Protein Differences Between Omnivores And Vegetarians Or Vegans

The recommended daily allowance for dietary protein intake appears to be the same for the elderly as for young adults, at 0.75 g/kg of body weight for both age groups.10 This is surprising, because the protein content of the adult body diminishes with age, with apparent exchanges of fat for muscle. Changes in muscle mass are related to whole-body protein turnover and changes in the rate of protein synthesis. The recent work by Campbell et al.,11 recommends 1.0 gm/kg of protein daily to maintain nitrogen balance in the elderly. Stressful physical and psychological stimuli in the elderly can induce a negative nitrogen balance.

The percentage of calories contributed by protein in human diets, from vegan to omnivore, can vary from 8 to 18%, with approximately 2 to 10% greater intake of protein by the omnivore subjects vs. the vegetarians. The lowest level of protein intake is in vegans.12 Animal protein is considerably higher in essential amino acids and sulfur amino acid content and promotes a higher rate of growth in a growing animal than will any single dietary plant protein. When two or more dietary plant proteins are combined in a single meal or meals for the day, however, this potential for growth difference is offset in growing children.13,14

The adult vegans in the Haddad et al. dietary study15 had higher serum albumin levels than the omnivore controls, which demonstrated vegan dietary protein adequacy. The vegans demonstrated this while maintaining lower blood urea nitrogen values. Long term, this pattern aids in the reduction of the incidence of chronic renal failure.

Animal protein contains considerably more sulfur as methionine and cysteine. These amino acids cause a higher acid load with the omnivore diet, which appears to be partially retained as a major component of the pathogenesis of bone disease and muscle wasting in aging.16 This helps to explain the improved mineral balance and skeletal metabolism in postmenopausal women treated orally with potassium-bicarbonate.16 Also, lipid peroxidation is enhanced with decreasing extracellular pH. Acidic pH releases iron from "safe" binding sites where this free iron then enhances free radical activity.17 Thus, the lower sulfur and phosphorus intake on an all-plant diet appears to provide a significant longevity advantage for elderly subjects.

Protein Source

Figure 11.1 Serum cholesterol is lower in rabbits fed plant proteins compared with animal proteins. Adapted from Sanchez and Hubbard.19

Protein Source

Figure 11.1 Serum cholesterol is lower in rabbits fed plant proteins compared with animal proteins. Adapted from Sanchez and Hubbard.19

Additional advantages from the decrease or elimination of animal protein for the elderly can be seen in the lower serum lipid levels of vegetarians or vegans. The type of dietary protein, animal or plant, is the primary factor in the effect of proteins on serum cholesterol levels. Thus, the amino acid composition of the protein is a controlling factor for serum cholesterol levels.19 Sixteen references in this cited paper indicate that the lysine:arginine ratio in proteins explains the effect of proteins on the level of serum cholesterol and, in turn, atherogenesis.19 Figure 11.1 shows the relationship between animal and vegetable protein on serum cholesterol levels. Soy protein from the soybean is a plant protein source that has been successfully used in long-term studies to decrease the level of serum cholesterol in hypercholesterolemic subjects. A high saturated fat intake however, can mask the soy protein effect.20 As the level of cholesterol in serum is decreased, the plasma levels of arginine, glycine, serine, and threonine are significantly increased while lysine, leucine, valine, pheny-lalanine, tyrosine, and histidine are decreased. In this regard, the index interpretation of the leucine:arginine ratio change appears to be similar and just as significant as the lysine:arginine ratio.19

Glucagon secretion responds to arginine levels.21 The secretion of insulin is in response to leucine.19,22 Massive doses of arginine, however, cause the secretion of both glucagon and insulin.23 Following the feeding of soy protein, mild increases in plasma arginine levels and, in turn, mild increases in glucagon, inhibit HMG CoA reductase, the rate limiting enzyme

Soy Casein Protein-free

Meals

Soy Casein Protein-free

Meals

Figure 11.2 Plasma insulin/glucagon ratio in normocholesterolemic and hyper-cholesterolemic men after ingesting a single protein-free meal or one containing soy protein or casein. From Hubbard, et al.26

in cholesterol biosynthesis.24 Leucine is a secretagogue of insulin22 and insulin in turn activates HMG CoA reductase.25

Utilizing test meals containing animal protein as casein from cow's milk, and soy protein from the soybean, while keeping the lipid and carbohydrate in the test meals constant, demonstrated that normocholes-terolemic vs. hypercholesterolemic subjects showed lower responses in insulin:glucagon ratios with both test meals. The soy protein significantly (p < 0.001) lowered this ratio in contrast to the effect of casein for the hypercholesterolemic subjects.26 Figure 11.2 shows these differences plotted as the ratio of insulin:glucagon. High levels of insulin are recognized as a possible risk factor in coronary heart disease27 and diabetes.28 Recent reports suggest that Type 2 diabetes is probably preventable by reducing insulin secretion by the use of dietary plant protein during the growth and early adult years.29 This, plus a consistent exercise program throughout life, is highly likely to protect the elderly from NIDDM.3031

Increased thyroid activity is evident from the higher resting metabolic rate of vegans32 and of the elevated thyrotropin levels in vegans.33 This serves to explain why vegans are, on the average, leaner with lower body mass indexes (BMI).32 Also, the immune status in vegans is elevated compared with omnivore subjects in terms of significantly elevated cytokine 2 and gamma interferon levels.34

Additionally, Parkinson patients also benefit from the low-leucine vegan diet because the low-leucine intake decreases the competition with their oral DOPA intake, so they can use lower amounts of DOPA medication to suppress their Parkinson disease symptoms without undesirable side effects from the higher DOPA intake.35

A recent complication in assessing the adequacy of dietary protein intake has been that a major proportion of our well-fed population demonstrates an inability to absorb dietary protein. This can be particularly severe in the elderly. Plasma amino acid patterns show an excess of alanine and urinary amino acids show low alanine, isoleucine, leucine, tyrosine, phenylalanine and lysine values.36 This indicates Cori-cycle gluconeogenesis37 or skeletal muscle breakdown, demonstrating inadequate protein intake related to decreased amino acid absorption from digested protein. Successful treatment of this condition with n-6 essential fatty acids from evening primrose oil or borage oil as a source of gamma-linolenic acid reduces or eliminates the evidence of skeletal muscle breakdown36 with regain of lost weight. This topic is covered in more detail relative to fatty acids in the lipid section that follows. Obviously malnourished elderly subjects, of course, need to be treated promptly with protein and energy supplements to activate nitrogen kinetics.38

A varied diet based on plant proteins is adequate, yielding growth and body maintenance results equivalent to a diet based on meat protein.39 The lower incidence of obesity, constipation, lung cancer, hypertension, coronary artery disease, type 2 diabetes, gallstones, reduced risk of breast cancer, diverticular disease, colon cancer, calcium kidney stones, and osteoporosis appear to be obvious advantages —particularly of the well balanced vegan diet — for the elderly.3,39,40 Key et al.,40 (Table 11.1) show the protective effect of daily fresh fruit intake in ischemic heart disease, cerebrovascular disease, and lung cancer, and daily raw salad protection for ischemic heart disease. They also presented a higher incidence of breast cancer in the vegetarian women, but the confidence interval was broad. The smokers in their study population demonstrated a higher rate of ischemic heart disease, cerebrovascular disease, and, of course, lung cancer, to emphasize the disease problems associated with smoking.

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