A Canadian study last year found no overall effect on type 2 diabetes control by using a low-glycemic-index diet and lower-carbohydrate diet, although the low-glycemic-index diet did reduce post-meal glucose levels and C-reactive protein.
For many years, a high-fat, low-carbohydrate diet was recommended for type 2 diabetics. Then in 1979 the American Diabetes Association recommended a high-carb, low-fat diet. Later, the ADA allowed more fat, mostly monounsaturated.
The experts are still debating how much and what kind of carbohydrate people with diabetes should eat. Recent years have seen a trend towards lower carbohydrate intake and lower-glycemic-index eating. Much of the supportive evidence we have is based on short-term studies – six to 12 weeks.
A Cochrane review in 2004 concluded that there was no high-quality data on the effectiveness of dietary treatment of diabetes.
The authors of the Canadian study at hand wrote:
Although almost everyone would agree that diet is the cornerstone of diabetes therapy, there is marked disagreement about what kind of dietary advice is best, particularly with respect to dietary carbohydrate.
We can put a man on the moon, but still aren’t sure what’s the best diet for people with diabetes despite years of experience and experimentation.
The Canadian researchers aimed to compare the effects of altered glycemic index and amount of carbohydrate on hemoglobin A1c, blood glucose, lipids, and C-reactive protein in men and women with type 2 diabetes.
162 subjects with mild diabetes, 35-75 years old, managed by diet alone, were randomly assigned to one of three diet groups:
- high-carb, high-glycemic-index (“high-GI“): 47% of calories from carb, 31% of cals from fat, glycemic index 63
- high-carb, low-glycemic-index (“low-GI“): 52% of cals from carb, 27% of cals from fat, glycemic index 55
- low-carb, high-monounsaturated fat (“low-CHO“): 39% of cals from carb, 40% of cals from fat, glycemic index 59
Average body mass index was 31 (mildly obese); average weight 83 kg (183 lb). The study lasted one year, a major strength of the study.
Results One Year Later
Hemoglobin A1c rose from 6.1% to 6.3%, with no difference between the various diet groups. There were no differences in insulin levels, whether fasting or two hours after an oral glucose tolerance test. Blood sugar levels after a glucose tolerance test were 7% lower with the low-GI diet compared to the other diet groups. No difference in LDL cholesterol levels. Little effect on triglycerides and HDL cholesterol. No differences in weight. C-reactive protein in the high-GI group fell from3.34 mg/L to 2.75. C-reactive protein in the low-GI group fell from 2.64 to 1.95. [All these C-reactive protein readings are in the normal range.]
Nearly all the people with diabetes I encounter are very different from this study cohort: they are on drug therapy for diabetes. So the results here don’t necessarily apply to the more typical cases of moderate or severe diabetes that require one or more glucose-control drugs.
Low-carb diet advocates can justifiably argue that the carb intake was still too high, and that’s why their numbers weren’t better. Vernon and Eberstein in their book, Atkins Diabetes Revolution, note that many people with type 2 diabetes will have to limit carboydrates (“net carbs”) to 40-60 grams a day. In the study at hand, the low-carb diet aimed for 39% of calories from carbohydrates. On a 2000-calorie diet, that’s 195 grams – a far cry from 60 grams.
Low-Gi advocates also can justifiably argue that the glycemic index was not low enough to make a difference. The researchers admit that the test diet reductions in carb intake and glycemic index were “modest.” Perhaps they thought that more drastic reductions were unsustainable.
Attempts to control diabetes with low-carb or low-glycemic-index eating should make more dramatic changes.
The low-glycemic-index diet lowered two-hour glucose levels on the glucose tolerance tests. The authors state that this parameter is a better indicator of heart disease risk – lower in this case – than are fasting glucose levels. Findings suggests improvements in insulin resistance and/or pancreas beta cell function. This finding may have no real-world clinical significance: remember that hemoglobin A1c levels were the same across all groups.
The changes in C-reactive protein just don’t seem clinically significant to me (nor to an editorialist in the same journal issue).
The aforementioned editorialist, Dr. Xavier Pi-Sunyer, had an interesting comment:
This finding suggests that we must be careful about disrupting subjects’ or patients’ diets with radical , doctrinaire changes that may actually be counterproductive. Furthermore, the diets had carbohydrate contents that varied from 39% to 52% of energy intake, and yet this variability had no effect on the subjects’ HbA1c. This finding confirms previous reports that the proportion of carbohydrate in the diet is not very important in determining the concentration of fasting blood glucose and that variations of 10% to 15% of total calories make little difference to overall control in patients with early type 2 diabetes.
I would emphasize “. . . in patients with early type 2 diabetes.”
A Mediterranean-style diet, then, could be just as effective as, if not better than, all the other “diabetic diets” out there.
Steve Parker, M.D.
References: Wolever, Thomas, et al. The Canadian Trial of Carbohydrates in Diabetes (CCD), a 1-y controlled trial of low-glycemic-index dietary carbohydrate in type 2 diabetes: no effect on glycated hemoglobin but reduction in C-reactive protein. American Journal of Clinical Nutrition, 87 (2008); 114-125.
Additional Resource: Michael R. Eades, M.D. Making worthless data confess. The Blog of Michael R. Eades, December 13, 2008. Accessed July 10, 2009. [Highly critical analysis from a leading low-carb, high-protein advocate.]