the skeleton and the hypothesis remains unproven. Recent studies from our laboratory demonstrate that dietary protein affects intestinal calcium absorption. We conducted a series of short-term nutrition intervention trials in healthy adults where dietary protein was adjusted to either low, medium or high. The highest protein diet resulted in hypercalciuria with no change in serum parathyroid hormone. Surprisingly, within 4 d, the low protein diet induced secondary hyperparathyroidism that persisted for 2 wk. The secondary hyperparathyroidism induced by the low protein diet was attributed to a reduction in intestinal calcium absorption (as assessed by dual stable calcium isotopes). The long-term consequences of these low protein-induced changes in calcium metabolism are not known, but they could be detrimental to skeletal health. Several recent epidemiological studies demonstrate reduced bone density and increased rates of bone loss in individuals habitually consuming low protein diets. Therefore, studies are needed to determine whether low protein intakes directly affect rates of bone resorption, bone formation or both.

Nutrition in bone health revisited: a story beyond calcium

Osteoporosis is a complex, multi-factorial condition characterized by reduced bone mass and impaired micro-architectural structure, leading to an increased susceptibility to fractures. Although most of the bone strength (including bone mass and quality) is genetically determined, many other factors (nutritional, environmental and life-style) also influence bone. Nutrition is important modifiable factor in the development and maintenance of bone mass and the prevention and treatment of osteoporosis. Approximately 80-90% of bone mineral content is comprised of calcium and phosphorus. Other dietary components, such as protein, magnesium, zinc, copper, iron, fluoride, vitamins D, A, C, and K are required for normal bone metabolism, while other ingested compounds not usually categorized as nutrients (e.g. caffeine, alcohol, phytoestrogens) may also impact bone health. Unraveling the interaction between different factors; nutritional, environmental, life style, and heredity help us to understand the complexity of the development of osteoporosis and subsequent fractures. This paper reviews the role of dietary components on bone health throughout different stages of life. Each nutrient is discussed separately, however the fact that many nutrients are co-dependent and simultaneously interact with genetic and environmental factors should not be neglected. The complexity of the interactions is probably the reason why there are controversial or inconsistent findings regarding the contribution of a single or a group of nutrients in bone health.

Changes in bone turnover in young women consuming different levels of dietary protein

Although high protein diets are known to increase urinary calcium excretion and induce negative calcium balance, the impact of dietary protein on bone turnover and fractures is controversial. We therefore evaluated the effect of dietary protein on markers of bone turnover in 16 healthy young women. The experiment consisted of 2 weeks of a well balanced diet containing moderate amounts of calcium, sodium, and protein followed by 4 days of an experimental diet containing one of three levels of protein (low, medium, or high). On day 4, serum and urinary calcium, serum PTH, 1,25-dihydroxyvitamin D, serum osteocalcin, bone-specific alkaline phosphatase, and urinary N-telopeptide excretion were measured. Urinary calcium excretion was significantly higher on the high than on the low protein diet. Secondary hyperparathyroidism occurred on the low protein diet. Urinary N-telopeptide excretion was significantly greater during the high protein than during the low protein intake (48.2 +/- 7.2 vs. 32.7 +/- 5.3 nM bone collagen equivalents/mM creatinine; P < 0.05). There was no increase in osteocalcin or bone-specific alkaline phosphatase when comparing the low to the high diet, suggesting that bone resorption was increased without a compensatory increase in bone formation. Our data suggest that at high levels of dietary protein, at least a portion of the increase in urinary calcium reflects increased bone resorption.

Nutrition, osteoporosis, and aging

Loss of bone is an almost universal accompaniment of aging that proceeds at an average rate of 0.5-1% per annum from midlife onwards. There are at least four nutrients involved in this process: calcium, salt, protein, and vitamin D, at least in women. The pathogenesis of osteoporosis in men is more obscure. Calcium is a positive risk factor because calcium requirement rises at the menopause due to an increase in obligatory calcium loss and a small reduction in calcium absorption that persist to the end of life. A metaanalysis of 20 calcium trials shows that this process can generally be arrested by calcium supplementation, although there is some doubt about its effectiveness in the first few years after menopause. Salt is a negative risk factor because it increases obligatory calcium loss; every 100 mmol of sodium takes 1 mmol of calcium out of the body. Restricting salt intake lowers the rate of bone resorption in postmenopausal women. Protein is another negative risk factor; increasing animal protein intake from 40 to 80 g daily increases urine calcium by about 1 mmol/day. Low protein intakes in third world countries may partially protect against osteoporosis. Vitamin D (sometimes called a nutrient and sometimes a hormone) is important because age-related vitamin D deficiency leads to malabsorption of calcium, accelerated bone loss, and increased risk of hip fracture. Vitamin D supplementation has been shown to retard bone loss and reduce hip fracture incidence in elderly women.

Increasing intake of soybean protein or casein, but not cod meal, reduces nephrocalcinosis in female rats

Female weanling rats were fed diets with soybean protein, casein or cod meal at 171, 342 or 513 mmol nitrogen/100 g for 3 wk. The diets were isonitrogenous and balanced for fat, cholesterol, calcium, magnesium and phosphorus. Cod meal feeding at 171 and 342 mmol nitrogen/100 g diet produced lower kidney calcium concentrations than the feeding of either soybean protein or casein. Increasing protein intakes were associated with reduced kidney calcium concentrations in the rats fed either soybean protein or casein but not in those fed cod meal. The anti-nephrocalcinogenic effect of increasing intakes of soybean protein may relate to the lowering of urinary phosphorus concentration. Increasing intakes of casein probably inhibited nephrocalcinogenesis by lowering urinary pH and raising urinary magnesium concentration. Increasing cod meal concentrations in the diet lowered urinary pH and raised urinary magnesium and calcium concentrations, but the effects on nephrocalcinogenesis of these changes probably counteracted each other.

Calcium absorption and calcium bioavailability

Calcium is important for bone health. It has been customary to focus on dietary calcium intake, but of central importance for the body needs in the individual patient is the actual calcium absorption. This absorption consists of an active vitamin D-mediated component and a passive diffusional component. A number of different methods are available for the evaluation of calcium absorption. At present the calcium absorption tests using calcium isotopes (radioactive or stable) appear to be the most reproducible way of determining calcium absorption. The major nutrient sources for calcium are milk and milk products, whereas some of the green vegetables have a low bioavailability of calcium. When deciding whether an increased calcium intake is advisable, the following questions must be answered. What is the calcium absorptive status of the patient? How should the calcium supplement be dispensed? What calcium salt should be used? When should calcium supplements be taken? What is the compliance of the patient? When should the treatment be evaluated? The calcium supplement might be taken as milk (or milk products) or, in patients with lactose intolerance, as calcium supplements. Quite a number of calcium supplements are available on the market, and many of them are marketed without proper knowledge of the bioavailability of the actual preparation. For the benefit of our patients it is now reasonable to demand such investigations before marketing calcium supplements.

Sources of protein-induced endogenous acid production and excretion by human adults

The origin of the increase in endogenous acid production and excretion associated with the calciuretic action of high protein intakes was investigated in human adults. Eight subjects, 4 males and 4 females, aged 25-36 years, were fed a low protein diet (50 g/day) and a high protein diet (120 g/day in males and 106 g/day in females) for 7 days each. The high protein diet was formulated by supplementing the low protein diet with a mixture of four purified proteins. Increased protein intake was associated with increases in urinary Ca, sulfate, titratable acidity (acid phosphates) and ammonium, and decreases in urinary pH and bicarbonate. There was no increase in organic anion excretion. The increases in endogenous acid production (EAP) and net acid excretion (NAE) were entirely attributable to the oxidation of excess sulfur amino acids (SAA), which yields 2 moles of hydrogen ions per mole of amino acid catabolized. The results differ in this respect from those reported for studies on the effect of SAA loading, which indicate that non-SAA make a major contribution to the increase in EAP seen under these conditions.

Effect of a high protein intake on acid-base balance in adult rats

Adult rats are able to maintain Ca balance under protein loads that produce Ca loss in adult humans. This species difference was investigated by determining the relationship between protein intake, endogenous acid production (EAP), net acid excretion (NAE), and urinary Ca in adult rats for comparison with a similar study on adult humans. Diets containing 10, 30, and 50% casein were fed in conjunction with proportionate increments in the sulfur amino acid (SAA) methionine (0.6, 1.8, and 3.0%). Urine volume, Ca, sulfate, organic anions, TA (titratable acidity as acid phosphates), and ammonium increased progressively with increases in protein intake, and pH decreased. When protein intake was increased at a constant level of SAA, no increase in urinary Ca, sulfate, and TA or decrease in pH was observed. Both SAA and non-SAA enhanced ammonium excretion but only non-SAA enhanced organic anion excretion, an indicator of incomplete oxidation of organic acids. SAA were responsible for 89 and 91% of the increase in EAP and Ca excretion, respectively, caused by increasing protein intake from 10-30% of the diet. In a comparison experiment, human adults on a high protein intake exhibited a much smaller increase in acid excretion as ammonium, a greater increase as TA, no change in organic anion excretion, and no increase in EAP from non-SAA. The importance of these species differences in acid-base response to a high protein intake in the greater ability of rats to maintain Ca balance on high protein intakes is unclear; however, the smaller fraction of endogenous Ca excreted in the urine of rats is probably an important factor.

Effect of dietary protein and minerals on calcium and zinc utilization

The utilization of most minerals can be altered not only by varying the dietary levels of protein, carbohydrate, fiber, fat, and vitamins but also by varying the forms of macronutrients fed. Interactions among minerals are frequent and sometimes complex. Thus, all assessments of mineral requirements should include discussions of the effects of interactions on mineral bioavailability. Some of these interactions occur in the gut and affect the absorption of minerals. Other interactions affect the metabolism, transport and storage of minerals, and ultimately their excretion in urine or in endogenous gut secretions.

[The effect of long-term increased protein administration on mineral metabolism and kidney function in the rat. I. Renal and enteral excretion of calcium, magnesium, phosphorus, sulfate and acid]

The influence of continuous imbalanced high protein intake on the metabolism of minerals (calcium, magnesium, phosphorus) and renal function was the subject of a long-term experiment with rats. In the first part of the study particular attention was directed to the contribution of protein-induced endogenous acid production and renal excretion of hydrogen ions and sulphate to the development of hypercalciuria. For 61 weeks 200 male Wistar rats in eight groups were fed isocaloric diets, whose protein contents were increased from 13 to 26 and 40 J% at the expense of carbohydrate intake. The fat content of the diets was 40 J%. In two groups with 13 and 26 J% protein the effect of different kinds of animal protein was also studied, replacing casein by beef. Mineral contents were kept constant in these diets. To examine the excretion mechanisms of calcium and phosphorus especially under conditions of excessive protein intake, the ratio of calcium to phosphorus was varied in three diets with 40 J% protein by increasing both minerals alternatively or together from 0.6 to 1.2%. An increase in dietary protein content from 13 to 26 or 40 J% produced a sustained hypercalciuria and also hypermagnesiuria over a period of more than 400 days (after 58 weeks: 3.3, 5.9, and 6.8 mg calcium/day; 2.2, 3.3, and 3.4 mg magnesium/day; p less than or equal to 0.05). No adaptation to high protein intake occurred. Hypermagnesiuria, which equally hasn't been described before as a result of high protein intake, was accompanied by a reduced fecal excretion of magnesium. With increased protein intake (casein and beef) hypercalciuria and also hypermagnesiuria were positively correlated with an increased formation and renal excretion of hydrogen ions and sulphate, which resulted from protein catabolism. The dietary protein source influenced the extent of hypercalciuria, irrespective of a constant phosphorus intake. Although leading to equal increases in renal total acid and sulphate excretion, beef as the main protein source caused a lower calciuria than casein. High phosphorus intake caused the highest total acid excretion of all groups, but resulted in a reduced hypercalciuria and hypermagnesiuria and counteracted the influence of an increased protein intake.

[The effect of long-term increased protein administration on mineral metabolism and kidney function in the rat. II. Kidney function and bone mineralization]

In the Federal Republic of Germany the average daily protein intake exceeds the Recommended Dietary Allowances for adults (0.8 g protein/kg body weight) by about 100%. On the other hand calcium intake is below the recommendations for certain age groups. Protein-induced hypercalciuria involves the risk of depletion of skeletal calcium stores, especially for older people who have a decreased absorption capacity for calcium. As a result of our study we postulate, that an altered renal function probably is one inducing factor of hypercalciuria. While urea excretion and serum urea concentration increased with an elevated dietary protein content from 13 to 26 or 40 J%, glomerular filtration rate remained unchanged. Fractional tubular reabsorption of calcium was significantly reduced by about 3% with increased endogenous acid production and renal excretion of hydrogen ions (first part of the study), which were accompanying a higher protein intake of 40 J% compared to 13 J% protein in the control group. Increasing the phosphorus content of the diet improved the reabsorption of calcium and magnesium. The kidneys of rats fed diets high in protein and phosphorus were hypertrophied. Histology of the kidneys showed signs of glomerulonephrosis. While the calcium content of the femora was slightly reduced with a higher protein intake of 40 compared to 13 J%, the magnesium content was increased (after 61 weeks: calcium from 261.4 to 257.1 mg/g dry fat-free wt [p less than or equal to 0.05]; magnesium from 3.2 to 3.5 mg/g dry fat-free wt [p less than or equal to 0.001]). Calcium and magnesium metabolism depends not only on the level of protein intake, but also on its interrelation with the dietary phosphorus content. With continuous high protein intake higher intakes for calcium, phosphorus and magnesium should be recommended, especially for older people.