Dietary Fats Rich in Saturated Fatty Acids (12:0, 14:0, and 16:0) Enhance Gallstone Formation Relative to Monounsaturated Fat (18:1) in Cholesterol-fed Hamsters

To test the possibility that dietary palmitic acid (16:0) may be lithogenic, different fats were blended to exchange 18:1 in olive oil with either 16:0 in palm stearin, 12:0 + 14:0 in coconut oil, or 14:0 + 16:0 in butterfat. Dietary 18:2 was held constant at 1.2% energy (en) (with extra safflower oil as needed) in these four purified diets containing low fat (11% of total energy) and 0.4% cholesterol. A fifth, high-fat diet provided 40% of the total energy as the 16:0-rich blend. All hamsters fed the low-fat 16:0-rich blend for six weeks developed cholesterol gallstones (8/8). Although the gallstone incidence was lower for the 12:0 + 14:0-rich diet (5/8), the severity of stone formation in affected hamsters was equal to that in the low-fat, 16:0-rich group. Mucin accumulation in gallbladder bile was often associated with cholesterol gallstones in diets containing 16:0, but was minimal in 18:1-rich and 12:0 + 14:0-rich groups. Neither the lithogenic index (all > 1.0), plasma lipids, nor liver cholesterol was a selective predictor of stone formation. The high-fat, 16:0-rich diet actually decreased cholesterol stone incidence (3/8) and severity, but yielded a high incidence of pigment stones (5/8). Thus, saturated fat and 16:0 per se were not responsible for the exaggerated lithogenesis. Because the antilithogenic 18:1-rich diet also normalized the 18:2 intake (1.2% en) relative to previous butter diets (0.3% en), the potential importance of essential fatty acids (EFA) deficiency in the model was tested in a second study by feeding graded amounts of 18:2 (0.3, 0.6, 0.9, and 1.2% en) as safflower oil in four low-fat, butter-rich diets (11% en as fat) without alleviating gallstone incidence or severity. These studies indicate that substitution of 18:1 for saturated fatty acids in low-fat diets reduces gallstone formation without affecting the lithogenic index. Furthermore, intake of 18:2 at or below the EFA requirement does not appear to be a major factor in this model.