The nutritional deficiency disease pellagra results from niacin deficiency. Diarrhea is one symptom of pellagra. All carcinoid syndrome patients have subnormal blood levels of tryptophan, an evidence of its depletion by being diverted to make large amounts of serotonin. Hence niacin deficiency can occur. Taking niacin in low doses prevents this; high doses are not necessary and can cause flushing. Tryptophan is obtained only via ingested food, particularly meat.