Re: Low carbohydrate-high protein diet and incidence of cardiovascular diseases in Swedish women: prospective cohort study
This paper has attracted much interest, but there are several reasons to be cautious in considering its conclusions.
The "opt-in" enrolment is a methodological weakness, and, although the study had more than 40 000 subjects, it also had a less than 50% recruitment rate, which creates the potential for bias. The authors make no comment on this matter.
Dietary habits and the presence of cardiovascular risk-factors were self-reported via a questionnaire at enrolment but there are no follow up data to look at changing trends for any of these indices, despite the mean follow-up being 15.7 years. This represents a significant shortcoming and, again, the authors do not address this issue.
The study does seem to confirm the well-established risks of hypertension, obesity, and smoking in terms of subsequent cardiovascular disease, with the relative risks between 2.48 and 2.73 for the presence of these conditions at enrolment.
However, it is less clear that the 5% increase in risk (RR 1.02-1.08) found in those on the lowest carbohydrate and highest protein diets can be considered a real finding, given the lack of longitudinal data on the more significant risk factors, and especially as the cases were defined by the occurrence of a cardiovascular event, and then mapped to the dietary habits. It would be interesting to know the current/exit blood pressure, BMI, and smoking status of these cases.
These women (the cases) developed premature cardiovascular disease, and it is suggested that dietary habits conferred a small excess risk. That is possible, but it is certainly true that the cases must have had significant predisposition to cardiovascular disease - one wonders what would have happened to them if they had had different dietary habits, and if they subsequently became obese and developed the metabolic syndrome?
The study design makes it impossible to answer this key question. Randomised controlled trials, of adequate power, with appropriate analysis of risk factors are necessary, but are lacking at this stage.
Competing interests: No competing interests