Re: Low carbohydrate-high protein diet and incidence of cardiovascular diseases in Swedish women: prospective cohort study
The conclusions of the paper are not supported by the data. The evidence does not demonstrate that low carbohydrate diets increase the risk of cardiovascular disease.
The fundamental problem with this research, as with most diet studies, is that the food intakes of subjects were not adequately measured (1) . Indeed, it suffers a double problem. First, it used an inaccurate measuring instrument at baseline. Second, it conducted no measure of intake at all over the following 15+ years.
The initial instrument was a food frequency questionnaire (FFQ), popular in large studies because results are easily processed. But FFQs are just another form of “self-report”. They rely on subjects honestly telling researchers what they eat.
But they do not. Subjects consistently report a healthier diet than they actually eat, smaller in volume and a more nutritious mix. Technically, this is known as “under-reporting”. In plain English, it is lying.
By whatever name, the problem is well-known, long established and large. In Britain, the accuracy of national diet surveys is checked by using “double-labelled water”, a biochemical measure independent of subjects’ self-reports. Among adults, average under-reporting of calorie intakes is 25% (2). No such independent measure was employed in the study reported here.
Instead, the authors claim they used a “validated” FFQ. But such “validations” only compare FFQs with other forms of self-report, vulnerable to the same problem of misrepresentation. The only thing “validated” is the errors of both.
Subjects’ intakes were never measured again, by any instrument. Nonetheless, the authors imply that their initial diets were maintained for a decade and a half, concluding that low carbohydrate diets are risky if used “on a regular basis”. This is crucial to their argument because they dismiss supportive evidence for such diets as only coming from “short-term” studies.
The authors acknowledge the difficulty, but try to defend themselves by claiming that lack of repeat intake measures is common in large cohort studies. They are certainly right about that.
Despite their limited intake data, the authors perform numerous statistical analyses to establish relationships with various forms of cardiovascular disease. None of these relationships are credible.
Like many before them, they conduct sophisticated secondary analyses of bad primary data. This is a statistical smokescreen that conceals the fundamental problem.
And the problem truly is fundamental. Like many others, they seek to show that bad diets cause diseases, without measuring diets accurately. This a a fundamental flaw in scientific method: trying to establish a causal relationship without measuring the independent variable properly.
In sum, this study does not allow us to conclude anything about low carbohydrate diets, for or against. It is just another partisan shot in the long-running ideological war between the proponents of low fat and low carbohydrate diets.
(1) Winkler J, The Fundamental Flaw in Obesity Research, Obesity Reviews, 2005, 6, 199-202. jtw@blueyonder.co.uk
(2) Rennie K, Coward A, Jebb S, Estimating under-reporting of energy intakes in dietary surveys using an individualised method, BJN, 2007, 97, 1169-1176.
Rapid Response:
Re: Low carbohydrate-high protein diet and incidence of cardiovascular diseases in Swedish women: prospective cohort study
The conclusions of the paper are not supported by the data. The evidence does not demonstrate that low carbohydrate diets increase the risk of cardiovascular disease.
The fundamental problem with this research, as with most diet studies, is that the food intakes of subjects were not adequately measured (1) . Indeed, it suffers a double problem. First, it used an inaccurate measuring instrument at baseline. Second, it conducted no measure of intake at all over the following 15+ years.
The initial instrument was a food frequency questionnaire (FFQ), popular in large studies because results are easily processed. But FFQs are just another form of “self-report”. They rely on subjects honestly telling researchers what they eat.
But they do not. Subjects consistently report a healthier diet than they actually eat, smaller in volume and a more nutritious mix. Technically, this is known as “under-reporting”. In plain English, it is lying.
By whatever name, the problem is well-known, long established and large. In Britain, the accuracy of national diet surveys is checked by using “double-labelled water”, a biochemical measure independent of subjects’ self-reports. Among adults, average under-reporting of calorie intakes is 25% (2). No such independent measure was employed in the study reported here.
Instead, the authors claim they used a “validated” FFQ. But such “validations” only compare FFQs with other forms of self-report, vulnerable to the same problem of misrepresentation. The only thing “validated” is the errors of both.
Subjects’ intakes were never measured again, by any instrument. Nonetheless, the authors imply that their initial diets were maintained for a decade and a half, concluding that low carbohydrate diets are risky if used “on a regular basis”. This is crucial to their argument because they dismiss supportive evidence for such diets as only coming from “short-term” studies.
The authors acknowledge the difficulty, but try to defend themselves by claiming that lack of repeat intake measures is common in large cohort studies. They are certainly right about that.
Despite their limited intake data, the authors perform numerous statistical analyses to establish relationships with various forms of cardiovascular disease. None of these relationships are credible.
Like many before them, they conduct sophisticated secondary analyses of bad primary data. This is a statistical smokescreen that conceals the fundamental problem.
And the problem truly is fundamental. Like many others, they seek to show that bad diets cause diseases, without measuring diets accurately. This a a fundamental flaw in scientific method: trying to establish a causal relationship without measuring the independent variable properly.
In sum, this study does not allow us to conclude anything about low carbohydrate diets, for or against. It is just another partisan shot in the long-running ideological war between the proponents of low fat and low carbohydrate diets.
(1) Winkler J, The Fundamental Flaw in Obesity Research, Obesity Reviews, 2005, 6, 199-202. jtw@blueyonder.co.uk
(2) Rennie K, Coward A, Jebb S, Estimating under-reporting of energy intakes in dietary surveys using an individualised method, BJN, 2007, 97, 1169-1176.
Competing interests: No competing interests