Increase in human exposure to methylamine precursors of N-nitrosamines after eating fish.

Consumption of fish has been encouraged recently because it may prevent mortality due to heart disease. Fish contains methylamines, which are precursors of N-nitrosamines. Nitrosamines can act as potent carcinogens in a wide variety of animal species, and there is no reason to assume that humans are resistant. Human subjects (n = 5) ingested a diet of known methylamine content for 2 days. On Day 3, they ate fish at the luncheon and dinner meals. On Day 4, they again ate the control diet. A single portion of fish contained as many methylamines as were normally excreted by the human in 2 days. Urinary excretion of monomethylamine remained constant (1.3 to 1.5 mumol/24 h/kg of body weight) throughout the study. Dimethylamine excretion increased more than 4-fold after fish was eaten (from 5.6 to 24.1 mumol/24 h/kg of body weight), while trimethylamine excretion increased more than 8-fold (from 0.2 to 1.6 mumol/24 h/kg of body weight). We conclude that the consumption of fish significantly increased exposure to methylamines, particularly to dimethylamine. Although there is potential for the in vivo conversion of dimethylamine to nitrosodimethylamine, a carcinogen, we know of no studies that have determined that the ingestion of fish increases the risk of cancer. This should be carefully investigated prior to recommending that humans change their eating habits.