Butter improves glucose tolerance compared with at highly polyunsaturated diet in the rat

Background: In several recent studies, it has been shown that markers of milk-fat intake, are strongly correlated to a low fasting-insulin level, indicating a positive correlation between milk-fat intake and improved glucose-tolerance [1, 2]. This is in contradiction to the general findings in epidemiological studies, where the typical fatty acid composition of milk-fat, i.e. a high level of saturated fatty acids (SFA) and low concentration of polyunsaturated fatty acids (PUFAs), has been correlated to increased insulin-resistance. It is therefore essential to characterize the impact of milk-fat on glucose-tolerance in intervention studies. Methods: 16 rats were divided into two groups and fed a semisynthetic diet containing 31 E-% fat, either as butter or highly polyunsaturated grapeseed oil. After 12 weeks on the diets, glucose-tolerance was assayed with the oral-glucose tolerance test (OGTT). Results and Discussion: The OGTT revealed that the rats on the butter-containing diet, had a substantially higher glucose tolerance than the rats, which were fed grapeseed oil (area under the curve =195  31 mM*min-2 vs. 310  13 mM*min-2, n= 8, p=0.004). There were no differences in serum triacylglycerol (TAG), serum free fatty acid and leptin between the groups. However, the butter-fed rats had a lower content of TAG in the white gastrocnemius muscle (7.7  1.5 vs. 23.1  6.2 mg/g tissue, p=0.01), and a much higher n-3 PUFA content (total n-3 PUFAs 1,43  0.06 vs 0.73  0.02g/mg tissue, p <0.0001) and lower n-6/n-3 PUFA ratio (2.29  0.09 vs 7.09  0.24, p