Influence of Dietary Fat Level on the Enzymatic and Lipogenic Adaptations in Adipose Tissue of Meal-fed Rats

Abstract

The effect of increasing dietary fat levels on the response to meal-eating (restriction of food ingestion to a single daily 2-hour meal) in the rat was investigated. The studies were conducted with diets supplying fat up to levels which would approximate the percentage of fat calories supplied in human diets. These levels were selected to evaluate whether the typical response to meal-eating observed in the rat could be anticipated in human subjects consuming diets considerably higher in fat content. Male rats were fed diets containing 10, 20 or 30% fat, these levels supplying from 21 to 52% of the ingested calories. Increasing the level of fat from 10 to 30% appeared to facilitate the adjustment of food consumption in meal-fed rats. Following the first week of the experiment, during which meal-eaters fed the 10 or 20% fat diets lost weight, meal-fed animals gained weight at essentially the same rate as did nibbling (ad libitum-fed) rats, despite a reduced food intake. The activities of the enzymes studied (glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and malic enzyme) and of fatty acid synthesis from acetate-1-¹⁴C or glucose-U-¹⁴C were increased by meal-feeding in rats fed diets containing 10 or 20% fat levels, but not in animals consuming 30% fat diets. The activities of glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and malic enzyme and the lipogenic capacity decreased with increasing levels of dietary fat in adipose tissue of both meal-fed and nibbling rats. The significance of these observations in relation to the homeostatic control of lipid metabolism is discussed. It is suggested that the inability to demonstrate meal-feeding effects in human subjects may be due to the high level of fat in normal human diets.