Amino acids are nitrogenous compounds required by the body to form proteins. Many people are aware that protein is an important structural component of the body. Protein makes a good construction material because it can be twisted and bonded to form various shapes and this allows flexibility in its role to provide structures such as hair, muscle, collagen and tendons. However as well a structural role in protein, amino acids can be used by the body to make neurotransmitters. These chemicals are pivotal in the communication between neurones in the brain and as such changing the levels of neurotransmitters can have a significant influence on brain function. For example, tyrosine and phenylalanine are required for the formation of the neurotransmitters adrenaline, noradrenaline and dopamine, excitatory neurotransmitters that can increase mood, motivation and movement control. In contrast tryptophan is required for the formation of serotonin and in this role can improve mood, induce sleep and decrease pain.
Tyrosine, tryptophan and phenylalanine can therefore affect mood directly because they are precursors of neurotransmitters. Phenylalanine and tryptophan are considered essential and as such are not synthesised in the body but must be obtained in the diet, and tyrosine is synthesised directly from phenylalanine. Dietary intake of these large neutral amino acids (LNAA) is therefore important in neurotransmitter synthesis. Other LNAA also play a role in neurotransmitter formation, although their role is indirect. This is because in order to be able to form neurotransmitters, tryptophan, phenylalanine and tyrosine must be able to cross the blood brain barrier. However, other LNAA can compete with this transport and limit their uptake. In particular, the branched chain amino acids valine, isoleucine and leucine (which are also LNAA) can inhibit the entry of neurotransmitter precursors to the central nervous system. The balance of these amino acids in the blood is therefore a determinant of the brain’s ability to synthesise neurotransmitters.
The ability of diet to feed the brain with its requirement of amino acids, and for foods to affect mood in this way has long been known. The quality, type and quantity of protein is able to affect the synthesis of neurotransmitters directly by supplying the raw materials to the plasma. For example, in one study seven healthy subjects were given 0, 75 or 150 grams of egg protein per day, and the plasma was sampled for LNAA1. Significant correlations were observed between plasma levels of the LNAA and the protein content of the diet. Generally as protein intake rose, levels of tyrosine, phenylalanine, valine, leucine and isoleucine rose concomitantly. However levels of tryptophan did not. Because plasma levels of LNAA are known to modulate neurotransmitter synthesis, these result show that the protein content of the diet can directly influence neurotransmitter synthesis rates and have effects on mood and cognition. But why did tryptophan levels not rise and how can seratonin be increased in this case?
Most protein sources do not cause large elevations in plasma tryptophan, but it is known that diet can modulate serotonin levels in the brain. This apparent paradox is explained by the ability of dietary carbohydrate, not protein, to elevate serotonin levels. This is not a result of the carbohydrate foods supplying tryptophan to the plasma, but an artifact of the ability of carbohydrate to raise the relative plasma levels of tryptophan indirectly. When carbohydrates are consumed, they cause the release of insulin, and this drives the branched chain amino acids leucine, isoleucine and valine into skeletal muscles. As a result of this the relative ratio of tryptophan to other large neutral amino acids increases and competition for entry of tryptophan to the brain via the LNAA transporter decreases. This causes more tryptophan to enter the brain and increases serotonin levels. This is one reason that carbohydrate foods increase tiredness, as serotonin is converted to melatonin, the sleep promoting hormone, and the reason that low carbohydrate diets increase wakefulness.
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