Fats may influence insulin sensitivity.

Physicians typically recommend that patients with Type II (non-insulin-dependent) diabetes eat foods low in fat, especially saturated fat. These guidelines aim primarily at preventing obesity - a condition that can aggravate diabetes -- and heart disease, which frequently accompanies the metabolic disorder. A new Australian study now indicates a more direct reason for encouraging diabetics to limit saturated fats: They may foster a diabetic's resistance to insulin.

Together with animal data, these new findings hold out the prospect of one day altering "insulin action by changing the fatty-acid composition of the diet," says Leonard H. Storlien at the University of Sydney, who led the study,

People with Type II diabetes often possess plenty of insulin, a hormone that helps transport glucose (blood sugar) into cells. However, because these diabetics develop resistance to insulin, glucose and insulin may build to unhealthy concentrations in blood, eventually promoting potentially life-threatening damage. This insulin resistance, a hallmark of Type II disease, is also common in obesity and can precede the onset of diabetes by a decade or more.

Storlien and his co-workers biopsied skeletal muscle from 20 men and seven women between the ages of 50 and 65; all were nondiabetic and undergoing coronary-bypass surgery. The researchers correlated fatty acids in the cell membranes with each patient's sensitivity to insulin. In general, they report in the Jan. 28 NEW ENGLAND JOURNAL OF MEDICINE, the more saturated fatty acids present, the more resistant a patient proved to insulin.

The Sydney team also biopsied skeletal muscle from 13 healthy men, each about 30 years old, and tested their sensitivity to insulin. Again, "we found exactly the same relationship," Storlien says: The more long-chained polyunsaturated fats in muscle-cell membranes, the more responsive these individuals were to insulin.

In earlier work with rats, Storlien says, his group found that diets rich in long-chained omega-3 polyunsaturated fatty acids -- from fish oils -- were the most effective in overcoming "very profound whole-body insulin resistance."

Animal and cell-culture studies show that increasing the polyunsaturated fatty acids in cell membranes not only increases the amount of insulin bound by a given tissue or cell, but also insulins action, notes Mark Yorek of the Veterans Administration Medical Center in Iowa City, Iowa. "And this study would support that [this also occurs] in humans." However, he cautions, fishy fats are not a panacea; studies show diabetics on diets high in fish oils can experience trouble regulating glucose levels.

Indeed, the Sydney researchers point out, the fatty-acid profile of muscle-cell membranes may be due as much to some internal defect as to diet. They noted, for instance, that the reduced activity of one enzyme - a delta-5 desaturase -"was directly related to estimates of insulin resistance in this study." This suggests, they say, that some enzyme-related defect might impair insulin action by reducing levels of long-chain polyunsaturated fatty acids in cell membranes.