5. DIETARY FIBRE
Proteins are made from building blocks known as amino acids. The most complex of the major nutrients, proteins are composed of carbon, hydrogen, oxygen, nitrogen and sulphur. Twenty-two amino acids, linked together in chains of varying lengths and sequences, account for an almost endless range of possible structures and functions. For instances, the hormone insulin and fingernails are both proteins made of the same 22 amino acids, but they differ so vastly in form and activity that their protein kinship is unrecognizable.
After water, which makes up about 70% of body weight, protein is the most abundant material in the
human body. It constitutes over 50% of the body’s dry weight or 15% of its total weight.
The human body makes over 100,000 different proteins, with widely varying shapes and functions. Some provide structure (bone). Others carry out complex biochemical reactions (enzymes). Still others prevent and combat infection and disease (antibodies). Others are hormones with regulating functions over metabolic processes (insulin). Yet others provide ornamental value (hair, nails). Others provide protective covering (skin). Others contract, to make movement possible (muscle).
Dietary protein provides amino acids, the “building blocks” for the unique proteins that the body
produces. Of the 22 amino acids found in nature, only 8 (10 for children) must be provided by foods,
because our body cannot make these 8 or 10 from other substances in quantities sufficient for our
requirements. These 8 or 10 amino acids are called essential amino acids. The body can produce other amino acids, known as non-essential amino acids, and therefore it is not necessary that our foods provide them. Note, the term “essential” and “non-essential” relate only to whether they must be present in the foods we eat, not to the relative importance of each individual amino acid. Essential and non-essential amino acids are listed below.
During protein synthesis within cells, free amino acids are linked in specific sequences to form thousands of proteins unique to human physiology and, more precisely, to each individual’s unique genetic make-up. The body’s free amino acid “pool” must therefore contain all amino acids necessary to build each particular protein.
Essential Amino Acids
** These are essential for children
Non-Essential Amino Acids
* Sulphur Amino Acids
Non-essential amino acids may originate either from digested dietary protein or from synthesis within the body. Essential amino acids must come from digested food proteins. If a diet is lacking even one essential amino acid, the body makes less of the protein that requires this essential amino acid. Just as a chain is only as strong as its weakest link, so a protein is only as efficient as its least abundant essential amino acid.
During periods of stress, illness or intense physical activity, the body may not make enough non-essential acids to meet its demands. It then depends more heavily on dietary sources of these amino acids.
A branched-chain amino acid (BCAA) is an amino acid having aliphatic side-chains with a branch (a carbon atom bound to more than two other carbon atoms). Among the proteinogenic amino acids, thereare three BCAAs: leucine, isoleucine and valine. The BCAAs are among the nine essential amino acids for humans, accounting for 35% of the essential amino acids in muscle proteins and 40% of the preformed amino acids required by mammals.
Nutritional efficiency of proteins
The nutritional value of a dietary protein is a measure of how adequately a protein supplies amino acids necessary to maintain positive nitrogen balance in humans. This value is now referred to as the “Protein Digestibility Corrected Amino Acid Score” (P.D.C.A.A.S.). This is the method now adopted by the World Health Organization, the US FDA and countries belonging to the United Nations. Proteins that completely satisfy the amino acid requirements for pre-school and school age children and for adults are assigned the value of 1, the highest possible score. Milk, egg and isolated soy protein are examples of complete proteins that are assigned the value of 1. Beef and chicken, on the other hand, have values of about .89.
In the past, the measure of a protein’s nutritional value was referred to as its Protein Efficiency Ratio
(PER). The PER was based on the protein requirements of weanling rats and was the standard promoted by the dairy industry as milk protein fulfilled the amino requirements of weanling rats.
Vegetable proteins would not completely meet the amino acid requirements of weanling rats and were
assigned a lower PER and therefore mistakenly thought to be “inferior” to milk proteins. Interestingly,
human milk scored an even lower PER then most vegetable proteins when it came to meeting the
nutritional requirements of weanling rats and by inference, would not be adequate for human needs.
This is of course a false assumption. It seems that few questioned the cows’ sole protein intake after weaning was from plant protein and that young cows would grow to become large and powerful animals solely on plant protein.
In the past, it was thought that most plant proteins were not “complete proteins” and that it was necessary to combine various plant proteins to obtain an adequate amino acid intake. Many clinical studies have demonstrated that this is an erroneous assumption and that a person following a varied vegan diet can maintain a positive nitrogen balance. The 1988 position paper of the American Dietetic Association emphasized that, because amino acids obtained from foods can combine with amino acids made in the body, it is not necessary to combine protein foods at each meal. Adequate amounts of all essential amino acids will be obtained if a varied vegan diet - containing unrefined grains, legumes, seeds, nuts and vegetables – is eaten on a daily basis (1).
1. Havala, S and Dwyer, J. (1988). “Position of The American Dietetic Association: vegetarian diets- technical support paper”, J. Am. Diet. Assn., 88, 352-355.
Dietary protein is important for nitrogen balance. Nitrogen balance, a measure of whether the body is
building up or deteriorating, is the result of the continual entry and loss of nitrogen containing amino acids in and out of the body. (Nitrogen in the air is inert and therefore plays no part in human biochemistry).
Nitrogen is necessary for the synthesis of many important substances in the body, including the nonessential amino acids necessary for making proteins, the nucleotides (purines and pyrimidines) from which genetic material and energy-carrying ATP are made, creatine, the vitamin nicotinic acid is required to provide the nitrogen necessary for all of the nitrogen-containing molecules.
Human protein requirement
Adequate protein is essential to support fitness training and hard physical labour. Protein requirement is based on total energy needs, body mass, type of activity, level of training, age, sex and climate,
temperature and altitude. Exercise results in an increased oxidation rate for the branched chain amino
acids (leucine, isoleucine and valine). Adequate levels of testosterone, growth hormone and insulin in the blood influence the anabolic effect of training. Arginine, lysine and ornithine are amino acids that may stimulate the release of testosterone, growth hormone and insulin and promote muscle development.
Adequate protein intake is necessary to provide these amino acids and to maintain positive nitrogen
balance and normal biochemical parameters. Protein, however, is not the only macronutrient necessary for maintaining good health and for improving athletic performance. Carbohydrates, essential fatty acids and dietary fibre also play essential roles. Most research indicates that dietary protein should make up between 15% and 30% of total calories, carbohydrates between 50% and 70% and essential fats between 15% and 20% of total calories (saturated fat should be as close to zero as possible). Dietary fibre should be at least 12 grams per 1000 calories per day.
Protein levels of 30% of total calories are optimal only for the most advanced strength athletes involved in a rigorous training programme. For example, a 100 kg advanced bodybuilder who consumes 4000 calories a day can use a maximum of 300 grams of protein a day. Any excess of this amount will be either burned as calories, stored as fat or excreted. A moderately active athlete who consumes 3200 calories a day would need approx. 25% of total calories from protein or 200 grams of protein a day. On the other hand, the average active individual who consumes 2000 calories a day would require only 15% of total calories from protein or 75 grams of protein a day.
In 1985, The World Health Organization (WHO) published figures for human protein requirement is 56 g of protein a day for a 75 kg man and 48 g for a 64 kg woman (1). The UK Department of Health and Social Security recommends 68 g a day for sedentary men and 54 g a day for sedentary women (2). The NACNE report (3) recommends a protein intake of 11% of calories. These recommendations are based on animal protein intake. Some plant based proteins may be less digestible because of intrinsic differences in the nature of the protein and the presence of other factors such as fibre, which may reduce protein digestibility by as much as 10%. Isolated soy protein, on the other hand, is equivalent to egg and milk protein in digestibility and meets the WHO/FAO/UNU minimum requirements for all essential amino acids for all pre-school and school age children and for adults.
Whey Proteins have the highest possible “Protein Digestibility Corrected Amino Acid Score” (P.D.C.A.A).
Food and Agriculture Organization/World Health Organization/United Nations University (1985). ‘Energy and
protein requirements’, WHO Technical Report Series 724. Geneva, WHO.
Department of Health and Social Security (1979). Recommended Daily Amounts of Food Energy and
Nutrients for Groups of People in the United Kingdom. London, HMSO.
National Advisory Committee on Nutrition Education 1983). Proposals for Nutritional Guidelines for Health
Education in Britain. London, Health Education Council.
Physiological effects of amino acids
Certain amino acids have profound individual effects in addition to their function as protein building
blocks. This emerging area of nutritional research has applications in treating and improving:
• Food and chemical allergies;
• Central nervous system disorders;
• Sleep disorders;
• Immune function;
• Anti-oxidant protection;
• Nerve and heart membrane stability;
• Efficiency of fat metabolism;
• Muscle mass;