We are all aware that we need to include protein in our daily diet. However, most of us pay very little attention to the source of the protein; we assume that all proteins have the same effects the upon our brain chemistry. This assumption is apparently not true.
Scientists have known for many years that diets lacking in certain amino acids can lead to significant alterations in the levels of neurotransmitters that influence how we think and feel. Most of the past research on this question utilized casein as the principle source of protein. A natural choice given that casein is the main protein found in cow’s milk. No attention was given to the fact that the levels of tyrosine and tryptophan, two amino acids that are critical to the formation of dopamine, norepinephrine and serotonin, are present in quite varied levels in different dietary proteins. For example, the other major protein in milk is lactalbumin, which contains a high proportion of tryptophan relative to many other amino acids has been shown to elevate the level of serotonin in the brain.
A recent study published in Nutritional Neuroscience by Choi et al compared the ability of different common dietary proteins to raise the levels of brain neurotransmitters that influence our mood, i.e. the catecholamines dopamine and norepinephrine or serotonin. Rats were fed a diet containing normal amounts of protein as either zein, found in maize, lactalbumin or casein, found in milk, soy protein or gluten, obtained from wheat, barley and rye. All of the proteins were equally, but only very modestly, effective at raising the level of dopamine and norepinephrine levels. Surprisingly, lactalbumin was extremely effective at raising the synthesis of serotonin while zein and casein had almost no effect.
The differences between the effects of the different diets were not trivial. In the cortex, the level of serotonin synthesis was ten times higher after eating lactalbumin, as compared to eating zein. Such large effects are similar in magnitude to those achieved following the administration of drugs that directly modify serotonin neurochemistry, such as many common anti-depressant and anti-anxiety drugs. The source of dietary protein does influence how we feel because the balance of amino acids alters our brain chemistry.
© Gary L. Wenk, Ph.D. Author of Your Brain on Food (Oxford, 2010)
