Current British dietary recommendations are to reduce total fat intake to less than 30 % of total energy intake and saturated fat to less than 10 %. The energy lost by this suggested decrease in saturated fat intake is partially replaced by increasing polyunsaturated fat intake. A high intake of total dietary fat has been shown to cause fasting hyperinsulinaemia [1] and to reduce the ability of insulin to suppress endogenous glucose production [2]. Dietary studies have, however, provided conflicting evidence about the beneficial effects of a diet rich in polyunsaturated fat (PUFA diet) on lipoprotein and glucose metabolism.In non-diabetic subjects a PUFA diet could improve total plasma cholesterol concentrations [3] but this could be at the expense of a decrease in HDLcholesterol [4]. On the other hand, in patients with Diabetologia (2002) Abstract Aims/hypothesis. British dietary recommendations are to decrease total fat intake to less than 30 % of daily energy intake and saturated fat to less than 10 %. In practice, it is difficult for people to make these changes. It may be easier to encourage people to switch from a diet rich in saturated fatty acids to one rich in polyunsaturated fatty acids. Methods. A total of 17 subjects ± six people with Type II (non-insulin-dependent) diabetes mellitus, six nonobese and five obese people without diabetes ± were randomised to spend two 5-week periods on a diet rich in saturated or in polyunsaturated fatty acids, in a crossover design. At the start of the study and after each dietary period, we assessed abdominal fat distribution using magnetic resonance imaging, insulin sensitivity using hyperinsulinaemic-euglycaemic clamps and fasting lipid parameters.Results. Dietary compliance, assessed by weekly 3-day dietary records and measurement of biochemical markers, was good. Energy and fat intake appeared to be reduced on the diet rich in polyunsaturated fatty acids although body weights did not change. Insulin sensitivity and plasma low density lipoprotein cholesterol concentrations improved with the diet rich in polyunsaturated fatty acids compared with the diet rich in saturated fatty acids. There was also a decrease in abdominal subcutaneous fat area. Conclusion/interpretation. If this result is confirmed in longer-term studies, this dietary manipulation would be more readily achieved by the general population than the current recommendations and could result in considerable improvement in insulin sensitivity, reducing the risk of developing Type II diabetes. [Diabetologia (2002) 45: 369±377]

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