Protein has a vital role to play in the maintenance and repair processes constantly occurring in the body. However, as the body’s protein requirements can be met by a relatively small amount of protein-rich foods, it is easy to overestimate how much protein you need, and to rely too heavily on protein from animal sources. By making small changes in your diet, you can ensure that your body receives sufficient protein from a variety of nutritious foods.
Approximately 25 per cent of the body consists of protein – it is the basic material of all living cells, and has several important functions. Proteins are made from chains of individual units called amino acids; the number of ‘essential’ amino acids a food contains determines the quality of the protein supplied. To meet the body’s need for a constant supply of good quality proteins, people should choose from a wide range of plant and animal sources.
THE ROLE OF PROTEIN
Every cell in the body needs protein for the growth, maintenance and repair of tissue -protein is needed to make everything from hair, nails, skin, blood, muscles and bones to hormones and neurotransmitters (the brain’s chemical messengers). Protein also forms enzymes that enable the body to digest food and to produce antibodies that fight infection and disease.
Most of the protein eaten in the daily diet is used to build and repair muscle, while some is used as fuel by the body.
Excess protein is converted to fat. However, this conversion produces toxic by-products – ketones and urea – which put a strain on the liver and kidneys. A high animal-protein diet may also interfere with the absorption of certain minerals such as calcium; this raises the risk of developing the bone-weakening disease osteoporosis and may also cause the excretion of more minerals in the urine. Animal proteins are often rich in calories in the form of saturated fat and this can contribute to heart disease.
THE COMPOSITION OF PROTEIN
Proteins are made up of chains of individual units called amino acids. As protein is the hardest food to digest and most dietary proteins are too large to be absorbed, the body has to break them down into their individual amino acid building blocks to enable them to be digested. The body then constructs its own new proteins from these amino acids. The number of essential amino acids in a food determines its protein quality.
Amino acids link together in innumerable combinations to form chains. There are 24 different amino acids but, since many are present several times, a protein molecule can actually consist of 500 or more amino acid units. The human body is capable of manufacturing most amino acids, but there are eight ‘essential’ ones (nine in children) that can be obtained only from food. The term ‘essential’ does not mean that these amino acids play a more essential role in the body – rather that they have to be obtained from the diet.
Whereas any single animal food (such as red meat, poultry and fish) contains all the essential amino acids in the correct proportions required by the body, plant foods contain only some of these amino acids. For this reason, animal proteins were once considered to be ‘complete’ protein sources while plant proteins were said to be ‘incomplete’. This terminology has now been revised, and proteins are defined in terms of their biological value (BV) – that is, how efficiently they can be used by the body. The higher the BV, the better the quality of protein. Explained in another way, if a dietary protein supplies the optimal amount of amino acids (in theory, 100 per cent), it is defined as ‘high quality’.
In practice, a food with a BV of more than 70 per cent is recognized as high quality, as long as it also provides adequate calories and supports growth. Foods of this quality include most animal foods – meat, fish, eggs, milk and other dairy products. Some plant foods, such as amaranth, quinoa and soya beans, have BVs that come close to those of meat. A food with a BV of less than 60 per cent is described as ‘low quality’ and is deficient in at least one essential amino acid. Low quality protein foods are plant-based and include vegetables, most grain, pulses, nuts and seeds. The protein quality of meals based on low quality plant proteins can be improved if a mixture of plant proteins is consumed, either at the same meal or later on in the day.
INCORPORATING PROTEIN INTO YOUR DIET
In the West, many people eat twice as much protein as their body needs. Most men have consumed their daily protein requirements by the time they have eaten lunch. The average man or teenage boy needs 55g (2oz) of protein a day – a target that can be achieved simply by eating a bowl of wheat cereal with skimmed milk, two slices of bread, a typical 85g (3oz) serving of salmon and a side serving of peas. The average woman or teenage girl needs 45g (1-1/2oz) of protein a day, and a child aged between 7 and 10 years needs 28g (1 oz).
Many athletes and body builders believe that eating a high-protein diet or taking protein-powder supplements will help them to build larger and stronger muscles, but this is a misconception. Although athletes do need slightly more protein than relatively sedentary people, this extra need is easily met by their generally greater food intake. The only effective way to increase muscle size and strength is through exercise. When an athlete takes in more protein than his or her body needs, the excess is transformed into fat and stored in the body.
Many people rely very heavily on animal products for their protein intake. The following suggestions can help you to adjust this imbalance.
• Plan meals around grains and vegetables instead of meat; this not only cuts down on saturated fats in the diet, but also boosts the intake of carbohydrates and fibre.
• If you do include meat in a meal, make it a small portion and serve it with a large helping of grains, pulses or fresh vegetables.
• Improve the quality of plant proteins by combining a variety of sources in one meal or throughout the course of the day – for example, pasta with chickpeas; rice with lentils, tofu with vegetables and quinoa. These combinations can be particularly useful for vegetarians.
• Eat dairy alternatives made from soya beans (milk and yoghurts, for example).
• Eat more fish and shellfish in place of red meats to provide sufficient protein while reducing saturated fat consumption.