The Influence of Age, Dietary Protein, and Calorie Restriction on Serum Cholesterol and Triglycerides in the Male Rat

The effects of age, dietary protein, and caloric restriction on serum cholesterol and triglycerides were studied in l and 2 year old male Wistar rats fed ad libitum or restricted. Ad libitum animals were fed diets low (12%), medium (20%), or high (28%) in casein throughout the life cycle. Restricted animals were fed diets either low (18%), medium (30%), or high (42%) in casein. The latter groups were fed the same absolute amounts of protein but two-thirds of the calories as determined from the feed intakes of the ad libitum-fed rats. Two additional ad libitum-fed groups were fed diets increased or decreased in protein. After the second month of life animals fed variable amounts of protein were fed a 20% casein diet which was either increased or decreased at the rate of 0.8% per month until a level of 28 or 12% casein was obtained.

Age had an effect on both serum cholesterol and triglycerides. The young animals had a significantly (p < 0.05) lower mean serum cholesterol level than the old animals whereas the old animals had a lower (p < 0.01) mean serum triglyceride level than their younger counterparts.

The various levels of dietary protein (low, medium, and high) resulted in no significant differences in either serum cholesterol or triglycerides. The overall effect of protein on serum cholesterol was quadratic (p < 0.01) but not linear. The overall effect of protein on triglycerides was neither linear nor quadratic, but the interaction of protein and calories on triglycerides was linear by group (p < 0.05) and quadratic by group (p < 0.05).

Combined dietary treatments, regardless of age, significantly (p < 0.01) affected both serum cholesterol and triglycerides. When all ad libitum-fed versus restricted-fed animals were contrasted, both parameters were significantly (p < 0.01) lower for restricted animals. Variation in the ad libitum level (20→12% casein and 20→28% casein) had no effect on the serum cholesterol or the triglyceride level. In general, the significantly lower serum lipids recorded can be attributed to calorie restriction.