Low-fat diet vs high-monounsaturated fat diet and type 2 diabetes

Nutrition Research Newsletter, Sept, 2004

The optimal diet for persons with diabetes has long been a subject of controversy. Dietary therapy was the only treatment available in the era before insulin therapy. As elucidated by Joslin, dietary carbohydrate had to be restricted in patients with type 1 diabetes because of impaired carbohydrate metabolism. Such diets were ketogenic and were composed largely of fat and protein--the good foods of life. Joslin also commented that this diet was atherogenic and made the point that, if patients did not die of diabetic ketoacidosis, they would probably die of coronary artery disease. With the discovery of insulin in 1921, it became possible to introduce carbohydrate-containing foods into the diabetic diet, but this approach was poorly accepted. Research in the 1960s, however, pointed out that a higher-carbohydrate, low-fat diet could be used in diabetic patients to lower cholesterol without increasing the plasma triacylglycerol concentration. More recently, a diet low in saturated fat has become accepted, but controversy has focused on whether the diabetic diet should be higher in monounsaturated fat or higher in carbohydrates. It was thought that a high-monounsaturated fat (high-mono) diet would avoid the possible plasma triacylglycerol- and glucose-elevating effects of a high-carbohydrate diet and still contain less saturated fat and cholesterol than the earlier diabetic diet. This study was therefore, carried out to define more precisely the optimal energy distribution of monounsaturated fat and carbohydrates in the diabetic diet. The need for such a definition comes from the recommendation of the American Diabetes Association that 60-70% of total energy be derived from a combination of monounsaturated fat and carbohydrates. It was suggested that more monounsaturated fat and less carbohydrate be prescribed, especially in diabetic patients with lipemia. On the other hand, it was recommended that dietary fiber be increased, and fiber is present in carbohydrate-containing plant foods.

Although high-mono diets have been associated with improvement in glycemic control and dyslipidemia in controlled metabolic studies, such results are not necessarily applicable to free-living subjects. In those metabolic studies, subjects were not allowed to regulate their own energy intake. The researchers hypothesized that, if patients were allowed to adjust their energy intake on the basis of satiety, a high-carbohydrate, high-fiber, low-fat diet might be superior to a high-mono diet. Accordingly, the purpose of the present study was to compare 2 ad libitum diets in diabetic patients--one high in monounsaturated fat and the other low in fat and high in fiber and complex carbohydrates--to ascertain which diet would lead to greater weight loss and greater improvements in dyslipidemia and glycemic control.

Eleven subjects (8 women and 3 men) with type 2 diabetes mellitus treated with oral glucose-lowering medication, diet, or both were recruited for the study. The subjects had fairly good glycemic control at baseline. The subjects were fed low-fat or high-mono metabolic diets in random order for 6 wk, and the two diets were separated by a 6-12-wk washout period. Both diets were offered at 25% above estimated energy requirement to allow self-selection for quantity of food. Diets were fed by using a 4-d menu cycle. Menus for the two diets were similar, with the fat and carbohydrate composition changed by differences in recipes and serving sizes; the subjects were thus blinded to dietary treatment, insofar as this was possible. In general, high-fat items and oils on the high-mono diet were partially replaced on the low-fat diet with fat-free oils and foods higher in complex carbohydrates. All meals were prepared by the Metabolic Kitchen of the General Clinical Research Center at Oregon Health & Science University. Subjects consumed one meal per day at the Clinical Research Center. The other meals including the weekend meals were packaged for home consumption. The subjects were instructed to eat to satisfaction and return uneaten foods, which were weighed to allow calculation of the total energy intake and nutrient consumption by using a nutrient analysis database. The subjects were encouraged to consume their meals on a regular schedule and were instructed to maintain their usual exercise level during the study.

The low-fat diet provided 20% of energy as fat, and the high-mono diet provided 40% of energy as fat (26% of energy was monounsaturated fat). The low-fat diet provided 65% of energy as carbohydrates compared with 45% as carbohydrates for the high-mono diet; refined sugar made up 10% of energy intake in both diets. The low-fat diet was higher in fiber and water content, weighed more, and had a lower energy density (kcal/g diet) than did the high-mono diet. Although both diets were low in saturated fat, the low-fat diet was lower than the high-mono diet in saturated fat and cholesterol. The difference in saturated fat and cholesterol between the two diets was intentional, in that the researchers wished to study the effects of a diet in its entirety (low-fat compared with high-mono), rather than the effects of individual dietary components. A low-fat diet will generally be lower in saturated fat and cholesterol and higher in dietary fiber than will a high-mono diet, so that the composition of the two diets in the study likely mirrored the composition of these diets as they would be eaten in the "real world." Laboratory analyses were performed on blood samples collected after a 12-h fast before and after each dietary period.