Circulating free fatty acids inhibit food intake in an oleate-specific manner in rats

Previous rodent studies showed that when injected into the brain, free fatty acids (FFAs) reduced food intake in an oleate-specific manner. The present study was performed to test whether food intake is regulated by circulating FFAs in an oleate-specific manner. Male Wistar rats received an intravenous infusion of olive, safflower, or coconut oil (100mg/h), together with heparin, to raise circulating oleate, linoleate, or palmitate, respectively, and their effects on overnight food intake were evaluated. Compared to other oils, olive oil infusion showed a significantly greater effect to reduce food intake (P<0.01). Total caloric intake, the sum of the calories from the diet and infused oil, was significantly reduced with olive oil (P<0.01) but not with coconut or safflower oil infusion, suggesting an oleate-specific effect on caloric intake. To further test this idea, different groups of rats received an intravenous infusion of oleate, linoleate, or octanoate (0.5mg/h). Oleate infusion decreased overnight food intake by 26% (P<0.001), but no significant effect was seen with linoleate, octanoate, or vehicle infusion (P>0.05). The effects of olive oil or oleate infusion could not be explained by changes in plasma glucose, insulin, leptin, or total FFA levels. The olive oil effect on food intake was not reduced in vagotomized rats, suggesting that oleate sensing may not involve peripheral sensors. In contrast, olive oil's effect was attenuated in high-fat-fed rats, suggesting that this effect is regulated (or impaired) under physiological (or pathological) conditions. Taken together, the present study provides evidence that circulating oleate is sensed by the brain differentially from other FFAs to control feeding in rats.