High-Carb Diets and Diabetes

Back in 1988 Dr. Garg published a study and concluded that a diet high in monounsaturated fat was preferable to one high in carbohydrate (CHO) for people with type 2 diabetes because the high-CHO diet raised the blood sugar levels and triglyceride levels and lowered the HDL level compared to the higher-fat diet.1 I wrote a letter to the editor pointing out that this study’s results would not apply in the real world where people, rather than researchers, determine their calorie intake. Why? Because a higher fat diet would be expected to provide less satiety per calorie than a diet higher in carbohydrate.2

Since then many other studies have been published using the same flawed experimental design and coming to the same largely irrelevant conclusions. Based on these flawed studies, many now believe diets higher in monounsaturated fats improve blood lipids and blood sugar control in diabetics and are even better for preventing cardiovascular diseases (CVD) than are diets higher in CHO.3,4,5 I have written letters to the editors repeatedly explaining why short-term studies with controlled calorie intakes lead to conclusions that are largely irrelevant to the planning of diets for people with CVD or diabetes. In the real world over the long term, most people consume more calories and gain weight on diets higher in fat and lower in CHO.6 Results from two long-term trials in subjects with impaired glucose tolerance (IGT) demonstrated that a diet higher in CHO combined with exercise led to a reduction in body weight and about a 60% reduced risk of developing diabetes.7,8 No comparable data exist to demonstrate that higher-fat diets slow the progression of IGT to type 2 diabetes. Two studies in normal subjects showed that the adverse changes seen in blood lipids do not occur if the subjects, rather than the researchers, determine how much they eat when offered either a high-fat or a high-CHO diet.9,10,11 Now a study from the University of Oregon has looked at the impact of feeding subjects with type 2 diabetes either a high-CHO or high-monounsaturated-fat diet ad libitum. Not surprisingly, the adverse impacts of the higher-CHO diet on blood lipids and blood sugar levels failed to materialize. Why? Because subjects spontaneously ate fewer calories and lost more weight on the high-CHO diet than on the higher-fat diet. The authors concluded, “Contrary to expectations, the ad libitum, low-fat, high-fiber diet promoted weight loss in patients with type 2 diabetes without causing unfavorable alterations in plasma lipids or glycemic control.”12 Bottom Line: Losing excess body fat remains the most effective way to prevent and treat type 2 diabetes. Diets very low in fat and high in unrefined carbohydrate have been shown repeatedly to promote weight loss, improve blood lipids and reduce the risk of developing type 2 diabetes and CVD when fed ad libitum. Indeed, only a very-low-fat diet has been shown to reverse angina pain13 and reverse the atherosclerotic process.14 By contrast, diets with more fat and less CHO will generally be more energy dense and lower in fiber, which makes them more likely to lead to weight gain in the long run. Higher fat diets are less effective for preventing the development of type 2 diabetes and CVD over the long term. While diets high in unsaturated fats do favorably impact blood lipids compared to high-saturated-fat diets, there remains no evidence that diets high in unsaturated fats can reverse angina or the atherosclerotic process. Certainly there is no evidence a high-unsaturated-fat diet leads to a reduction in ad libitum calorie intake and promotes weight loss. Until such data are published, a diet very low in fat and high in unrefined CHO appears preferable to one with more monounsaturated fat. By James Kenney, PhD, RD, LD, FACN. References: 1. N Engl J Med 1988;319:828-34 2. N Engl J Med 1989; 320:536 3. Am J Clin Nutr 1997;65:1027-33 4. J Am Diet Assoc 1997;97:151-6 5. Am J Clin Nutr 1999;69:411-8 6. Am J Clin Nutr 1999;70:423 7. N Engl J Med 2002;346:393-403 8. N Engl J Med 2001;334:1343-50 9. JAMA 1995;274:1450-5 10. Arterioscler Thromb 1994;14:1751-60 11. Arterioscler Thromb 1994;14:1751-60 12. Am J Clin Nutr 2004;80:668-73 13. J Cardiac Rehab 1983;3:183-90 14. JAMA 1998;280:2001-7