Abstract
The hypothesis that dietary fat can influence glucose tolerance and insulin sensitivity adversely or beneficially was tested in a series of experiments in rats fed variable amounts and types of fat for periods up to 6 weeks. Studies included the development of an intravenous glucose tolerance test (ivGTT) and measurement of weight gain, food intake, organ weight including adipose tissue in terms of epididymal, suprarenal and mesenteric fat pads, and in some studies membrane fatty acid profile in liver and red blood cells. Insulin sensitivity was measured by relating changes in glucose concentration to measured insulin levels. A crude measure of the B cell response to glucose, the insulinogenic index, was evaluated from the relationship between insulin levels and glucose concentrations. After an initial series of pilot studies with 20% (w/w) com oil, olive oil, and fish oil/olive oil diets to establish the methods, glucose tolerance and insulin responses and action were measured in three studies. In experiment I 20% (w/w) com oil, olive oil, butter oil and fish oil/olive oil (1:4) diets were fed for 5 weeks. Although there were no significant differences (p=0. 05) in food intake rats fed olive oil and butter were heavier with larger fat pads and less glucose tolerant than those fed com oil or fish oil/olive oil, although fasting glycaemia did not vary with diet. Insulin levels measured by ELISA were judged unreliable in this study. Membrane fatty acids reflected the dietary fatty acid composition in terms of amounts of SFA, MUFA and n-6 and n-3 PUFAs. In a second study the fats were fed corn-oil, olive oil, butter or fish oil/olive oil (1:1), at 10% (w/w) for 5 weeks. In this case there were no sustained diet effects on bodyweight or food intake but fat pad size was again increased in the olive oil and butter groups. Glucose tolerance was only impaired in the butter group which exhibited a hyperinsulinaemia and impaired insulin sensitivity assessed by a reliable assay. To resolve the discrepant responses to the MUFA diet in a third experiment 20% (w/w) diets of corn-oil, olive oil, butter, fish oil/olive oil (1:1), or fish oil/butter (1:1) were fed for 5 weeks with a chow-fed control group, all diets fed at a fixed intake close to the maximum ad lib level. Rats fed olive oil and butter were heavier with larger fat pads than all other groups. Compared with the chow fed group glucose tolerance was only impaired by butter which also resulted in an hyperinsulinaemia and impaired insulin sensitivity not observed in the fish-oil butter diet. Membrane fatty acid profiles in hepatic and red-cells indicated that glucose tolerance was not influenced by the n6-n3 PUFA ratio with only total SFA reflecting glucose tolerance. These results indicate that in this rat model although both SFA and MUFA high fat diets increase fatness compared with n6 or n3 PUFA diets only SFA impairs glucose tolerance or insulin sensitivity, effects not observed when SFA is fed with fish oil. MUFA or PUFA diets maintain glucose tolerance although only PUFA diets prevent increased fatness.