Normal young adult male rats were either force-fed or allowed to eat ad libitum a moderate carbohydrate diet for 3–4 weeks. The force-fed animals were given either the amount of diet consumed by the animals eating ad libitum (pair-fed) or 80% of this amount (underfed). After a 2-week period of observation, we found that the rats eating ad libitum gained 65 gm of body weight, the pair-fed, force-fed 62 gm and the underfed, force-fed 40 gm. On the basis of the water, fat and protein content of the skin, viscera and carcass of control animals killed at the beginning of the feeding regimen and of similar constituents of the experimental animals after 2 weeks of feeding, the composition of the newly formed tissues of the various groups of animals consisted of the following: a) the rat with free access to food—water = 67.8%, fat = 7.8% and protein = 22.4%; b) the pair-fed, force-fed animal—water = 55.5%, fat = 23.6% and protein = 17.7%; c) the underfed, force-fed animal—water = 64.4%, fat = 7.9% and protein = 20.0%. The ratio of calories retained in newly formed tissue to the calories ingested over the 2-week period was 11.9% for the animals eating ad libitum, 20.6% for the pair-fed, force-fed animals and 9.5% for the underfed, force-fed rats. Force feeding appears to change intermediary metabolic pathways in the direction of increased ‘efficiency’ with resultant greater fat deposition.

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Normal young adult male rats were allowed to eat a diet of a specified protein content ad libitum, or were pair-force-fed the same diet twice a day. At the end of 14 days of controlled feeding, the animals were killed and analyzed for total body lipids. These analyses showed that the fat content of the force-fed animals, relative to those that ate ad libitum, became progressively greater as the dietary protein was increased in quantity from 0 to 67% of the diet. Furthermore, it was observed that force-fed animals excreted about 37% more urea nitrogen over a 7-day period than pair-fed animals eating ad libitum. Abruptly reversing the feeding habits of the rats immediately reversed the relative amounts of urinary nitrogen they excreted. The results of both types of experiments suggest that the intermediary metabolism of protein is related to feeding frequency (or load of absorbed nutrients to be metabolized per unit time). With fewer feedings but with total 24-hr intake constant, less dietary protein appears to participate in protein anabolic reactions since larger amounts of nitrogen are lost in the urine.

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