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.     

Further studies of the effect of a high protein diet as meat on calcium metabolism

Previous studies in this Unit have shown that a high protein intake, given as meat, did not induce hypercalciuria, except for the initial and temporary increase in two subjects. In the present investigation the long-term effect of a high meat diet on calcium metabolism was studied for 78 to 132 days in four adult males and the short-term effect for 18 to 30 days in three subjects. Calcium and phosphorus balances and calcium absorption studies, using 47Ca as the tracer, were carried out. During the long-term high meat intake and during the short-term high meat studies, there was no significant change of the urinary or fecal calcium nor of the calcium balance. There was also no significant change of the intestinal absorption of calcium during the high meat intake. These long- and short-term studies have confirmed our previous results that a high protein intake, given as meat, does not lead to hypercalciuria and does not induce calcium loss.     

Potassium intake and the calcium economy

BACKGROUND: Dietary potassium intake (K) lowers urinary calcium (Ca) excretion and, in short-term studies, may improve Ca balance. PURPOSE: Our objective was to assess K effects on the Ca economy under steady-state conditions. DESIGN: 8-day, inpatient metabolic studies of nitrogen, phosphorus, and Ca balance, combined with dual isotopic Ca tracer kinetics studies. Study diet matched to prestudy nutrient intakes. SUBJECTS: 191 single women studied from 1-5 times at approximately 5-year intervals, for a total of 644 inpatient studies. Median age at time of study: 50.2 yrs; 301 studies were performed postmenopausally without hormone replacement; 343 were either premenopausal or postmenopausal but on estrogen replacement therapy. RESULTS: Dietary K was highly significantly associated with urinary Ca excretion, with a coefficient of -0.0109 mmol urine Ca/mmol diet K. However, dietary K was negatively correlated with dual-tracer Ca absorption (coefficient for Ca absorption fraction: -0.00094/mmol dietary K), and was not associated with urine Ca after adjustment for Ca absorption. CONCLUSION: While a high K diet (i.e., one rich in fruits, vegetables, and dairy products) has multiple health benefits and clearly lowers urine Ca, it does not seem to exert any appreciable net influence on the Ca economy, largely because the reduced calciuria is offset by reduction of intestinal absorption. We note, however, that since the high K intakes in our studies come more from milk and meat than from fruits and vegetables, we cannot exclude a possible balance effect for different food sources of K.     

Effects of high protein diet and sodium bicarbonate supplementation on calcium metabolism in rats

This study was conducted to determine the effect of a high protein diet on calcium metabolism in rat. Wistar strain male rats (50 days old) were divided into 5 groups (day 0): control diet (18% casein); high protein diet (18% casein +20% lactalbumin); high protein and 0.1% sodium bicarbonate diet; high protein and 0.2% sodium bicarbonate diet; and high protein and 0.4% sodium bicarbonate diet. On days 0, 1, 3, 5, 7, 9, urine samples were collected and, at the same time, feces were collected from half of the animals in each group. Urinary titratable acidity (TA-HCO3-), ammonium ion (NH4+), and net acid excretion (NAE) were measured as an index of acid-base balance in rat body. Urinary volume was rapidly increased and the increase of urinary volume continued throughout the study in rats fed the high protein diet. Urinary excretions of calcium and phosphorus were increased after day 3 and day 1, respectively, in rats fed the high protein diet. The high protein diet depressed calcium absorption and elevated phosphorus absorption from the digestive tract in rats fed the high protein diet. The high protein diet decreased TA-HCO3-, which was closely correlated with the decrease of NAE. Sodium bicarbonate supplementation to the high protein diet had little effect on urinary calcium excretion and NAE. This study suggested that there was no relationship between metabolic acidosis and hypercalciuria in rats fed the high protein diet.     

Effects of protein intake or exercise on 24 h urinary solute excretion

The effects of protein intake or exercise on 24 h urinary solute excretion, were evaluated in 10 female 18-19 yr of age. This study was performed during four periods: a low-protein diet (30 g x 5 days), a normal-protein diet (control, 60 g x 5 days), a high-protein diet (90 g x 5 days), and exercise loading with a normal-protein diet. (The amount of plant protein was kept constant to be 24 g/day) The following results were obtained: 1. In the case of exercise loading, urinary potassium (K) and nitrogen (N) excretions decreased significantly, while urinary sodium (Na), chlorine (Cl), calcium (Ca), and phosphate (P) excretions showed no significant differences compared with control values. 2. With the low-protein diet, urinary Ca excretion decreased significantly compared with those in normal or high-protein diet. 3. The apparent fractional absorption of Na, Cl, and Ca in the female on the high-protein diet was significantly higher than that in those on the low-protein diet. These results suggest the following: 1. The amount of urinary K excretion is not only directly influenced by K intake, but also by K metabolism, such as K+ transport between extra- and intracellular spaces. 2. Although urinary Ca excretion was not increased by the increment in protein in the diet from 60 g/day to 90 g/day, it is necessary to evaluate both quantity and quality of a protein diet. 3. Protein intake of more than 60 g/day is necessary for an effective increase in Ca and NaCl absorption.     

A diet high in meat protein and potential renal acid load increases fractional calcium absorption and urinary calcium excretion without affecting markers of bone resorption or formation in postmenopausal women

Our objective in this study was to determine the effects of a high-protein and high-potential renal acid load (PRAL) diet on calcium (Ca) absorption and retention and markers of bone metabolism. In a randomized crossover design, 16 postmenopausal women consumed 2 diets: 1 with low protein and low PRAL (LPLP; total protein: 61 g/d; PRAL: -48 mEq/d) and 1 with high protein and high PRAL (HPHP; total protein: 118 g/d; PRAL: 33 mEq/d) for 7 wk each separated by a 1-wk break. Ca absorption was measured by whole body scintillation counting of radio-labeled (47)Ca. Compared with the LPLP diet, the HPHP diet increased participants' serum IGF-I concentrations (P < 0.0001), decreased serum intact PTH concentrations (P < 0.001), and increased fractional (47)Ca absorption (mean ± pooled SD: 22.3 vs. 26.5 ± 5.4%; P < 0.05) and urinary Ca excretion (156 vs. 203 ± 63 mg/d; P = 0.005). The net difference between the amount of Ca absorbed and excreted in urine did not differ between 2 diet periods (55 vs. 28 ± 51 mg/d). The dietary treatments did not affect other markers of bone metabolism. In summary, a diet high in protein and PRAL increases the fractional absorption of dietary Ca, which partially compensates for increased urinary Ca, in postmenopausal women. The increased IGF-I and decreased PTH concentrations in serum, with no change in biomarkers of bone resorption or formation, indicate a high-protein diet has no adverse effects on bone health.     

High urinary isoflavone excretion phenotype decreases plasma cholesterol in golden Syrian hamsters fed soy protein

Apparent absorption of isoflavones varies greatly among individuals but is relatively stable within an individual. We hypothesized that high urinary isoflavone excreters would show less plasma non-HDL cholesterol (non-HDL-C) than low isoflavone excreters after soy protein feeding. Fifty Golden Syrian hamsters were fed a high-fat/casein diet (n = 10) or a high-fat/soy protein diet (n = 40) for 4 wk. We identified 2 distinct urinary isoflavone excretion phenotypes based upon HPLC analysis of urinary glycitein using a pairwise correlation plots analysis, or based upon total urinary isoflavone using a hierarchical cluster test. High isoflavone excreters showed greater urinary isoflavones (P < 0.05) than did low isoflavone excreters at wk 1 and 4. The low urinary glycitein excretion phenotype was more stable than the high urinary glycitein excretion phenotype by McNemar's test. High urinary isoflavone excreters had significantly less non-HDL-C than did the low isoflavone excreters or casein-fed controls (P < 0.05). Plasma total and non-HDL-C were negatively correlated with urinary daidzein, glycitein, and total isoflavone excretion (r = -0.45 to -0.58, P < 0.05). Urinary isoflavone excretion phenotypes predicted the cholesterol-lowering efficacy of soy protein. Isoflavone absorbability, probably due to gut microbial ecology, is an important controllable variable in studies of effects of soy protein on blood lipids.     

Acute effects of increased meat protein on urinary electrolytes and cyclic adenosine monophosphate and serum parathyroid hormone

The effect of a high meat protein diet on urinary electrolytes, cyclic AMP, and serum immunoassayable parathyroid hormone was studied in six subjects fed a high meat protein diet (1.5 to 2.9 g/kg) for 7 days. The diet produced minor decreases in serum calcium and phosphorus but increased endogenous creatinine clearance by about 20% (p less than 0.02) and urinary calcium by about 80% (p less than 0.01) without changing urinary sodium. Urinary calcium correlated (p less than 0.01) with urinary sulfate (r = 0.60), ammonia (r = 0.72), and nitrogen (r = 0.60). Urinary cyclic AMP increased (p less than 0.01) 14% while serum parathyroid hormone (measured by C-terminal assays) decreased (p less than 0.05) by more than 30%. It was concluded 1) that this diet acutely altered renal handling of calcium at a site different from that of sodium, 2) that the excretion of acidic products of protein metabolism contributed to these changes, and 3) that parathyroid hormone secretion was not changed acutely.     

Soy protein isolate increases urinary estrogens and the ratio of 2:16alpha-hydroxyestrone in men at high risk of prostate cancer

Specific estrogen metabolites may initiate and promote hormone-related cancers. In epidemiological studies, significantly lower excretion of urinary estradiol (E2) and lower ratio of urinary 2-hydroxy estrogens to 16alpha-hydroxyestrone (2:16 OH-E1) have been reported in prostate cancer cases compared to controls. Although soy supplementation has been shown to increase the ratio 2:16 OH-E1 in women, no studies to our knowledge have investigated the effects of soy supplementation on estrogen metabolism in men. The objective of this randomized controlled trial was to determine the effects of soy protein isolate consumption on estrogen metabolism in men at high risk for developing advanced prostate cancer. Fifty-eight men supplemented their habitual diets with 1 of 3 protein isolates: 1) isoflavone-rich soy protein isolate (SPI+) (107 mg isoflavones/d); 2) alcohol-washed soy protein isolate (SPI-) (<6 mg isoflavones/d); or 3) milk protein isolate (MPI), each providing 40 g protein/d. At 0, 3, and 6 mo of supplementation, the urinary estrogen metabolite profile was measured by GC-MS. Both soy groups had higher E2 excretion than the MPI group at 3 and 6 mo. After 6 mo of supplementation, the SPI+ group had a significantly higher urinary 2:16 OH-E1 ratio than the MPI group. Increased urinary E2 excretion and 2:16 OH-E1 ratio in men consuming soy protein isolate are consistent with studies in postmenopausal women and suggest that soy consumption may be beneficial in men at high risk of progressing to advanced prostate cancer as a result of effects on endogenous estrogen metabolism.     

Hepatic cysteine dioxygenase activity and sulfur amino acid metabolism in rats: possible indicators in the evaluation of protein quality

Experiments were carried out to examine the possibility that the sulfur amino acid metabolism of rats may be an indicator of the nutritional value of dietary protein. Rats were fed diets containing 8, 16 or 24% of gluten, soy protein or casein for 3 wk. Hepatic cysteine dioxygenase activity, hepatic concentration of glutathione, cysteine and taurine and urinary taurine were examined. In addition, the sulfur amino acid metabolism of rats fed these diets fortified with the appropriate first limiting amino acid for 7 d was also examined. High urinary taurine excretion was observed in the three gluten groups, whereas very low urinary taurine excretion was observed with up to 24% soy protein or up to 16% casein. The hepatic hepatic cysteine dioxygenase activities of the gluten diet groups were higher than those of corresponding soy protein or casein diet groups, except that of rats fed the 24% casein diet. The hepatic concentrations of both glutathione and cysteine in gluten diet groups were also higher than those of corresponding soy protein or casein diet groups, except 24% soy protein and 16 and 24% casein diet groups. In rats fed the casein or soy protein diets urinary taurine excretion and hepatic cysteine dioxygenase activity increased with increasing methionine supplementation, the first limiting amino acid. Conversely, in rats fed the gluten diet both urinary taurine excretion and hepatic cysteine dioxygenase activity decreased with increasing lysine supplementation, the first limiting amino acid. These findings suggest that urinary taurine excretion and hepatic cysteine dioxygenase activity may be useful as sensitive indicators of the nutritional value of dietary protein.     

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.     

Dietary animal and plant protein and human bone health: a whole foods approach

Urinary calcium excretion is strongly related to net renal acid excretion. The catabolism of dietary protein generates ammonium ion and sulfates from sulfur-containing amino acids. Bone citrate and carbonate are mobilized to neutralize these acids, so urinary calcium increases when dietary protein increases. Common plant proteins such as soy, corn, wheat and rice have similar total S per g of protein as eggs, milk and muscle from meat, poultry and fish. Therefore increasing intake of purified proteins from either animal or plant sources similarly increases urinary calcium. The effects of a protein on urinary calcium and bone metabolism are modified by other nutrients found in that protein food source. For example, the high amount of calcium in milk compensates for urinary calcium losses generated by milk protein. Similarly, the high potassium levels of plant protein foods, such as legumes and grains, will decrease urinary calcium. The hypocalciuric effect of the high phosphate associated with the amino acids of meat at least partially offsets the hypercalciuric effect of the protein. Other food and dietary constituents such as vitamin D, isoflavones in soy, caffeine and added salt also have effects on bone health. Many of these other components are considered in the potential renal acid load of a food or diet, which predicts its effect on urinary acid and thus calcium. "Excess" dietary protein from either animal or plant proteins may be detrimental to bone health, but its effect will be modified by other nutrients in the food and total diet.     

Effects of a high protein diet on bone formation and calcium metabolism in rats

The effects of a high protein diet on bone formation and calcium (Ca) metabolism were evaluated in rats using an ectopic endochondral bone induction model. A control diet (18% casein) or a high protein diet (18% casein + 20% lactalbumin) was given to 50-day-old rats. Ten days after the feeding of the experimental diet, rats were intramuscularly implanted with demineralized bone powder (day 0). On day 14 and day 21, the implanted bone powder was harvested, and blood and urine samples were also obtained. Urinary Ca excretion was not increased on day 12-14; however, it was elevated on day 19-21 in rats fed the high protein diet compared with rats fed the control diet. The high protein diet remarkably stimulated urinary sulfate excretion in both sampling periods, which reflected dietary sulfur-containing amino acids contents. Also, rats fed the high protein diet exhibited a decrease in serum Ca concentrations. There was little difference in Ca contents and the activities of alkaline phosphatase and acid phosphatase in the implants between control group and high protein diet group on day 14 and day 21. Histological examination in the implanted demineralized bone powder on day 14 indicated only cartilage in rats fed the high protein diet in contrast to the occurrences of osteogenesis and remodeling in those fed the control diet. Retarded bone formation in rats fed the high protein diet might be owing to, in part at least, a restricted amount of Ca utilized at the stage of cartilage calcification.     

Effect of dietary lactose on salt-mediated changes in mineral metabolism and bone composition in the rat

The effects of salt (sodium chloride) supplementation of rat diets (80 g/kg diet), with or without lactose (150 g/kg), were studied in weanling rats over 14 d. Dietary salt increased water intake and reduced weight gain and food conversion efficiency, but these variables were unaffected by lactose. Salt-supplemented rats exhibited a three- to fivefold increase in urinary calcium excretion and a small increase in urinary magnesium and phosphorus excretion, irrespective of dietary lactose content. In addition, salt supplementation reduced plasma alkaline phosphatase (EC 3.1.3.1) activity. Lactose increased urinary Ca and Mg excretion and plasma Ca and P concentrations. Salt reduced tibia mass but not tibia mass expressed relative to body-weight, but neither variable was affected by lactose. Both tibia Mg content and concentration were reduced by salt but unaffected by lactose, and neither tibia P content nor concentration was affected by salt or lactose. Tibia Ca content was reduced by salt but this was prevented by lactose. Tibia Ca concentration was unaffected by salt or lactose, although there was a reduction (not significant) in tibia Ca concentration in animals fed on the lactose-free diet. These results show that lactose had no independent effect on bone and that reduced accretion of bone mass and mineral content in rats fed on the high-salt diets was due, at least in part, to reduced growth. Failure to offset sodium-induced hypercalciuria by a compensatory increase in net Ca absorption may have contributed to reduced bone Ca accretion. The protective effect of lactose against reduced bone Ca accretion may be due to increased Ca absorption.     

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.     

Effect of soy protein/animal protein ratio on calcium metabolism of the rat

OBJECTIVE: This study examined the effects of a ratio of soy protein to animal protein on bone metabolism of rats. METHODS: Experimental groups were a high soy protein group (200 g of soy protein and 0 g of casein per kilogram of diet; HSoy), a middle soy protein group (100 g of soy protein and 100 g of casein per kilogram of diet; MSoy), a low soy protein group (50 g of soy protein and 150 g of casein per kilogram of diet; LSoy), and a no soy protein group (0 g of soy protein and 200 g of casein per kilogram of diet; NSoy). Calcium excretion and retention, biochemical parametrically related calcium metabolism, and bone mineral density were measured. Statistical analysis was performed with SAS software. RESULTS: Urinary excretion of calcium was significantly high in the LSoy and NSoy groups, and there was no difference in absorption of calcium across experimental groups. Calcium retention was significantly higher in the HSoy and MSoy groups than in the LSoy and NSoy groups. Experimental groups showed no differences in the activity of alkaline phosphatase. The casein group (NSoy) showed a remarkably lower degree of serum osteocalcin concentration. The concentration of deoxypyridinoline in urine showed an increasing tendency, i.e., HSoy and MSoy < LSoy < NSoy, indicating that at a higher degree of animal protein/soy proteins, its concentration increases. Wet weight of the femur appeared to be significantly greater in the MSoy and LSoy groups than in the NSoy group. Ash content of the femur and bone density were highest in the MSoy group, with the ratio of 1:1 between soy protein and animal protein. CONCLUSIONS: This study indicated that deoxypyridinoline concentration was lower and the density of osteocalcin was higher in the MSoy group than in the NSoy group, and that calcium retention was high and bone mineral density was the highest in the MSoy group. The ratio of soy to animal protein that seemed to have the most positively significant effect on calcium metabolism was 1:1.     

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.     

Altering dietary protein type and quantity reduces urinary albumin excretion without affecting plasma glucose concentrations in BKS.cg-m +Lepr db/+Lepr db (db/db) mice

Protein restriction is used conventionally in the prevention and treatment of diabetic nephropathy. Recently, the use of soy protein instead of animal protein has been postulated as a new preventive and treatment option. The aim of this study was to determine the qualitative and quantitative effects of dietary protein on biomarkers of diabetic nephropathy in a Type 2 diabetes mellitus mouse model (BKS.cg-m +Lepr(db)/+Lepr(db) mice). Diabetic (+Lepr(db)/+Lepr(db)) and control (m+/m+) mice (n = 24/group) consumed one of four different diets ad libitum [20% casein, 20% soy protein, 12% casein or 12% soy protein (energy-based percentages)] from 35 +/- 4 d of age until termination (184-217 d of age). Blood and urine were collected throughout the study to measure biomarkers of diabetes and diabetic nephropathy. Kidney tissue was collected at the end of the study for weight. In diabetic mice, a 20% casein diet increased urinary albumin excretion to macroalbuminuric levels, whereas a 20% soy protein diet led to no major changes in urinary albumin excretion. Low protein diets (12%), independently of protein type, decreased urinary albumin excretion to low microalbuminuric levels. There were no significant differences in plasma glucose concentrations. These findings show lower urinary albumin excretion when a soy protein diet or a low casein diet is fed, suggesting a delay in the progression of diabetic nephropathy.     

Calcium metabolism, intestinal calcium-binding protein, and bone growth of rats fed high protein diets

The rat was used as a model for investigating the mechanism by which the consumption of high diets causes calciuria. Using a combined balance and kinetics study, calcium (Ca) metabolism was studied in 56-day old male rats which had been consuming a control (18% casein) or a high protein (36% casein) diet for 2, 14, or 28 days. Urine Ca was significantly increased to 1.7 mg/day and 1.1 mg/day in rats which consumed the high protein diet for 2 or 14 days respectively. After 29 days of consuming the high protein diet, urinary Ca excretion was 0.7 mg/day, the same as that of controls. No other criteria of calcium metabolism were significantly affected by the high protein intakes. Intestinal calcium-binding protein activity was not affected by consumption of the 36% casein diet for 7 days, nor was bone mineralization after consumption of this diet for 32 days. Since the rat excretes a low percentage of dietary Ca via the urinary route, it is not a useful model for studying Ca kinetics in protein-induced calciuria.     

The effect of dietary protein on nitrogen and sulfur metabolism in portacaval-shunted rats

The effect of varying the amount of protein in the diet on postoperative recovery, plasma ammonia, urinary orotic acid and metabolism of sulfur-containing amino acids was examined in rats with portacaval shunts (PCS). Food intake and weight gain were lower in both PCS and control rats fed a low (6%) casein diet unsupplemented with methionine compared with rats fed an adequate (18%) casein diet. PCS rats fed 60% casein ate slightly less and took longer to recover their preoperative body weight compared to 60% controls. Shunted rats were consistently hyperammonemic and orotic aciduric compared to controls. Increasing protein in the diet elevated plasma ammonia and urinary orotic acid in all rats to levels above those of the rats fed 18% casein, but the effect was greater in rats with PCS. After i.p. injection of L-[35S]methionine or L-[35S]cysteine, urinary 35S and [35S]sulfate excretion increased and [35S]taurine and total taurine excretion decreased in all rats fed 60% casein. These changes are consistent with our observation that hepatic activities of cysteine dioxygenase and cysteine sulfinate:alpha-ketoglutarate aminotransferase increased and that of cysteine sulfinate decarboxylase decreased in rats fed the high protein diet. The effect of dietary treatment on both urinary taurine excretion and decarboxylase activity was greater in PCS rats than in controls. Although PCS rats fed a high protein diet may have a decreased taurine-synthesizing capability compared to controls, their ability to oxidize a methionine or cysteine load to sulfate is not compromised by feeding either an 18 or 60% casein diet.