Abstract: The invention develops a weight management system which comprises administering tyrosine in the desired amount to change and maintain the body weight of an animal. If the animal is overweight, the animal would be fed a low level tyrosine food composition. If the animal is underweight, the animal would be fed a high level tyrosine food composition. The invention also relates to an low tyrosine animal food which contains, on dry matter basis: (a) from about 0.01% to less than about 0.4% of tyrosine, (b) from about 0.42% to about 3% of phenylalanine (c) from about 7% to about 70% of protein, (d) from about 1% to about 60% of fat, (e) from 0 to 90% of carbohydrate, (f) from 0 to about 40% of dietary fiber, and (g) from 0 to about 15% of nutritional balancing agents.