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Water Restriction in Nutrition Studies

I. Level of Fat and Protein Utilization¹

Department of Biochemistry, College of Agriculture, University of Wisconsin, Madison

1. The use of high-fat or high-protein diets or both under conditions of water restriction in almost all instances was responsible for larger weight loses and a greater frequency of fatalities of young rats than occurred with high carbohydrate diets. These results are in part attributed to the little recognized fact that fat, as well as protein, contributes less metabolic water per calorie than carbohydrate.

2. Progressive substitution of fat for carbohydrate (sucrose) in suboptimum protein diets fed ad libitum to weanling rats resulted in (a) increased caloric intake, (b) increased efficiency of protein utilization, (c) increased growth and (d) no change in total body specific gravity, although the ratio of protein to energy was kept constant.

3. At an optimum level of protein, progressive substitution of fat (above 5%) for carbohydrate (sucrose) resulted in (a) decreased efficiency of protein utilization, (b) reduced growth and (c) no change in total body specific gravity, with the ratio of protein to energy maintained constant.

4. A number of possible explanations for the protein-sparing action of high-fat diets are discussed.

5. A study of organ to body-weight ratios showed (a) reduced thymus ratios and increased kidney ratios with intensification of water-restrictive measures and (b) larger thymus ratios concomitant with increased amounts of dietary protein. Liver ratio values were little affected by changes in water or protein levels of the diet. Spleen ratios were somewhat similar to those of the thymus.

6. The growth-promoting potential of an adequate diet cannot be based solely on the percentage of (high quality) protein. The level of protein/calorie must also be considered.

Manuscript received 12 March 1955.