arbohydrates can cause drowsiness because they increase the uptake of L-tryptophan to the brain which subsequently causes an increase in the production of melatonin (here). Protein can counter the calming effects of carbohydrates and increase alertness for a number of reasons. Because undigested protein in the stomach decreases the speed of digestion, a high protein meal can reduce the speed that glucose appears in the blood. This results in slower rates of insulin release and thus reduces the disappearance of branched chain amino acids from the plasma, thereby decreasing L-tryptophan entry to the brain. If eaten alone protein also generally causes a lower release of insulin compared to carbohydrates, which has a similar effect. Further, protein consumption increases plasma levels of L-tyrosine and L-phenylalanine, both of which compete for absorption across the blood brain barrier with L-tryptophan on the L-type large neutral amino acid transporter. In the brain, L-tyrosine and L-phenylalanine are converted to adrenaline, dopamine and noradrenaline, which increase alertness, countering the effects of melatonin.
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