In patients with type 2 diabetes, 14 weeks of a high-carbohydrate diet modestly raises blood pressure compared to a diet high in monounsaturated fat, new study findings indicate.

Studies evaluating the effects of high-carbohydrate and high-monounsaturated fat diets have yielded conflicting results, Dr. Abhimanyu Garg and colleagues note in their report, published in the journal Diabetes Care. They suggest that these studies may have been limited by their short duration.

Their own study compared the effect of two same-calorie diets: a high-carbohydrate diet consisting of 55 percent of calories as carbohydrate, 30 percent as fat, and 10 percent as monounsaturated fat; and a high-monounsaturated fat diet deriving 40 percent of calories from carbohydrate, 45 percent from fat, and 25 percent from monounsaturated fat.

The 42 patients with type 2 diabetes participating in the study consumed each diet for 6 weeks, with about 1 week between the two periods, with the order of the diets randomly assigned. Subjects were invited to continue the second diet for an additional 8 weeks. Eight patients continued on the high-monounsaturated fat diet and 13 continued on the high-carbohydrate diet.

Garg, from the University of Texas Southwestern Medical Center at Dallas, and his associates found that, after the initial 6-week periods, there were no significant differences between diets in systolic or diastolic blood pressure, the upper and lower numbers on a standard reading, respectively, or in heart rate.

However, after the 8 week-extension, the high carbohydrate diet was associated with a diastolic blood pressure that was 7 points higher than at the end of both 6-week phases, systolic blood pressure was 6 points higher, and heart rate was higher by 7 to 8 beats per minute.

In contrast, the 8-week extension of the high-monounsaturated fat diet led to a significant lowering of heart rate compared with the end of the initial 6-week periods. Systolic and diastolic blood pressure were 3 to 4 points lower after 14 weeks on the high-monounsaturated fat diet, but the difference did not reach statistical significance.

“The most plausible mechanism for an increase in blood pressure and heart rate on a high-carbohydrate diet compared with a high-monounsaturated fat diet might be the accentuation of” high insulin levels, the authors propose.

SOURCE: Diabetes Care, November 2005.