The massive intake of carb-heavy calories stimulates the release of insulin, which in turn triggers the uptake of most amino acids from the blood into the muscles except for tryptophan.

With other amino acids swept out of the bloodstream, tryptophan — from turkey or ham or any meat or cheese, for that matter — can better make its way to the brain to produce serotonin. Without that insulin surge, tryptophan would have to compete with all the other kinds of amino acids in the big meal as they make their way to the brain via a common chemical transport route.

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