Estimates for daily protein requirements have undergone a considerable evolution in recent times. In the past, it was thought that a high-protein diet would impart strength and stamina, and this concept is to this day still entrenched in the minds of most people. Evidence has, however been mounting that a high-protein diet, and particularly a diet high in animal proteins, is detrimental to health. Conversely, a diet too low in proteins will lead to protein malnutrition. Low protein diets are normally associated with less affluent societies which in addition to low protein concentrations also suffer from energy deficient diets. Protein-energy malnutrition has been linked to growth abnormalities and lasting detrimental effects on mental development. Children that have experienced such malnutrition have lower IQS than their adequately nourished siblings.17
Hunger and malnutrition are major problems facing the world’s poor. Nearly 200 million children suffer from protein-energy malnutrition and over 2 000 million experience micronutrient deficiencies.18 In affluent societies, protein-energy malnutrition is rare, and protein-energy over utilization which leads to obesity, cardiovascular disease, diabetes
cancer and other degenerative diseases is more prevalent. These facts have prompted many to change their lifestyles, and such changes can lead to nutrient shortages if these are not properly conducted. In this regard, minimum protein requirements become important not only to people in poorer societies, but also to those who are more affluent.
In view of the association between high protein consumption and disease, the recommended daily requirement for protein has been considerably reduced in terms of what was considered essential when the first international recommendation of 1g protein per kilogram body weight per day was made by the League of Nations in 1936. The recommended daily allowance (RDA) for protein is currently being revised, but it is generally accepted that a daily consumption of a mere 56g for men and 44g for women is adequate.19 The protein requirements are, however, not the same for all age groups, and there is an age-related decline in protein needs. These age-related protein requirements were reflected in the recommended levels of protein consumption proposed by the Food and Agricultural Organization (FAO), The World Health Organization (WHO) and the United Nations University (UNU) in 1985.20 These recommendations are presented in table 1.1. and are for an animal protein source.
Table 1.1 Safe levels of protein intake as proposed by the FAO/WHO/UNU. Values are uncorrected for the nutritional value of the protein. (Ref. 20.21)
In both infants and adults, a combination of plant protein sources will supply adequate quantities of all the essential amino acids, but variety-poor diets would be restrictive, even if a good protein source such as soy protein is used. Soy protein isolates, as sole protein source, would supply sufficient amino acids for adults but might be insufficient for children, but a restrictive diet relying solely on grains as a source of protein would be inadequate to meet the needs of even adults.21 The combination of grains with legumes, seeds, or nuts will, however, supply a high quality protein with adequate concentrations of essential amino acids to meet the needs of all age classes.
The amino acid lysine seems to be one of the most important limiting amino acids, and differences between amino acid sufficiency in diets of affluent and poor societies will be greatest for lysine. Diets based largely on cereals can thus lead to shortages, and the revised estimates for lysine (30 mg/kg/day or some estimates are as high as 50 mg/kg/day) are probably closer to the actual requirements.23,24 Comparisons between major food groups in terms of their amino acid composition per gram of protein are presented in table 1.3.
| Food group | |||||
| Amino acid | Animal mean ±SD | Cereals mean ± SD | Legumes mean ± SD | Nuts/seeds mean ± SD | Fruit & veg mean ± SD |
| No. samples | 1 726 | 170 | 153 | 153 | 572 |
| Isoleucine Leucine Lysine Saa Aaa Threonine Tryptophan Valine | 46.7 ± 4.7 79.6 ± 6.0 84.3 ± 7.1 37.7 ± 3.3 74.9 ± 8.2 43.4 ± 2.6 11.4 ± 1.5 51.2 ± 5.6 | 39.8 ± 4.6 86.3 ± 26.3 30.5 ±9.8 41.1 ± 4.8 83.0 ± 9.2 33.6 ± 5.4 12.1 ±3.3 51.1 ± 6 9 | 45.3 ± 4.2 78.9 ± 4.2 67.1 ± 3.8 25.3 ± 2.8 84.9 ± 6.3 40.0 ± 3.3 12.3 ± 2.4 50.5 ± 4.0 | 42.8 ± 6.1 73.5 ± 9.0 43.5 ± 12.7 37.7 ± 11.7 88.0 ± 16.9 37.9 ± 5.4 15.4 ± 4.6 55.6 ± 10.3 | 38.5 ± 10.8 59.1 ± 19.6 49.2 ± 13.3 23.6 ± 7.2 64.0 ± 18.4 35.1 ± 8.7 10.8 ± 3.9 45.9 ± 12.6 |
Saa = sulphur amino acids, Aaa = aromatic amino acids
Table 1.3 Amino acid composition of major food groups. Data is compiled from the Massachusetts Nutrition Data Bank and is presented in mg/g protein. (From reference 24)
For the majority of amino acids the differences between the means are small, but levels of leucine and aromatic amino acids are low in fruits and vegetables and levels of tryptophan are high in nuts compared to the other foods. The major differences between groups are to be seen in lysine and to a lesser extent in sulphur amino acids. Lysine concentrations in cereals are only 30.5 mg/g protein compared to 84.3 mg/g protein in animal products and 67.0 mg/g protein in legumes. Vegan vegetarians with a varied diet containing grains, legumes and nuts or seeds as protein source will have no problem in meeting daily needs of essential amino acids, but restricted diets based largely on grains can be insufficient, particularly if daily intakes are low as in poorer societies.
Most people in industrialized countries consume far in excess of the recommended daily allowance for proteins, and in the United States, most adults consume 105g to 120g of protein per day,25 most of which is derived from animal sources. Such a high concentrations of
Figures for infants are not included, but breast fed infants should have a more than adequate supply from mothers milk. There is reason to believe, that the figure of 0.75 g/kg/day of good quality protein for adults may be too low to meet the needs of adults under all circumstances. Although it is true that if people can consume enough of their traditional diets to meet their needs, then protein quantities are normally sufficient as well. However, problems can arise when illness or poverty prevents people from consuming sufficient quantities.22 In 1994 the UNU- sponsored International Dietary Energy Consultative Group (IDECG) together with WHO and FAO representatives convened a meeting where it was concluded that the requirements for adults needed to be reassessed, but that the figures for children were probably adequate.. It seems desirable at this stage to round the figure to 0.8 g/kg/day for adults, and due to the lower efficiency of protein utilization in the elderly to propose a protein intake of 1.0 g/kg/day for this group.22
The quantitative need for essential amino acids also declines with age, but this need is thought to decline more rapidly than the need for total protein. Adults thus need lower concentrations of essential amino acids, per unit of protein, to maintain nutritional adequacy than do infants and young children.21 Recent evidence, however, shows that these figures may need revision and that requirements may be somewhat higher, even for adults. In table
1.2 the amino acid requirements for children and adults are presented, and the revised estimates for adults are also included.21
Table 1.2. Amino acid requirements for children and adults. (Ref.20,21)
amino acids in the intestine will stimulate the production of more amino acid receptor sites in the intestinal epithelium and will thus enhance amino acid absorption.26 Only a fraction of these amino acids is utilized to meet the body’s protein requirements, and the remainder must be converted into a form which the body can either store or utilize as an energy source. Excess proteins cannot be stored as such, as the body is geared largely for storing fat in the adipose tissues or carbohydrates, in the form of glycogen, in the liver and muscles. To meet these criteria, the amino acids must be metabolized resulting in the overproduction of potentially detrimental byproducts of amino acid metabolism. It would be wiser to limit the production of these compounds in the first place by reducing protein consumption to levels more in line with the daily requirements and increasing carbohydrate consumption to compensate for the concomitant energy decrease
Many recent studies have confirmed the adverse effects of dietary excesses of proteins, particularly animal proteins. High-protein diets are not only associated with cancer, but are also associated with kidney stone formation and progressive deterioration of renal function.27,28 Diets that are deficient in proteins can lead to the formation of bladder stones, but a strong correlation exists between the consumption of animal proteins and the formation of kidney stones. This is particularly apparent in affluent societies. In northern and western regions of India, animal protein intake is 100% higher than in the poorer southern and eastern regions and, consequently, the incidence of kidney stones is more than four times as high. Similar trends have been observed in a variety of other countries including Germany and Austria.29,30 Diets rich in animal protein also lead to the formation of calcium oxalate crystals because the urine composition is altered in a way which inhibits its ability to prevent crystals from forming.27 The urine levels of calcium and uric acid are increased when animal proteins are consumed whereas the levels of citrate are decreased, and it is the reduction in citrate levels which decreases the ability of the urine to inhibit crystal formation.
High-protein diets, particularly animal proteins, also have a significant calciuretic effect, that is, they cause the loss of calcium in urine,31,32,33 and urinary calcium loss is linked to osteoporosis. Diets that are deficient in proteins have also been shown to have a negative influence on bone formation, but this is certainly not the reason for the high incidence of osteoporosis in affluent societies. Rather, it is the high consumption of proteins that gives cause for concern in these societies. Prevention of osteoporosis, through following a sensible diet, is absolutely essential, because by the time osteoporosis is generally diagnosed, 50% to 75% of the original bone material has been lost.34
Animal protein sources contain greater concentrations of sodium than do plant protein sources, and they also have higher concentrations of sulphur-containing amino acids both of which cause calcium loss.29,35,36 The katabolism of dietary sulphur-containing amino acids increases the rate of acid excretion via the kidneys and this acid stress directly inhibits the renal reabsorption of calcium and leads to calcium loss. In a study done on young children, it was found that a high-protein diet increased calcium loss and the net acid excretion on the high-protein diet was nearly threefold higher than that observed with a low-protein diet.37 Sodium and calcium are reabsorbed at various common sites along the renal tubule, and a high sodium intake reduces the amount of calcium that can be reabsorbed from the renal filtrate, thus also leading to calcium loss. In contrast, plant protein sources such as soy protein, tend not to induce the loss of calcium,36 and soy protein sources such as tofu and soya milk maintain calcium equilibrium. A twofold increase in protein consumption causes a 50% increase in urinary calcium, but a soy based diet maintains calcium balance at a calcium intake of 457mg/day in spite of a 90g protein intake. Calcium plays an important role in many physiological functions including the metabolism of proteins, and if excessive amounts of calcium are lost, because of a high-protein uptake, the body will call on the reserves in the
bones, thus possibly laying the foundations for osteoporosis (for a more detailed discussion on osteoporosis see chapter 5).
Individual foods, even within food groups, vary in their protein and amino acid composition, and a comparison of these foods can thus help in the selection of food items which will ensure optimal supplies of proteins and essential amino acids, even in areas where food varieties are restricted. In table 1.4 the quantities of proteins and essential amino acids in a number of plant and animal foods are presented. Data for individual nuts and seeds is not included here as this information is dealt with later and is included in tables 7.14 and 7.15.
The total quantity of protein present, in the foods listed in table 1.4, is expressed in grams per 100g portions (% protein), but it must be remembered that percentage protein is not the best indicator of protein availability, as not all the protein is utilizable. Nevertheless, it does give an indication of protein quantities available, and together with the information on food combinations, that will provide optimum amino acid concentrations, can still serve a useful purpose. The extent to which proteins are actually utilized, is determined by a number of factors, including the concentrations of the essential amino acids present in the protein. The availability of a protein was termed its biological value, but the NPU (net protein utilization) is a better way of expressing the availability of proteins. The NPU is a combination of the biological value and the coefficient of digestibility of a protein, and is thus a more useful parameter than just the biological value. Both of these parameters, however, emphasize the importance of adequate concentrations of essential amino acids. The NPU can be improved by making the protein more digestible and by including a variety of plant protein sources in the diet to ensure a balanced supply of essential amino acids. The ways in which these objectives can be achieved, are discussed in chapter 7.
Table 1.4. The protein and amino acid composition of selected protein foods. The figures are for 100g edible portions. (Ref.38)