In this fact sheet: Structure and function, Dietary sources of protein, Protein combining, Dietary requirements for protein
Proteins are the building blocks of the body and an important part of our diet. A balanced diet should include a daily intake of around 0.75grams of protein for every kilo of body weight. There are many protein rich foods for vegetarians, rivalling the protein content of meat products. Popular concern that vegetarians lack protein is misplaced and plant protein has the advantage of containing reduced saturated fat associated with meat protein.
‘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 the body's structure providing cell renewal and growth; muscle accounts for around half of the protein in the body. Protein collagen makes up a major part of the skeleton, connective tissues, nails and hair. Proteins also manages metabolism and body processes. Hormones, controlling growth and appetite, enzymes that break down food, and haemoglobin that carries oxygen in the blood are all forms of protein.
The body makes proteins by combining substances called amino acids. From 20 amino acids the body can produce a vast array of proteins. The body must maintain and renew proteins from the food we eat, therefore the proportions of the amino acids in our foods is important. Although half of the 20 amino acids can be broken down and re-assembled in the cells into other amino acids, there are 8 amino acids that cannot be produced this way and must be present in the diet. These are known as the 8 essential amino acids: Isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine (histidine is also regarded as essential for infants).
Dietary sources of protein and the essential amino acids
The protein content of plant origin foods 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 as the perfect protein food for its ideal balance of amino acids.
Whether dairy products and/or eggs are included in our diet, plant foods provide a major part of protein intake. Pulses, such as quinoa, can form the basis of many types of meals and soya products such as milk, tofu, miso or ready made products like burgers and sausages are probably the most versatile source of protein. Nuts can be incorporated into breakfast and sweet or savoury dishes providing an energy-dense source of protein. QuornTM is a form of myco-protein – an edible fungus - and is sold in a range of different forms from mince to fillets.
Everyday foods that are normally regarded as carbohydrates such as rice and grains, pasta, breakfast cereals and bread 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 eaten in quantity also provide useful amounts of protein.
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