Enlargement of the high density lipoprotein pool in rats by exposure to cold and by feeding a high fat diet.

Adult rats fed diets containing either 2 or 40% lipids such as lard were acclimated to environmental temperatures of 28 degrees C (warm), 22 degrees C (control) or 5 degrees C (cold) for 8 weeks. All animals adapted perfectly to both diet and temperature. Feeding of the high fat diet was found to increase plasma triglycerides, cholesterol and phospholipids, but also to enlarge the pool of HDL lipoproteins by 46% and 50% in the control and cold environment, respectively. Exposure to cold did not depress serum triglycerides in these adapted animals, but increased serum cholesterol and phospholipid. The HDL pool increased by 43% in animals fed the control diet and by 47% in the fat-fed. The effects of diet and environment were additive and resulted in a 113% increase in the HDL pool of fat fed rats exposed to 5 degrees C over that of rats fed the low lipid diet at 22 degrees C. Exposure to a warm environment led to an increase in plasma triglycerides and cholesterol in both diet groups and to a decrease in phospholipids. This was accompanied by a reduction in the size of the HDL pool which was significant (p less than 0.05) in the rats fed the high fat diet. Enhanced incorporation of 3H Leucine into HDL apoproteins suggests that lipid feeding and cold exposure act by increasing synthesis. This increase must be specific since other plasma proteins were not affected.