Method of addition of cellulose to experimental diets and its effect on rat growth and protein utilization

The growth response of rats to experimental diets limiting in protein was used to evaluate two modes of addition of fiber. The first mode (trial A) used the more conventional approach of substituting cellulose for starch. The second mode (trial B) proceeded by total diet dilution, by which cellulose was added to a cellulose-free stock diet. Rats fed both types of diets compensated for the energy dilution and their digestible energy intake did not change markedly. However, both protein intake and growth increased with rising dietary cellulose levels in trial A and the protein efficiency ratio (PER) decreased. These did not change in trial B, except at the highest cellulose level, where growth and PER tended to decrease. Not only did the total diet digestibility and gross energy digestibility diminish upon the addition of cellulose to the diet, but the apparent protein digestibility also decreased in response to increasing dietary cellulose. We concluded that, in the design of experiments using high-fiber diets, careful consideration should be given to the effect which the method of dietary fiber addition has on the effective concentration of nutrients in the non-fiber portion of the diet and to the effect which variations in the parameter might have on nutrient utilization or biological responses to them. Our results indicate that, under conditions where protein is in limiting amounts in the low-fiber diets, total diet dilution is the more appropriate mode of dietary fiber addition. This conclusion may apply to other nutrients as well, but that remains to be proved.