Determinants of high-fat diet hyperphagia: experimental dissection of orosensory and postingestive effects

High-fat diets often promote greater caloric intake and/or weight gain than high-carbohydrate diets in both laboratory animals and humans. Because altering the fat content of a diet simultaneously changes both its sensory properties and postingestive effects, it is unclear whether high-fat hyperphagia is due to the diet's palatability, its postingestive effects, or both. The present studies isolated the independent capacity of the orosensory and postingestive effects of a liquid high-fat diet (High-Fat) to produce overeating relative to an isocaloric liquid high-carbohydrate (High-CHO) diet. Rats fed High-Fat orally ate more calories and gained more weight over 16 days than rats fed High-CHO orally. One-bottle sham-feeding intake of High-Fat and High-CHO did not differ, but in two-bottle sham-feeding tests High-Fat was clearly preferred. When orosensory influences on intake were equated via chronic self-regulated intragastric feeding, High-Fat still promoted greater intake than High-CHO, although absolute intake across both diets was lower during intragastric feeding relative to oral feeding. An analysis of short-term intake revealed that rats accustomed to infusion of High-CHO increased meal size immediately when switched to High-Fat. The present results, coupled with previous findings, suggest that the postingestive effects of fat enhance daily caloric intake in two ways: 1) during a meal, fat produces less suppression of intake per calorie than carbohydrate; and 2) after a meal, fat produces less suppression of intake per calorie during the intermeal interval than carbohydrate.