The effect of vitamin B12 on the tolerance of chicks for high levels of dietary fat and carbohydrate.

Three experiments were conducted with White Leghorn chicks hatched from hens fed diets varying in levels of protein, fat, and vitamin B12. Adding animal fat at a level of 10% in the chick diet caused growth depression of vitamin B12 deficient chicks, regardless of protein or energy level of hen or chick diet. Increasing the level of fat to 20% in the chick diet caused further growth depression and increased mortality. Feed efficiency of vitamin B12 deficient chicks was severely depressed by each additional increment in the fat level. Increasing protein content from 20 to 30% in the chick diet resulted in severe growth depression and poor feed efficiency. Although the added fat in the 30% protein chick diet depressed growth of chicks hatched from hens fed the 16 and 32% protein with added fat, it improved growth of those hatched from hens fed the similar diets with no added fat. Added fat in the 30% protein chick diet also improved feed efficiency of all chicks regardless of breeder diet treatments. Chicks hatched with an adequate carry-over of vitamin B12 from hens or chicks fed a diet with 10 micrograms of added vitamin B12/kg of feed did not show the growth depression caused by the high level of fat in the 20 and 30% protein chick diets. Feed efficiency was greatly improved by the addition of vitamin B12 to all chick diets. In a 22% protein vitamin B12 deficient diet, isocaloric substitution of glucose for fat depressed chick growth significantly and this growth depression was counteracted by supplementing the diet with 10 or 100 micrograms of vitamin B12/kg of feed. The vitamin B12 requirement was not increased by such substitution in the 22% protein diet. In contrast, isocaloric substitution of fat for glucose in the 32% protein chick diet increased the vitamin B12 need for optimum growth.