One common belief, often voiced by critics of vegan diets, is that plant foods don’t provide adequate tryptophan. This essential amino acid is needed to make the neurotransmitter serotonin, and low levels of serotonin are linked to depression. Meat is higher in tryptophan than plants, but a well-balanced vegan diet is almost guaranteed to provide more than enough of this amino acid. The recommendation for tryptophan is 5 milligrams for every kilogram of body weight. Adding in a factor for plant protein digestion, this translates to a vegan RDA of 5.5 milligrams per kilogram of body weight or
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