Whether fat has a role in preventing type 2 diabetes has been a vexed question for the past 50 years. There have been two very large prospective studies in women1,2 and one in men.3 The Nurses Health Study showed no relationship between the incidence of type 2 diabetes and intakes of total dietary fat, saturated fat or monounsaturated fat.1 However, polyunsaturated fat was protective and trans fatty acids were harmful, so that replacing 2% of energy from trans fatty acids with polyunsaturated fat lowered the incidence of type 2 diabetes by 40%. In Australia, where trans fatty acids are much less relevant (as they are not present in margarines and hardened fats as they are in the United States), this would translate to replacing 5% of energy from carbohydrates with polyunsaturated fat, resulting in a decrease of 37% in the incidence of diabetes. Similar but weaker findings in relation to vegetable oils came out of the Iowa Women's Study (although trans fatty acids were found to be protective).2 Some smaller studies have found a relationship between total fat intake and incidence of diabetes,4 while other studies have found a relationship between saturated fat intake and fasting glucose levels,5 fasting and postload insulin levels6 or levels of HbA1c.7 Insulin sensitivity may not be altered by fat intake, as the Insulin Resistance and Atherosclerosis Study found no such association,8 confirming the findings of most intervention studies. Only one recent study has shown that substituting saturated fat for monounsaturated fat improved insulin sensitivity in healthy men and women.9
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