Aims/hypothesisThe study aimed to compare the effects of a 2 year intervention with a low-fat diet (LFD) or a low-carbohydrate diet (LCD), based on four group meetings to achieve compliance.MethodsThis was a prospective randomised parallel trial involving 61 adults with type 2 diabetes consecutively recruited in primary care and randomised by drawing ballots. Patients that did not speak Swedish could not be recruited. The primary outcomes in this non-blinded study were weight and HbA1c. Patients on the LFD aimed for 55–60 energy per cent (E%) and those on LCD for 20 E% from carbohydrate.ResultsThe mean BMI and HbA1c of the participants were 32.7 ± 5.4 kg/m2 and 57.0 ± 9.2 mmol/mol, respectively. No patients were lost to follow-up. Weight loss did not differ between groups and was maximal at 6 months: LFD −3.99 ± 4.1 kg (n = 31); LCD −4.31 ± 3.6 kg (n = 30); p < 0.001 within groups. At 24 months, patients on the LFD had lost −2.97 ± 4.9 kg and those on LCD −2.34 ± 5.1 kg compared with baseline (p = 0.002 and p = 0.020 within groups, respectively). HbA1c fell in the LCD group only (LCD at 6 months −4.8 ± 8.3 mmol/mol, p = 0.004, at 12 months −2.2 ± 7.7 mmol/mol, p = 0.12; LFD at 6 months −0.9 ± 8.8 mmol/mol, p = 0.56). At 6 months, HDL-cholesterol had increased with the LCD (from 1.13 ± 0.33 mmol/l to 1.25 ± 0.47 mmol/l, p = 0.018) while LDL-cholesterol did not differ between groups. Insulin doses were reduced in the LCD group (0 months, LCD 42 ± 65 E, LFD 39 ± 51 E; 6 months, LCD 30 ± 47 E, LFD 38 ± 48 E; p = 0.046 for between-group change).Conclusions/interpretationWeight changes did not differ between the diet groups, while insulin doses were reduced significantly more with the LCD at 6 months, when compliance was good. Thus, aiming for 20% of energy intake from carbohydrates is safe with respect to cardiovascular risk compared with the traditional LFD and this approach could constitute a treatment alternative.Trial registration:ClinicalTrials.gov NCT01005498Funding:University Hospital of Linköping Research Funds, Linköping University, the County Council of Östergötland, and the Diabetes Research Centre of Linköping UniversityElectronic supplementary materialThe online version of this article (doi:10.1007/s00125-012-2567-4) contains peer-reviewed but unedited supplementary material, which is available to authorised users.

Weight-changes did not differ between the diet groups while improvements in HRQoL only occurred after one year during treatment with LCD. No changes of HRQoL occurred in the LFD group in spite of a similar reduction in body weight.