Enlargement of the High Density Lipoprotein Pool in Rats by Exposure to Cold and by Feeding a High Fat Diet

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Abstract
Adult rats fed diets containing either 2 or 40% lipids such as lard were acclimated to environmental temperatures of 28.degree. C (warm), 22.degree. C (control) or 5.degree. C (cold) for 8 wk. All animals adapted perfectly to both diet and temperature. Feeding of the high fat diet increased plasma triglycerides, cholesterol and phospholipids, but also enlarged the pool of HDL [high density lipoprotein] 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.degree. C over that of rats fed the low lipid diet at 22.degree. 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 < 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.