Our results indicate that after 1 year, overweight patients with type 2 diabetes had similar weight reduction with both a low-carbohydrate and a low-fat diet. Similar to prior studies, we observed a more rapid weight loss after initiation of a low-carbohydrate diet and equivalent weight loss after 1 year (2
). Despite modest weight reduction in both arms, there was no significant reduction in A1C. Participants in the low-carbohydrate arm achieved an initial mean reduction of A1C 0.6% in the first 3 months, but this was not sustained. Prior studies demonstrated that a 6-kg weight loss was associated with A1C reduction of 0.55% (22
). Study participants achieved a 3-kg weight loss, which may not have been sufficient to affect A1C. The lack of change in A1C should also be taken in the context of reduced medications. One-third of all participants were taking thiazolidinedione medications before randomization, which were discontinued during prerandomization and were not restarted. In addition, there was an overall reduction in insulin and sulfonylurea dose. Perhaps we would have observed greater reductions in A1C if we did not make medication adjustments during the study; however, because we were concerned about hypoglycemia, not making adjustments would not have been appropriate.
The initial difference in weight loss between arms was similar to that observed previously (2
). Although there is debate regarding the effects of macronutrient composition on weight loss, it would appear from our results that participants in the low-carbohydrate arm reduced their caloric intake to a greater extent than participants in the low-fat arm. In addition, low-carbohydrate arm participants had a greater reduction in insulin dose and because of the potential effect of insulin on weight gain, this reduction may have promoted a greater weight loss.
Dietary adherence is a key factor in achievement of weight loss with any diet (5
). Our diverse patient population, 80% of whom were black or Hispanic, had high carbohydrate and fat intake at baseline. Culturally, diets of Hispanics may have higher amounts of carbohydrate intake than those of the general U.S. population (23
) and following a low-carbohydrate diet may have posed an even greater challenge for this population.
We did not observe any change in blood pressure at 1 year but did observe an increase in HDL in participants in the low-carbohydrate arm, which is consistent with prior studies (2
). Participants in the low-carbohydrate arm increased their total and monounsaturated fat intake, which may have contributed to this increase. In contrast with previous studies, we did not observe significant reductions in triglycerides, which may be due to low triglyceride levels at baseline.
We did not have outcomes of A1C and weight for 19% of participants at 12 months. This attrition rate is lower than that found in many dietary interventions (13
). We analyzed the data, carrying forward baseline values for these missing outcomes, assuming that any weight lost during the study was regained. This analysis did not change our results. However, if participants withdrew because they gained weight beyond their baseline weight, then our weight results would be less favorable.
Several limitations should be considered in interpreting our findings. Despite randomization, participants in the low-fat arm were heavier at baseline than those in the low-carbohydrate arm. Although we controlled for this imbalance statistically, it raises the question of whether there were other unmeasured differences between the arms. We used single-day dietary recall or a single-day food record to assess dietary intake, either of which is subject to bias. Participants may not have fully recalled their dietary intake and, in addition, may have changed their dietary intake for the day before their scheduled appointment. We did not have objective measures of physical activity, which could be a confounder; however, given the similarity of our findings in both groups at 1 year, it is unlikely that there were significant changes in physical activity in either group.
In conclusion, our study demonstrates that among overweight patients with type 2 diabetes, there was no significant difference in the weight or A1C change in participants after a low-carbohydrate compared with a low-fat diet for 12 months. Participants in both arms achieved an average 3.4% weight reduction but did not reduce A1C. Differences in the short-term effects of each diet were not sustained.