Can a low carbohydrate diet improve HbA1c levels in prediabetes and type 2 diabetes?

The authors evaluated the impact of a moderate carbohydrate versus a low carbohydrate diet on blood glucose levels in type 2 diabetes and prediabetes.

Although there is agreement that diet is an important strategy for treating type 2 diabetes and prediabetes, there is little consensus about the optimal diet. Current recommendations from the American Diabetes Association suggest that ‘‘for weight loss, either low-carbohydrate, low-fat calorie-restricted, or Mediterranean diets may be effective”. However, the American Diabetes Association suggests the intake of 150 g of carbohydrates a day whereas some studies have suggested that further lowering carbohydrate intake can improve glycemic (blood glucose) control.

The authors of this study examined the impact of two different diets (medium carbohydrate, low fat calorie-restricted, carbohydrate counting diet compared to very low carbohydrate, high fat, non-calorie restricted diet) on diabetes over 3 months by estimation of influence on HbA1c levels (a measurement of average blood glucose levels over the past 3 months).

34 patients were randomly assigned to a moderate carbohydrate, calorie restricted diet (18 patients) or low carbohydrate, ketogenic diet (16). Under the moderate carbohydrate diet, participants were encouraged to derive 45 – 50% of their calorie intake from carbohydrates and were taught to count carbohydrates using 15 g as a unit and took in 165 g daily in total. Fat consumption was lowered and protein consumption was the same as before the beginning of the study. Under the low carbohydrate diet, participants lowered their carbohydrate intake to 20 – 50 g per day (not including fiber) with the goal of achieving nutritional ketosis (the body uses fat as a source of energy instead of glucose, producing higher than usual levels of ketone acids). Fat consumption was increased and protein consumption was the same as before the beginning of the study. All patients met for a 2-hour class weekly throughout the intervention for instruction on their particular diets.

By the end of the 3 month intervention, the moderate carbohydrate group had reduced their energy intake by 792.1 kcal, while the low carbohydrate reduced their energy intake by 696.9 kcal.

The average HbA1c levels did not change in the moderate carbohydrate group, but decreased by 0.6% in the low carbohydrate group. No one in the moderate carbohydrate group achieved a normal HbA1c level of less than or equal to 5.7% compared to 13% of the low carbohydrate group. 56% of the low carbohydrate group saw a clinically significant drop of 0.5% or greater compared to 22% of the moderate carbohydrate group.

44% of the low carbohydrate group discontinued one or more oral diabetes medications compared to 11% of the moderate carbohydrate group. 31% of the low carbohydrate group discontinued sulfonylureas (drugs used to increase insulin release; Glipizide, Glimepiride) compared to 5% in the moderate carbohydrate group.

Those in the low carbohydrate group lost an average of 5.5 kg (5.5% of their body weight) while those in the moderate carbohydrate group lost an average of 2.6 kg (2.8% of their body weight), but this was not deemed to be statistically significant.

Participants within both groups reported significantly reduced carbohydrate and sweet cravings, emotional eating and hunger.

The authors suggest that a very low carbohydrate diet coupled with skills to promote behavior change may improve glycemic control in type 2 diabetes while allowing decreases in diabetes medications.