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Meal composition and blood levels of amino acids

CAMBRIDGE, MASSACHUSETTS. Tryptophan and tyrosine are two important amino acids involved in the synthesis of serotonin and catecholamines (epinephrine and norepinephrine) respectively. Tryptophan must pass through the blood-brain barrier in order to be converted to serotonin. The blood-brain barrier's capacity for transporting amino acids is limited so tryptophan must compete with other large neutral amino acids (tyrosine, phenylalanine, leucine, isoleucine, valine and methionine) for access to the brain. This makes the ratio of tryptophan to the other amino acids (LNAAs) crucial in determining how much tryptophan will eventually be converted to serotonin. A similar situation exists for tyrosine.

Researchers at the Massachusetts Institute of Technology recently carried out a small trial to see if the composition of breakfast would influence the tryptophan:LNAA and tyrosine:LNAA ratios. Their experiment involved 8 healthy, young volunteers who consumed a carbohydrate-rich breakfast (69.9 grams of carbohydrates and 5.2 grams of protein) and a protein-rich breakfast (15.4 grams of carbohydrates and 46.8 grams of protein) 3 to 7 days apart. The researchers found that the carbohydrate-rich breakfast increased tryptophan:LNAA ratio by approximately 10% (not statistically significant) over baseline while the protein-rich breakfast decreased the ratio by about 30% (statistically significant). Similarly, while the carbohydrate-rich breakfast increased the tyrosine:LNAA ratio by a non-significant 5%, the protein-rich breakfast decreased it by a significant 20%. The total average difference in tryptophan:LNAA ratio between the carbohydrate-rich breakfast and the protein-rich one was 54% (28% for the tyrosine:LNAA ratio).

The researchers conclude that the composition of breakfast, or any meal for that matter, can result in substantial differences in plasma tryptophan ratio and thus probably in brain tryptophan concentration and serotonin synthesis. The meal composition also affects tyrosine ratio and may affect the synthesis of catecholamines. The researchers also noticed that while insulin concentrations rose sharply after the carbohydrate-rich meal there was very little, if any, increase after the protein-rich meal.
Wurtman, Richard J., et al. Effects of normal meals rich in carbohydrates or proteins on plasma tryptophan and tyrosine ratios. American Journal of Clinical Nutrition, Vol. 77, January 2003, pp. 128-32

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