Proteins in the Diet

A fat content of 20-35% is strongly recommended in the 2005 dietary guidelines for Americans released by the U.S. Department of Health and Human Services. The fat in today s diet is about 40% saturated, 40% monounsaturated, and 20% polyunsaturated. Lowering the saturated and monounsaturated fat and raising the polyunsaturated fat content of the diet is also strongly recommended. What is the basis for these recommendations Heart disease is the primary cause of death in the United States (Section 17.1), and atherosclerosis, the buildup of fatty deposits called plaque on the inner walls of arteries, reduces the flow of hlood to the heart. If a coronary artery is blocked by plaque, a heart attack occurs as a result of the reduced blood flow carrying oxygen to the heart. About 98% of all heart attack victims have atherosclerosis, and the major components of atherosclerotic plaque are saturated fatty acids and cholesterol. 

The relation between blood levels of cholesterol and heart disease is well established. The more saturated fats and cholesterol in the diet, the higher the blood cholesterol is likely to be. Cholesterol, a lipid, has a waxy consistency, so to be transported in the bloodstream it must bond to a more water-soluble substance. Cholesterol combines with proteins to form lipoproteins, which are water soluble because of their many —NHj and —COO ions. About 65 % of the cholesterol in the blood is carried by low-density lipoproteins (LDLs), whereas about 25% is carried by high-density lipoproteins (HDLs). (The density difference is caused by the ratios of lipid to protein.) 

LDLs are the bad cholesterol and HDLs are the good cholesterol referred to in discussions of heart disease. LDLs transport cholesterol away from the liver and throughout the body they are therefore bad because they distribute cholesterol to arteries, where it can form the deposits of atherosclerosis. HDLs are good because they transport excess cholesterol from body tissues to the liver, where it is converted to bile acids that are needed in digestion. 

Cholesterol occurs naturally only in dairy products, meat, and fish. There is no cholesterol in fresh vegetables, fruits, or vegetable oils. 

The heart disease risk-blood cholesterol connection. The values given here are blood levels for persons more than 20 years of age. The values for children and adolescents are about 15% lower. 

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A smaller number of individuals, 132 samples, was analyzed for stable isotopes of nitrogen. There were two objectives to the nitrogen isotope analysis 1) to determine the amount of animal protein in the diet and to look for variation between the sexes and 2) to determine when infants were weaned from the breast. These objectives were addressed by selecting at least ten individuals, when available, from a number of age classes. Figure 1.1 illustrates the age distribution of the sample. Table 1.3 shows the results of these analyses. Individual 5 N values are provided in Herring et al. (1998). 

Nitrogen is mainly taken in by humans in the form of proteins, so there is no possibility for ambiguity here nitrogen isotopic ratios must reflect those of the protein in the diet. 

All these factors make comparisons with other archaeological indicators of paleodiet more complicated. Nevertheless, nitrogen trophic level effects, together with measurement of the A CapaiU,.coii,g.n spacing, seems to be the best way to quantify proportions of animal protein in the diet. 

In words, the DIFF equation states that the observed collagen 8 values are determined by both the dietary protein (with additional isofractionation), and the non-protein in the diet (with a potentially different isofractionation), by an amount that may vary with the fraction of each component in the diet. Note that if dp and dN were both set equal to -i-5%o, and if the f(F) were set = 1, this would re-state the original DIFF you are (i.e., your collagen is) what you eat (protein and non-protein) plus five permil . 

Two amino acids—cysteine and tyrosine—can be synthesized in the body, but only from essential amino acid ptecutsots (cysteine from methionine and tyrosine from phenylalanine). The dietary intakes of cysteine and tytosine thus affect the requirements for methionine and phenylalanine. The remaining 11 amino acids in proteins are considered to be nonessential or dispensable, since they can be synthesized as long as there is enough total protein in the diet—ie, if one of these amino acids is omitted from the diet, nitrogen balance can stiU be maintained. Howevet, only three amino acids—alanine, aspartate, and glutamate—can be considered to be truly dispensable they ate synthesized from common metabolic intetmediates (pyruvate, ox-... 

Urinary Responses. In agreement with some recent findings (10, 11), the apparent absorption of calcium and phosphorus did not increase significantly as protein in the diet increased (data not shown) pro-... 

Where arable land is high cost, high value, or in scarce supply the production potential of protein becomes an especially important factor. Fruits and leaves from wild, undomesticated plants provide attractive sources of protein in the diet because of their natural acceptability by local inhabitants and their wide-spread accessibility due to native growth. However, for domesticated and commercialized situations the production of protein per unit area of arable land is an important factor. 

By the 1930s many workers had shown that nutritionally inadequate proteins, such as zein from maize, could be effective as a source of nitrogen if supplemented by additional amino acids (for zein, tryptophan). Even if it contained all the essential amino acids, the amount of protein in the diet influenced the results. Osbome and Mendel found that if the diet contained 18% by weight casein, which is low in cystine, young rats grew, but if the amount of protein was diminished, added cystine was required to offset the relative deficiency of this amino acid. Later, after methionine had been discovered, it was shown to replace the need for cystine. 

By the middle of the nineteenth century it was generally accepted that nitrogen was excreted by mammals as urea, and by 1905 Folin had shown that the amount of urea voided through the kidneys as urine was proportional to the level of protein in the diet. Normally 20-30 g urea are excreted by man per day protein intake also affects urine volume, which is usually 1.2-1.5 L per day. 

Neurotransmitter Production. Neurotransmitters are relatively simple chemicals, and our bodies make most of the ones that we use. The nerve cell receives precursor substances such as amino acids from proteins in the diet and chemically processes these precursors to form neurotransmitter chemicals. The neurotransmitter is then stored in small sacs inside the neuron called storage vesicles. These storage vesicles reside inside the axon terminals. 

The presence of proteins in the diet is essential for health. An important question, therefore, is what is the minimal amount of protein that must be provided to maintain health It is not an easy question to answer. Even when no protein or amino acid is consumed, in an otherwise adequate diet, urea is lost from the body due to body protein break down. The daily loss of protein is about 0.34 g per kg or about 24 g protein each day for a 70 kg person (i.e. when no protein is consumed). However, this amount does not represent the minimal intake required, since other factors, (such as the amount of energy consumed, other components in the diet, and trauma physical activity can affect this amount.) The recommended dietary allowance (RDA) for a young adult is 0.8 g per kg per day (Table 8.6). 

An increase in the protein content of the diet in rats increases the maximnm activities of all the enzymes of the cycle in the liver. It is assnmed that this represents increased amonnts of these enzymes in the liver (Table 10.4). Since a chronic increase in the protein in the diet in hnmans increases urea production over a long period and also a decrease in protein in the diet decreases urea production, it is assnmed that, as in the rat, this is due to changes in the concentrations and therefore activities of urea cycle enzymes. 

Fatty liver occurs in kwashiorkor, probably due to lack of protein in the diet, which reduces the synthesis of the structural protein for VLDL (apolipoprotein B). The increased triacylglycerol produced in the liver from fatty acids removed from the blood (i.e. the inter-tissue triac-... 

This has obvious advantages over the process seen for glutamate synthesis via the reductive amination of 2-oxoglutarate, in that it no longer requires the intervention of free ammonia. We thus have the situation that some organisms are able to carry out the fixation of ammonia via reductive amination, whereas others manipulate via transamination the amino acid structures obtained from protein in the diet. 

The amount of fat in the ration has little effect on the fat content of milk, but the fatty acid composition of the dietary fat greatly influences that of milk fat (Chapter 4). Neither the total protein content of milk nor the proportions of the individual proteins is greatly influenced by the amount or kind of protein in the diet except at very greatly reduced intakes. Overfeeding with protein does, however, increase the NPN content of the milk (Thomas 1980). Milk of normal composition with the normal content and proportions of proteins can be produced on protein-free diets with urea and ammonium salts as the only sources of nitrogen (Virtanen 1966). 

For the most part, adequate copper is received in diet and widespread human deficiencies do not occur, but deficiencies may arise because of antagonists. The metals Cd, Hg, Ag and Zn interfere with copper metabolism, probably by competing for copper-binding sites in proteins. Ascorbic acid depresses intestinal absorption of copper56 (in contrast to iron). Some proteins in the diet adversely affect utilization of copper. The sulfide ion is a well known inhibitor of copper absorption, since it forms copper(II) sulfide which is insoluble.56... 

Both marine and freshwater fish are often overfed in fish farms, so their own lipids are less likely to follow changes in the dietary lipid pattern. In the natural state, the influence of food on the lipids of the tissues is somewhat blurred by the fact that some of the tissue lipids are generated from carbohydrates and proteins in the diet and so synthesized de novo. In his classic work, Lovem (1964) showed that the impact of food on the fatty acid composition of herring lipids was most pronounced during intensive feeding. In winter, at the end of that period, the fatty acid composition became different from that of the zooplankton on which the fish had been feeding earlier. 

The nature of protein in the diet also appears to alter xenobiotic metabolism. Clinton and Visek ( / and Clinton and Visek (Zi.) observed that protein alters carcinogen metabolism and that fat level in the diet alters the absorption and distribution of DMBA in the tissue. The effects of protein nutriture also are observed when 1, 2-dimethylhydrazine is included in the diet. It appears that inadequate protein in the diet causes a significant decrease in hepatic MFO activity. 

Carroll, K.K., Giovannetti, P.M., Huff, M.W., Moase, O., Roberts, D.C., and Wolfe, B.M. 1978. Hypocholesterolemic effect of substituting soybean protein for animal protein in the diet of healthy young women. Am. J. Clin. Nutr. 31(8), 1312-1321. 

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