Our study did not confirm the finding that weight reduction is more efficient in individuals following an LCD than in those following an LFD, as has been found in some previous trials [6, 9, 12, 18–20]. An important difference in our study compared with these earlier studies [6, 9, 12, 18–20] was that we used comparatively fewer resources to achieve compliance. In our study, only four group meetings were offered during the first 12 months of the trial. The rationale for this design was to make the results more applicable to regular clinic care in which educational activities such as group meetings can be offered as a means to improve glycaemic control. No patients were lost to follow-up and data on glycaemic control were complete at 24 months, while data on weight were lacking for only one participant at this time point. This outcome of our study left minimal room for the selection of participants, who did indeed find either of the diets suitable, to affect the main outcomes. Our findings indicate that if patients are randomised to an LCD compared with an LFD with resources used to achieve changes in diet composition that are readily available for many providers of care, both diets induce similar weight reductions. This was also in line with the finding that both groups reported similar energy intake during the study. Westman et al have reported more efficient weight reduction with an LCD after 6 months when compared with an LFD . In that study, diet information was facilitated, compared with our design, by lack of energy restriction in the low-carbohydrate group. This could have affected the more beneficial findings compared with our study regarding weight loss. Also, Westman et al had a total of 18 group meetings during their 6 month study, and this could have affected the outcome. Information on increased exercise was also part of their lifestyle change programme, but was not included in ours. Specifically, we aimed to study the effects of macronutrient composition on glycaemic control and cardiovascular risk factors, which was why we aimed to achieve no differences in energy intake in the information we gave to the participants. Interestingly, we did find an increase in HDL-cholesterol after 6 months and a specific reduction in HbA1c levels in the low-carbohydrate group only, which suggests that these effects are dependent on macronutrient composition per se; this is in line with the findings of Westman et al .
We also acknowledge that we might have achieved better weight reduction if a design similar to that used in ‘Weight Watchers’ programmes had been incorporated. For regular care provided by the Swedish tax-based system, incentives used in Weight Watchers, such as public display of the body weight results of the participants, would not be feasible for general use in clinic care because of patient privacy. Also, one should keep in mind that there is selection and incentive in such commercially run programmes, as participants are willing to pay to participate. However, effective weight loss in a Weight Watchers group was recently shown in a study even when the cost for participation was reimbursed by the study organisers .
Although patients in our study who had been randomised to the low-carbohydrate group reported a lower intake of carbohydrates at baseline compared with the low-fat group, this was unrelated to weight changes in statistical analyses. In retrospect, this group difference in reported intake of macronutrients between the groups might have been a consequence of the participants being informed of the randomisation results before the diet record at baseline was performed. Consequently, some participants may have adjusted their diet to make it similar to that to which they had been allocated, ahead of the first group information meeting. Unfortunately the baseline difference was not elucidated until the end of the trial and it was thus judged to be of little meaning to ask participants with little intake of energy from carbohydrates in the low-carbohydrate group at baseline whether this was a consequence of the randomisation, which had occurred more than 2 years earlier.
The largest changes in macronutrient intake were seen in patients randomised to the low-carbohydrate group. Indeed, patients in the low-fat group had the same macronutrient composition at baseline as during the study, suggesting that this was indeed a traditional diet and that they, according to the diet records, had been given similar diet recommendations earlier.
The patients following the LCD increased the percentage of energy intake from both total and saturated fat throughout the 24 months of the trial according to diet records, in line with the study protocol. At 6 months, when weight reduction was most pronounced, only the LCD group had changes in blood lipid levels in the form of increased HDL-cholesterol. However, during the study there had also been changes in lipid-lowering therapy that make these findings inconclusive regarding whether they solely depended on changes in diet. At the end of the trial, several patients had been newly started on lipid-lowering therapy. This is an obvious limitation of our trial from a mechanistic point of view, but it was a consequence of our efforts to limit the resources necessary for the diets to be implemented in regular primary care, to allow the methods to translate easily to real-life application. However, as 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors (statins) mainly affect LDL-cholesterol levels and, as earlier trials have also found that diets high in fat elevate HDL-cholesterol to a greater extent than high-carbohydrate diets in type 2 diabetes [9, 11, 16], we find it likely that the increase in HDL-cholesterol in our trial was mainly an effect of the change in diet.
We acknowledge that the general applicability of our study results might be limited because of the high participation rate that was achieved. The study nurses had also taken care of the same patients ahead of the study start and when identifying potential participants according to inclusion and exclusion criteria it cannot be excluded that, prior to the study, they might have discharged patients judged not to have been suitable participants for various reasons. Another potential explanation for the high participation rate was that the study protocol was not very time-consuming for the patients as it involved only four group meetings. We also acknowledge the problems with diet records. Although diet records with notebooks and scales can be more detailed and precise than standardised food frequency questionnaires, results from surveys of food intake have low reproducibility and, in particular, there are systematic errors in underreporting energy intake . Thus, total energy intake might not be accurate in our study, but the lowering of HbA1c in only the low-carbohydrate group at 6 months and also differences in HDL-cholesterol changes at similar weight reductions suggest that the groups did indeed change their macronutrient intake differently in our trial.
The analyses of outcome in the participants who were compliant with either energy intake or with the E% from fat implied better long-term effects on weight loss than in the total cohort analysed on an intention-to-treat basis. Although this was a post-hoc analysis, and thus data should be interpreted with caution, it was of interest to note that HDL-cholesterol increased by 33% in patients reasonably compliant with fat intake, which was in line with data from Westman et al . However, blood pressure levels were not reduced in patients on the LCD at 24 months. It cannot be excluded that salt intake increased in parallel with ingestion of fat, as has been demonstrated in the general population , leading to less favourable blood pressure levels. Unfortunately, we did not collect urine for determination of the amount of sodium.
In conclusion, our findings support the use of an LCD with 20 E% from carbohydrates as an alternative to a traditional low-fat diet, if the aim primarily is to improve glycaemic control in type 2 diabetes. We achieved a weight loss of about 4 kg in both groups after 6 months based on group information on three occasions and there was only one more group meeting, which took place at 12 months’ study duration. However, as in many earlier studies, compliance with the LCD was reduced after 6 months, as judged by the increase in body weight and according to food records, and it cannot be ruled out that different results could have been obtained if more effort had been made to achieve compliance with the diet composition and reduction of energy intake.