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PubMed1981

Whole-body leucine and lysine metabolism: response to dietary protein intake in young men.

Whole-body leucine and lysine metabolism was explored in young adult men by a primed constant intravenous infusion of a mixture of L-[1-13C]leucine and L-[alpha-15N]lysine over a 4-h period. Subjects were studied after an overnight fast (postabsorptive state) or while consuming hourly meals (fed state) after adaptation to diets providing either a surfeit level of protein (1.5 g.kg body-1.day-1), a level approximating maintenance requirements (marginal intake) (0.6 g.kg body wt-1.day-1), or a grossly inadequate level (0.1 g.kg-1.day-1). The change in protein intake from a marginal to a surfeit level was associated with an increased leucine flux and incorporation of leucine into body protein. In the fed state, oxidation of leucine increased sharply and release of leucine from tissue protein diminished. When dietary protein intake was reduced from the requirement to inadequate level, leucine flux and body protein synthesis and protein breakdown were reduced, together with a smaller reduction in leucine oxidation. The response of the metabolism of [15N]lysine was responsible for maintenance of leucine and other essential amino acid economy, and they appear to be related to the nitrogen and amino acid requirements of the subject. These findings also demonstrate an effect of meals, modulated by their protein content, on the dynamics of whole-body amino acid metabolism.

Adolescent↗
PubMed2006

Immunoglobulin production is impaired in protein-deprived mice and can be restored by dietary protein supplementation.

Most contacts with food protein and microbiota antigens occur at the level of the gut mucosa. In animal models where this natural stimulation is absent, such as germ-free and antigen-free mice, the gut-associated lymphoid tissue (GALT) and systemic immunological activities are underdeveloped. We have shown that food proteins play a critical role in the full development of the immune system. C57BL/6 mice weaned to a diet in which intact proteins are replaced by equivalent amounts of amino acids (Aa diet) have a poorly developed GALT as well as low levels of serum immunoglobulins (total Ig, IgG, and IgA, but not IgM). In the present study, we evaluated whether the introduction of a protein-containing diet in 10 adult Aa-fed C57BL/6 mice could restore their immunoglobulin levels and whether this recovery was dependent on the amount of dietary protein. After the introduction of a casein-containing diet, Aa-fed mice presented a fast recovery (after 7 days) of secretory IgA (from 0.33 to 0.75 mg/mL, while in casein-fed mice this value was 0.81 mg/mL) and serum immunoglobulin levels (from 5.39 to 10.25 mg/mL of total Ig). Five percent dietary casein was enough to promote the restoration of secretory IgA and serum immunoglobulin levels to a normal range after 30 days feeding casein diet (as in casein-fed mice--15% by weight of diet). These data suggest that the defect detected in the immunoglobulin levels was a reversible result of the absence of food proteins as an antigenic stimulus. They also indicate that the deleterious consequences of malnutrition at an early age for some immune functions may be restored by therapeutic intervention later in life.

Animals↗
PubMed1986

Effects of dietary protein on theophylline pharmacokinetics and caffeine and aminopyrine breath tests.

The effects of low- and high-protein diets on theophylline kinetics and the time course of changes in 13C-labeled caffeine and aminopyrine CO2 breath tests were examined in six young men. With a low-protein diet, mean theophylline clearance fell 21% (P less than 0.04) and the t1/2 rose from 8.0 to 10.6 hours (P less than 0.02). With a high-protein diet, mean theophylline clearance rose 26% (P less than 0.004) and the t1/2 shortened to 7.4 hours (P less than 0.03). Theophylline volume of distribution and protein binding did not change. Renal clearance of theophylline was lowered during the low-protein diet. Theophylline clearance correlated with caffeine breath test values during the low- (r = 0.73) and high- (r = 0.70) protein diets. Theophylline clearance correlated less well with the aminopyrine breath test values during the low- (r = 0.47) and high- (r = 0.55) protein diets. Thus dietary protein significantly influenced theophylline clearance, but the caffeine and aminopyrine breath tests showed a differential response to this important environmental factor.

Adult↗
PubMed1990

Differing actions of dietary protein and enalapril on renal function and proteinuria.

Albuminuria (UalbV) increases in proportion to dietary protein in rats with passive Heymann nephritis. To determine whether a similar relationship existed in normal animals, 14 normal rats were switched from an 8.5% protein diet (LP) to a 40% protein diet (HP). Initially UalbV and glomerular filtration rate (GFR) increased in parallel, but GFR ceased to increase after 48 h while UalbV continued to increase, causing a significant increase in fractional renal clearance of albumin (FCalb). In contrast, HP for 4 days caused only a transient increase in GFR in nephrotic rats but effected a threefold sustained increase in UalbV. Pretreatment of nephrotic rats with enalapril blunted but did not entirely prevent the increase in UalbV after switching to HP and did not affect the increase or subsequent decline in GFR. Treatment with enalapril for 10 days reduced UalbV and FCalb in nephrotic rats fed either LP or HP. The similar pattern of changes in urinary albumin excretion in normal and nephrotic rats after dietary protein augmentation suggests that dietary protein may modify UalbV by the same process in both normal and nephrotic animals. The increases in UalbV and GFR resulting from dietary protein augmentation represent parallel but independent processes, since only the proteinuric response is modified by converting enzyme inhibition. Dietary protein restriction and converting enzyme inhibition exert an additive effect to reduce UalbV.

Albuminuria↗
PubMed2003

Development of alimentary zinc deficiency in growing rats is retarded at low dietary protein levels.

The aim of the present study was to determine whether the level of dietary protein would influence the onset of zinc deficiency in rats because zinc-deprived rats have problems metabolizing dietary protein. Young male Sprague-Dawley rats were fed isoenergetic Zn-deficient diets (0.8 mg Zn/kg diet) or control diets substituted with zinc sulfate (54 mg Zn/kg diet) and protein levels of 2, 5, 8, 10, 15, 20 or 25 g/100 g for 21 d to determine whether changing the protein level of Zn-deficient diets affects the Zn status of the rats. In rats fed low dietary protein levels of 2 and 5%, feed intake, growth and appearance did not differ between the Zn-deficient rats and the control rats because the low zinc requirement was met by mobilization of zinc from the skeleton. At higher dietary protein levels, the Zn-depleted rats developed marked signs of Zn deficiency and had reduced feed intake, growth, alkaline phosphatase activity in the serum and Zn concentrations in serum and femur compared with the control rats. The reduced feed intakes and decreased growth of Zn-depleted rats fed high dietary protein levels (20 and 25%) compared with control rats may be due to disturbed protein synthesis, as demonstrated by the increased activities of alanine aminotransferase, glutamate dehydrogenase and carbamoylphosphate synthetase in the liver. Zinc as an essential component of the diet is thus vital for the efficient utilization of dietary protein.

Alanine Transaminase↗
PubMed1990

Effect of dietary protein on rat renin and angiotensinogen gene expression.

Plasma renin activity varies with the level of dietary protein, being higher on a high protein diet. To explore the molecular mechanisms underlying this relationship we first examined the effect of dietary protein on renin and angiotensinogen gene expression at the level of steady state mRNA in male Sprague-Dawley rats. Renal renin mRNA was higher on a 50% (high) compared to a 6% (low) protein diet both 3 d (9.4 +/- 1.1 vs. 5.3 +/- 0.4 pg/micrograms of total RNA; P less than 0.02) and 21 d (6.8 +/- 1.0 vs. 3.5 +/- 0.4 pg/micrograms of total RNA; P less than 0.02) after dietary change. No change occurred in either renal or liver angiotensinogen mRNA. When three levels of dietary protein were examined, renal renin mRNA was elevated on a 50% and lowered on a 6% protein diet compared to a more standard 20% protein diet. Kidney weights and renal protein, RNA, and RNA/DNA increased with the level of dietary protein reflecting protein-induced renal hypertrophy. Uninephrectomy resulted in no change in renin mRNA compared to sham operation (3.7 +/- 0.1 vs. 3.4 +/- 0.1 pg/micrograms RNA; P = NS) despite renal growth in the uninephrectomy group implicating dietary protein and not hypertrophy as the major factor for stimulating renin mRNA. In conclusion, the level of dietary protein is a novel and specific stimulus for changes in renal renin mRNA. The increased plasma renin activity on a high protein diet is due at least in part to increased renin synthesis.

Angiotensinogen↗
PubMed1996

Dietary protein modifies oxidized cholesterol-induced alterations of linoleic acid and cholesterol metabolism in rats.

Effects of dietary protein on oxidized cholesterol-induced alterations in linoleic acid and cholesterol metabolism were studied in 4-wk-old male Sprague-Dawley rats, using casein and soybean protein as dietary protein sources. The rats were fed one of the two proteins in cholesterol-free, 0.3% cholesterol or 0.3% oxidized cholesterol mixture diets using a pair-feeding protocol for 3 wk. In the soybean protein-fed group, rats fed oxidized cholesterol did not have lower activity of liver microsomal delta6 desaturase, the rate-limiting enzyme in the metabolism of linoleic acid to arachidonic acid, compared with rats fed cholesterol-free diet, whereas in the casein-fed group the desaturase activity was significantly greater in rats fed oxidized cholesterol than in those fed cholesterol-free diet. This was in contrast to a significant reduction in liver microsomal delta6 desaturase activity by cholesterol, irrespective of protein source. In general, these changes were reflected in the desaturation indices of liver phospholipids. Furthermore, soybean protein significantly increased the fecal excretion of neutral and acidic steroids and tended to reduce (P = 0.082) the accumulation of oxidized cholesterols in the liver. Thus, soybean protein partly modified some of the undesirable effects of oxidized cholesterol through its hypocholesterolemic effect and possibly through the modulation of hepatic delta6 desaturase activity.

Animals↗
PubMed1982

Effect of dietary protein level and monensin on performance of chicks.

Four experiments were conducted to investigate a possible interaction between dietary protein level and monensin on growth performance of young chicks. Crossbred chicks (New Hampshire X Columbian) or commercial broiler chicks (Hubbard X Hubbard) were fed corn-soybean meal diets containing various protein levels with or without added monensin (160 or 121 mg/kg). Monensin had little or no effect on growth performance of crossbred chicks fed diets containing 24 or 20% protein. However, growth performance of chicks fed 16% protein diets was depressed 15 and 8% by 160 and 121 mg/kg monensin, respectively. With commercial broiler chicks a marked interaction was observed between dietary protein level and monensin. As dietary protein level decreased, the depression in performance from monensin increased. At 24.0, 21.5, 19.0, and 16.5% protein, weight gains at 4 weeks of age were depressed 20, 29, 35, and 45% by 160 mg/kg monensin and 7, 14, 19, and 28% by 121 mg/kg monensin, respectively. Linear regression analysis indicated that 121 mg/kg monensin depressed growth rate an additional 2.7% for each 1% reduction in dietary protein level. A large portion of the growth depression from monensin could be explained by reduced feed intake. The results of this study indicate that level of dietary protein has a profound effect on the magnitude of growth depression caused by addition of monensin to chick diets. Thus, deficient levels of dietary protein should be avoided when using this anticoccidial drug in broiler feeds.

Animal Feed↗
PubMed1995

Utilization of dietary protein in the vitamin B12-deficient rats.

Utilization of dietary protein in vitamin B12 (B12)-deficient rats was evaluated by determinating the content of plasma protein, urinary excretion of nitrogen compounds, and nitrogen-balance after the rats were fed on a B12-deficient soy bean protein diet by pair-feeding for 100 days. The severe B12-deficiency was confirmed in rats by a remarkable increase in urinary methylmalonic acid excretion and a remarkable decrease in the hepatic B12 level. Growth of B12-deficient rats was significantly retarded as compared both with ad libitum-feeding control rats and pair-feeding control rats. The growth retardation due to B12-deficiency was alleviated by the administration of 1 microgram/day of CN-B12 for 30 days. Plasma total protein and albumin levels in rats fed on a B12-deficient diet decreased, compared with those in pair-feeding control, and increase in urea-nitrogen was observed. The excretion of urinary nitrogen compounds, such as urea-nitrogen, allantoin, and creatinine, was significantly depressed by B12-deficiency compared with those in pair-feeding control. The administration of CN-B12 to B12-deficient rats for 30 days resulted in the recovery of the changes in plasma proteins and urinary excretion of nitrogen compounds. The above results suggested that the extreme B12-deficiency depressed the utilization of dietary protein in rats. Moreover, the decrease in urinary urea-nitrogen excretion was supposed to be due to the adaptation by the depression of the dietary protein utilization.

Animal Nutritional Physiological Phenomena↗
PubMed1987

Modulation of rat skeletal muscle branched-chain alpha-keto acid dehydrogenase in vivo. Effects of dietary protein and meal consumption.

The effects of dietary protein on the activity of skeletal muscle branched-chain alpha-keto acid dehydrogenase (BCKAD) were investigated. BCKAD is rate-limiting for branched-chain amino acid (BCAA) catabolism by muscle; its activity is modulated by phosphorylation-dephosphorylation. In rats fed an adequate protein (25% casein) diet, BCKAD was approximately 2% active postabsorptively and increased to 10% or 16% active after a 25% or 50% protein meal, respectively. Prolonged feeding of a 50% protein diet increased postabsorptive BCKAD activity to 7% with further increases to 40% active postprandially. On a low protein (9% casein) diet BCKAD remained approximately 2% active regardless of meal-feeding. Dose-dependent activation of BCKAD by intravenous leucine in postabsorptive rats was blunted by a low protein diet. We conclude that excesses of dietary protein enhance the capacity of skeletal muscle to oxidize BCAA, muscle conserves BCAA when protein intake is inadequate, and skeletal muscle may play an important role in whole-body BCAA homeostasis.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗
PubMed1998

Dietary protein affects intestinal calcium absorption.

BACKGROUND: Changes in dietary protein in adults are associated with changes in urinary calcium excretion. The mechanisms underlying this effect are not completely understood, but alterations in intestinal absorption of calcium are not thought to be involved. OBJECTIVE: We reexamined this mechanism by evaluating the effect of 2 amounts of dietary protein (low: 0.7 g/kg; and high: 2.1 g/kg) on fractional calcium absorption in 7 healthy, young women. DESIGN: The experiment consisted of 2 wk of a well-balanced diet containing moderate amounts of calcium, sodium, and protein followed by 5 d of an experimental diet that contained 1 of 2 amounts of protein and constant amounts of other nutrients known to influence calcium metabolism. Seven subjects received both amounts of dietary protein in random order. Blood and urine were sampled at baseline and on day 4. Fractional calcium absorption was measured by dual-stable calcium isotopes on day 5. In a second study of 5 additional women, we evaluated the effects of dietary fiber on calcitropic hormones. RESULTS: Subjects developed hypocalciuria and secondary hyperparathyroidism on day 4 of the low-protein diet. Urinary calcium excretion and the glomerular filtration rate were elevated significantly by day 4 of the high-protein compared with the low-protein diet. Fractional calcium absorption after the low-protein diet was 0.19+/-0.03, which was significantly lower than that after the high-protein diet (0.26+/-0.03, P=0.05). CONCLUSION: These data provide evidence that depressed intestinal calcium absorption explains, in part, low-protein-induced secondary hyperparathyroidism.

Adult↗
PubMedDate not supplied

Dietary protein and blood pressure.

OBJECTIVE: - To review published and presented data on the relationship between dietary protein and blood pressure in humans and animals. DATA SOURCES: - Bibliographies from review articles and books on diet and blood pressure that had references to dietary protein. The bibliographies were supplemented with computerized MEDLINE search restricted to English language and abstracts presented at epidemiologic meetings. STUDY SELECTION: - Observational and intervention studies in humans and experimental studies in animals. DATA EXTRACTION: - In human studies, systolic or diastolic blood pressure were outcome measures, and dietary protein was measured by dietary assessment methods or by urine collections. In animal studies, blood pressure and related physiological effects were outcome measures, and experimental treatment included protein or amino acids. DATA SYNTHESIS: - Historically, dietary protein has been thought to raise blood pressure; however, studies conducted in Japan raised the possibility of an inverse relationship. Data analyses from subsequent observational studies in the United States and elsewhere have provided evidence of an inverse relationship between protein and blood pressure. However, intervention studies have mostly found no significant effects of protein on blood pressure. Few animal studies have specifically examined the effects of increased dietary protein on blood pressure. CONCLUSIONS: - Because of insufficient data and limitations in previous investigations, better controlled and adequately powered human studies are needed to assess the effect of dietary protein on blood pressure. In addition, more research using animal models, in which experimental conditions are highly controlled and detailed mechanistic studies can be performed, is needed to help provide experimental support for or against the protein-blood pressure hypothesis.

Animals↗
PubMed1995

Marginal dietary protein restriction reduced nonprotein nitrogen, but not protein nitrogen, components of human milk.

OBJECTIVE: This study was designed to determine whether marginal dietary protein intakes of lactating women alter their milk production and composition. The study followed the observation that marginal dietary protein intakes lead to significantly negative nitrogen balances in these women. METHODS: Twenty-four healthy, mature lactating women were divided evenly into three groups on the basis of postpartum times; 1, 5, or 12 months. Each woman was given a controlled protein diet of either 1.5 (HP) or 1.0 (MP) g/kg body weight/day for 7 to 10 days. Milk production was measured for 72 hours by test weighing the infant before and after feeding. Milk samples from alternate breasts were collected for 24 hours, pooled, and analyzed for their protein nitrogen, nonprotein nitrogen, free and protein-bound amino acid, and lactoferrin concentrations. RESULTS: Reduced dietary protein intakes were associated with a decrease (p < 0.05) in the nonprotein nitrogen and total free amino acid fractions of milk. Although urea was unaffected, the concentrations of milk free leucine, phenylalanine, histidine, and ornithine were lower (p < 0.05) in the MP than in the HP group, while the output of leucine and histidine tended to be lower in the MP group. The amount of milk produced, as well as the amounts of protein nitrogen, protein-bound amino acid, and lactoferrin and their concentrations were unaffected by a reduction of dietary protein. CONCLUSIONS: Maternal milk production and the protein nitrogen, but not the nonprotein nitrogen, fraction of human milk were relatively well-preserved in the presence of a short-term, marginal reduction of dietary protein.

Adult↗
PubMed2004

Effect of dietary protein, zinc, and carbon monoxide on fetal zinc concentration in mice.

Dietary protein and zinc deficiencies known to be detrimental to the developing fetus are common in pregnant women in developing countries. Everyone in modern society is at risk of exposure to carbon monoxide (CO). This study was conducted to observe the effect of dietary protein, zinc, and exposure to CO on the fetal zinc concentrations by factorial experimentation. Pregnant mice of CD-1 strain were maintained on 17% (control) or 9% (deficient) protein diets mixed with deficient, normal (control), or supplemental zinc throughout gestation. The dams in each dietary group were exposed to air (control) or 500 ppm CO in air in environmental chambers from gestation day 8 to gestational day 18. The dams were sacrificed on d 18 and fetal zinc levels were measured by atomic absorption spectrophotometry. Carbon monoxide levels used in this study had no significant effect on fetal zinc concentration in any treatment group. When both dietary protein and zinc levels were normal, the mean fetal zinc concentrations were higher than all other dietary protein/zinc combinations (15.2 +/-6.0 and 14.2 +/- 4.1 microg Zn/g of tissue for 0 and 500 ppm CO levels). However, when dietary protein levels were deficient, supplemental zinc increased the fetal zinc concentrations significantly (12.7+/-3.8 and 13.1+/-0.3.6 microg Zn/g of tissue, in 0 and 500 ppm CO groups) as compared to zinc-deficient groups (8.7 +/- 3.0 and 10.0 +/- 3.3 microg Zn/g of tissue in 0 and 500 ppm CO groups). The results of this study may be relevant to populations that experience both marginal zinc and protein diets during gestation.

Animals↗
PubMed1982

Immunocompetence and dietary protein intake in early infancy.

Forty-one normal full-term infants were fed from birth, during the first 4.5 months of life, different diets based on two formulas similar in rough composition but basically different, the one being constituted only of cow milk protein and the other of soy protein. From each of the two formulas two different dilutions were prepared so that all diets supplied about 100 kcal/kg/day, but, respectively, 2.0 and 4.0 g/kg/day of cow milk protein and 2.0 and 5.0 g/kg/day of soy protein. After 4.5 months, growth in weight, length, and head circumference was normal and very similar in all infants. Gammaglobulin, immunoglobulin, transferrin, and some complement fractions (particularly C'3, C'1 INA, C'3 PA) were lower in infants receiving soy protein diets than in those receiving cow milk protein diets, and, within each type of diet, in those infants receiving lower amounts of protein. In particular, values of infants receiving 5.0 g/kg/day of soy protein were roughly comparable to those of infants receiving 2.0 g/kg/day of cow milk protein. B lymphocyte markers and reactivity did not show significant differences among the various groups. T lymphocyte markers and reactivity showed an impairment in soy protein diets (and in particular in 2.0 g/kg/day protein diets) with respect to cow milk diets. It was also observed that morbidity (mainly infections of upper respiratory tract) was higher in those infants who took soybean protein and in those who took lesser amount of protein.

B-Lymphocytes↗