About 14 per cent of the human body is protein, so it is obvious that a growing child must have an intake of protein to allow for an increase in the total amount of protein in the body as it grows. Similarly, a pregnant woman obviously needs an intake of protein to permit the fetus to grow. What is less obvious is why an adult, whose body weight does not change, nevertheless requires protein in the diet.
Studies in the 19th century showed that animals that were fed a protein-free diet could not maintain their body weight, but wasted away. Proteins contain the element nitrogen in their constituent amino acids, and a chemical method for measuring total nitrogen-containing compounds in the diet, and in urine and faeces was developed by Johan Kjeldahl in 1883—a method that is still in use today. This made it possible to investigate the balance between protein intake and excretion of the products of protein metabolism, and so to begin to estimate how much protein is required in the diet.
Nitrogen balance is the difference between the intake of nitrogen-containing compounds in the diet (mainly protein), and the excretion of nitrogen-containing compounds (mainly small molecules such as urea) from the body. An adult is normally in nitrogen balance or nitrogen equilibrium—the intake and excretion of nitrogenous compounds are equal and there is no change in the total amount of protein in the body. A growing child, a pregnant woman, or someone who is recovering from protein loss is in positive nitrogen balance—the excretion of nitrogen-containing compounds is less than the intake. In this case, there is retention of nitrogen-containing compounds (as protein) in the body, and an increase in the total protein content of the body.
Studies of nitrogen balance are still used to determine protein requirements. If someone is fed too little protein, they will not be able to maintain nitrogen equilibrium. Their excretion of nitrogen-containing compounds will be greater than their intake, and there will be a loss of body protein. This is negative nitrogen balance, and is also seen in response to trauma or infection, and in patients with advanced cancer.
Protein requirements are determined by feeding volunteers on different levels of protein intake, and measuring their nitrogen balance. If they are fed 20–30g of protein per day at the start of the experiment, they will be unable to maintain nitrogen balance. Their protein intake is then increased gradually (with time to adapt to each change in intake) until they can maintain equilibrium between intake and excretion. This intake is just meeting their requirement. As their intake is increased above their requirement level, initially they have a period of positive nitrogen balance while they replace the protein that was lost when their intake was inadequate. Once the lost protein has been replaced, they remain in nitrogen equilibrium. Eating a high protein diet does not increase the total amount of protein in the body, but breakdown or catabolism of protein increases, and the excretion of nitrogen-containing compounds increases, to match the intake.
The outcome of studies on nitrogen balance is that the average protein requirement for an adult is 0.65g of protein/kg body weight, or 45.5g of protein per day for a 70kg (11 st) person. There is individual variation around this average requirement, and in order to ensure that essentially everyone’s protein requirements are met, an appropriate level of protein intake is set at 2 x standard deviation above the average requirement. This gives a safe and adequate level of protein intake as 0.83 g/kg body weight, or 58g (2 oz) of protein/day for a 70kg (11 st) person. Average intakes of protein in developed countries are considerably higher than this—of the order of 90g (3½ oz)/day, so there is unlikely to be a problem of protein deficiency.
An alternative way of expressing protein requirements is to calculate the percentage of energy intake coming from protein, which yields 4 kcal (17 kJ)/gram. On this basis, the safe and adequate level of protein intake represents about 8.25 per cent of energy intake, and average Western diets provide 14–15 per cent of energy from protein. Even in developing countries, it is likely that protein intake as a percentage of energy intake will be adequate to meet an adult’s needs. Apart from cassava, yam, and rice, most starchy dietary staple foods provide more than 9 per cent of energy from protein, and it would only need a relatively small amount of meat, fish, or another protein-rich food to ensure an adequate intake.
It was not until the middle of the 20th century that the underlying reason for the need for an intake of protein in an adult became apparent. Rudolph Schönheimer fed animals diets containing amino acids labelled with a stable isotope as part of their protein intake. Since the animals were in nitrogen equilibrium, he expected to recover almost all of the isotopic label in urine and faeces. In fact he recovered less than half—the remainder was incorporated into tissue proteins. Although the total amount of protein in the body remained unchanged, there is continual breakdown of tissue proteins and replacement by newly synthesized protein. Schönheimer coined the term ‘dynamic equilibrium’ for this process.
We now know that different proteins in the body are broken down and replaced at different rates. Some, and especially enzymes that are control points in metabolic pathways, are broken down and replaced within a few hours; others, which can be considered to be ‘housekeeping’ enzymes and structural proteins turn over more slowly, with a half-life of days or weeks. Collagen, the structural protein of bones and connective tissue has a half-life of almost a year.
Studies with stable isotopically labelled amino acids have also shown that although an adult is overall in nitrogen equilibrium, this represents the average of periods of negative balance between meals and positive balance after a meal.
In the fasting state (starting about 4 to 5 hours after a meal), the rate of protein synthesis slows down. This is because protein synthesis is very energy expensive, and in the fasting state we are reliant on reserves of fat and carbohydrate laid down after a meal as the source of metabolic fuel—for this reason, we need to conserve energy. Protein breakdown continues at the normal rate, liberating amino acids that are not used for replacement protein synthesis, but are metabolized as metabolic fuel or used for glucose synthesis.
After a meal there is an abundant source of metabolic fuel (sufficiently in excess of immediate requirements to permit reserves of fat and carbohydrate to be laid down in preparation for the interval between meals). There is also an abundant supply of amino acids from protein in the diet. This permits a period of positive nitrogen balance to replace the protein that was broken down in the fasting state. It is this replacement of protein broken down in the fasting state that explains why an adult has a requirement for an intake of protein although overall there is no change in the total body protein content.
Sportspeople and athletes in training, and especially body builders, are increasing their muscle levels and, therefore, the total amount of protein in their body. Many people think that this means they need more protein in their diet, and this is correct to a certain extent. However, even children recovering from severe malnutrition, who show rapid catch-up growth, the gain in body protein is less than half the total amount of protein that is synthesized and broken down each day. Average intakes of protein are sufficiently in excess of requirements that there is no need for more protein in the diet (whether from eating large amounts of meat or from taking protein supplements) to permit an increase in the total amount of protein in the body as muscle is gained through training. On the other hand, there is a need for increased energy intake to meet both the cost of new protein synthesis and the energy cost of training and increased physical activity. If a person eats a larger quantity of their usual types of food, he or she will automatically increase not only energy but also protein intake.
The protein supplements that are marketed for sportspeople therefore seem to be unnecessary. The American Dietetic Association has stated that an intake of 1.2–1.7g of protein/kg body weight by endurance and strength-trained athletes ‘can generally be achieved through diet without the use of protein or amino acid supplements. Energy intake to maintain body weight is necessary for optimal protein use.’ There is also concern that some of these supplements may contain undeclared ingredients, including steroid hormones, which may increase tissue protein synthesis, but are banned substances in competitive sport.
The same argument applies to people recovering from a period of illness. They may have lost a considerable amount of body protein, as a result of both surgical trauma and prolonged bed-rest. However, assuming that they can eat enough of their normal foods to meet their energy requirements, they will have sufficient protein to permit replacement protein synthesis during convalescence. Nevertheless, in some cases it may be difficult for a sick person to eat enough food to meet requirements, and a dietitian may well prescribe a high-energy or high-protein supplement.
Proteins are made from 21 different amino acids. Twelve of these can be synthesized in the body from more or less common metabolic intermediates, as long as there is an adequate total amount of protein in the diet. However, the remaining nine cannot be synthesized in the body, but must be provided in the diet. These are called the essential or indispensable amino acids. If they are not provided in adequate amounts to meet the need for tissue protein synthesis then it is not possible to maintain nitrogen equilibrium. Once the essential amino acid that is present in least amount compared with the requirement has been used, the remaining amino acids, both essential and non-essential, will be metabolized as metabolic fuel. Requirements for the essential amino acids can be determined by studies of nitrogen balance, where people are fed adequate amounts of total protein, but with one or other of the essential amino acids in limited amounts, until the intake at which nitrogen equilibrium can just be maintained is found.
Knowing the requirement for each essential amino acid as a percentage of total protein intake allows us to define protein quality. A protein that provides only 50 per cent of the requirement for one of the essential amino acids will have a protein score of only 0.5. A protein that provides more than the requirement of all of the essential amino acids will have a protein score of 1.0. In some countries, food labelling legislation requires that not only must the protein content of a food be declared, but also the quality of that protein, which is expressed as the ‘protein score’ (also called the ‘amino acid score’), corrected for the digestibility of the protein. This is known as the ‘protein digestibility corrected amino acid score’ (PDCAAS).
Milk and eggs have a protein score of 1 and meat has a score of 0.8–0.9. However, individual plant proteins have a score as low as 0.4–0.5. This has led to a distinction between animal proteins—sometimes called ‘first class’ proteins—and vegetable proteins—sometimes called ‘second class’ proteins. While this is so when individual proteins are considered, it makes little or no difference when whole diets are considered. The usability of cereal proteins is limited by their content of the amino acid lysine, but they have a relative excess of the amino acid methionine. By contrast, the usability of the proteins of peas and beans is limited by their content of methionine, and they have a relative excess of lysine. In a judicious mixture of vegetable proteins (e.g. rice and peas, or pasta and beans), the excess amino acids in one protein will compensate for the deficit in the other, and the overall protein score will be 0.8–0.9, the same as that of meat. The protein score of diets in developing countries, with little meat, eggs, fish, or dairy produce, is almost as high as that of diets in Western countries. There is certainly no problem in obtaining sufficient protein, or in overall adequate quality, from a vegetarian diet. Even among omnivores in Western countries, just over one-third of protein intake comes from cereals, fruit, and vegetables.