Protein
In this fact sheet: Structure and function, Dietary sources of protein, Protein combining, Dietary requirements for protein
Protein is an important part of our diet. A balanced diet should include a daily intake of around .75grams of protein for every kilo of your body weight. There are a great variety of protein rich foods for the vegetarian, rivalling for protein content, those consumed in a meat based diet. Popular concern that vegetarians go short of protein is misplaced and in particular plant protein has the added advantage of being free from the saturated fat associated with meat.
‘The nutritional advantages of protein foods of animal origin over those of vegetable origin, lie in practice more in the presence of associated nutrients such as vitamin B12, iron and retinol than in the protein.’ [FSA Manual of Nutrition 11th Edition 2008]
Structure and function
Proteins perform a wide range of functions in the body. Protein is fundamental to cell renewal and growth; muscle accounts for around half of the protein in the body. The protein collagen makes up a major part of the skeleton; connective tissues, the nails and hair are all forms of protein. As well as contributing significantly to the body’s structure proteins also govern our body processes. Hormones, controlling such things as growth and appetite, enzymes that break down food during digestion and haemoglobin that carries oxygen in the blood are all forms of protein.
The body makes proteins by combining substances called amino acids. From twenty amino acids the body can produce a vast array of proteins. The body must maintain and renew proteins from the food we eat so the proportions of the amino acids in the foods we eat is important if the body is to make the proteins it needs in the necessary quantities. Although about half of the 20 amino acids that make up human proteins can be broken down and re-assembled within the cells into other amino acids there are eight amino acids that cannot be made in this way and must be present in the diet, these are known as the essential amino acids: Isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine (the amino acid histidine is also regarded as essential for infants).
Dietary sources of protein and the essential amino acids
The protein content of foods of plant origin such as nuts, peas and beans (including peanuts) is very high and rivals that of meat and fish. Some vegetarians include eggs and/or dairy products as part of their protein intake. Cheese has similar levels of protein to meat and fish by weight whilst egg is regarded technically by dietitians as the perfect protein food on account of its ideal balance of amino acids.
Whether dairy products and/or eggs are included in our diet, plant foods provide the major part of our protein intake as vegetarians. Pulses can form the basis of many types of meals and whilst soya in the form of milk, tofu, miso or ready made products like burgers and sausages is probably the most versatile source of protein. Nuts also can be incorporated into breakfast and sweet or savoury dishes making them invaluable as a satisfying energy-dense source of protein. QuornTM a form of myco-protein – an edible fungus - is now widely available and is sold in a range of different forms from mince to fillets.
Some everyday foods that are normally regarded as carbohydrates such as rice and other grains, pasta, breakfast cereals and breads contain significant amounts of protein and can play an important part in your intake. For example, 100g of wholemeal bread contains 9.4g of protein. Potatoes too, because they are eaten in quantity, provide useful amounts of protein.
Protein combining
Of the eight essential amino acids two– lysine and methionine are given special attention in vegetarian diets. This is because compared with foods of animal origin such as eggs, milk and cheese various food groups of vegetable origin have an imbalance of either lysine or methionine. The food groups mainly in question are; cereals, such as wheat, oats and rice, and legumes; beans, peas and lentils. Wheat and rice proteins are comparatively low in lysine but better sources of methionine whereas beans and peas are relatively high in lysine yet in lower methionine. This has naturally led to the idea of cereals and legumes as ‘complementary’ proteins. In practice this means that meals that combine for example beans and rice or houmous and bread will provide a biologically ‘complete’ protein intake. It was thought until relatively recently that, as the body does not readily store amino acids it was essential for vegetarians to combine ‘complementary proteins’ at each meal. There has been some debate over this which has concluded that this isn’t strictly necessary, however it still has some advantages and seems a sensible way to approach a varied and complete diet.
Dietary requirements for protein (RNI)
As with the other main food groups, fats and carbohydrates, an excess of protein in the diet will be treated by the body as a source of energy and in turn can be converted to body fat potentially contributing to obesity. Current official guidelines for protein intake suggest for adults a daily intake of 0.75g of protein per kilo of body weight. Pregnant women should add 6g to this total and add 11g whilst in the first 4 months of breast feeding and thereafter add 8g per day for the duration of breastfeeding.
Average adult (19+) woman = 45g per day
Average adult (19+) man = 55g per day