If we eat more protein than our requirements, it cannot be stored as amino acids, but must be added to our fat stockpiles. Although there are fewer calories in protein than fat, it is still possible to get fat on a high-protein diet if we eat too much. However, as protein makes us feel full faster, we tend to eat less. A number of effective weight loss diets make use of this action, including the Zone diet, Protein Power and the CSIRO diet. These diets do not generally increase the amount of protein in the diet (this would not work on its own), rather they increase the amount of energy coming from protein by reducing carbohydrate intake. In most diets, 10-15% of energy comes from protein, and this is doubled in most of these diets. The other obvious advantage of a high-protein diet is that it usually means eating less carbohydrate and fat.
High-protein diets also have a down-side. Too much animal protein always increases our exposure to animal fat, as well as leading to constipation if fibre intake is not increased at the same time. Vegetable protein doesn’t have these same problems. Very high protein diets (> 45% energy as protein) can also cause weakness, nausea and diarrhoea.
Amino acid supplements
A number of dietary supplements contain pre-digested protein (amino acid supplements, elemental diets). While this may be helpful in individuals with a damaged digestive tract, eating high biological value protein achieves the same result as expensive supplements in otherwise healthy individuals. Where possible, amino acids should always be obtained from whole food protein sources. A number of specific amino acid supplements are widely used in clinical nutrition and for health promotion. As few individuals on a conventional diet have inadequate levels of amino acids, these supplements mainly act by stimulating pathways involved in removing the excess load, or by altering the balance of amino acids in the body.
- Arginine is normally obtained from both the diet and cell metabolism. It is used in many proteins, as well as the synthesis of nitric oxide, one of the major regulators of blood flow in the body. Supplements have been used to support immune function, increase growth hormone and reduce erection problems. However, the impact of these effects is probably small when compared to more specific interventions.
- Glutamine is the most abundant amino acid in the body. It is used to make protein, as well as DNA and some brain neurotransmitters. Unlike other amino acids, it can be stored in muscles for later use. Although it can be synthesised in the body under conditions of increased protein turnover (e.g. illness, surgery), it becomes essential. In these states, supplements have been shown to be beneficial in aiding recovery. As glutamine readily crosses into the brain, supplements have also been widely used as a memory aid, especially in aging individuals.
- Tyrosine is a non-essential amino acid that is also used as the starting substrate for many important brain signalling chemicals, including dopamine and noradrenaline. Although supplements have little effect in healthy individuals, in individuals with an activated stress response (e.g. sleep deprived, over-worked, emotionally wrought), tyrosine can improve function and mood.
- Tryptophan is an essential amino acid used by the body to make proteins, Vitamin B3 and a number of important brain signalling chemicals. Although present in most protein, it is particularly plentiful in dairy products, meat, eggs, fish, chocolate, oats, bananas, mangoes, nuts and seeds. Tryptophan supplements can aid sleep, possibly by altering levels of the sleep hormones, serotonin and melatonin. It may also be useful in mental diseases associated with low serotonin levels, like depression.
- The branched chain amino acids (leucine, isoleucine and valine) are among the major components of muscle protein. Supplements containing branched chain amino acids, especially leucine, can help build muscle mass.