For over 40 years now we have been urged, by various bodies including the American Heart association, British Heart Foundation and various nutritional bodies to eat a low-fat diet, to aim for no more than 30 per cent calories from fat and less than 10 per cent from saturated fat and to eat plenty of ‘complex’ carbohydrates.
The increases in obesity, diabetes and heart disease seen in the thirty years since those guidelines were formulated would suggest that something, somewhere, was amiss and the fact that the guidelines themselves weren’t actually based on scientific evidence is somewhat concerning. Opponents of the low-fat paradigm hold that the true culprit is excess refined carbohydrates which, in combination with unhealthy fats, is the true cause of the above conditions. Both sides are less than complimentary about the other.
A new study has generated some interesting results. The large prospective cohort study (meaning patients were followed in real time) involved 135,000 patients who were followed up from recruitment for 7.4 years. Food intake was assessed by validated questionnaire. Outcomes of interest included total mortality, cardiovascular events including death, myocardial infarction (heart attack) and stroke.
Overall results are striking; higher carbohydrate intake (more than 60 per cent proportion of total calories) was associated with an increased risk of death from all causes (total mortality) of 28 per cent. Interestingly, the majority of these carbohydrates were refined carbohydrates, not sources such as fruit or vegetables. It’s worth remembering that higher amounts of refined carbohydrates in the diet have been implicated in both cardiovascular disease and other diseases including cancer.
A higher intake of saturated fat (e.g. butter) was associated with a 14 per cent decrease in death from all causes, monounsaturated fat (e.g. olive oil) a 19 per cent decrease and polyunsaturated fat (e.g. walnuts ) a 20 per cent decrease. Total fat intake was not associated with risk of myocardial infarction or cardiovascular disease mortality, and higher levels of saturated fat intake were associated with lower risks of stroke.
All studies of this type of course are sufficient to establish correlation, but not causation. Some will argue that as it was not a randomised control trial (gold standard to test the effect of interventions), the paper is insufficient for any meaningful recommendation to be made. The reality is that this kind of study is the best we can ever hope for. Nobody is willing to commit to the vast expense that a randomised control trial would require.
Despite the protestations of advocates of the traditional paradigm, there appears to be a consistent lack of association between lower fat consumption in diet and better health outcomes, in multiple studies of variable design but a large number of participants, like the Nurses Health Study and Health Professionals follow up study. Other high quality evidence supports this contention.
Furthermore there appears to be substantial benefit when increasing the intake of healthy fats that is not just limited to cardiovascular health.
Where does this leave us? It all depends. The same authorities who declare a single well-designed trial which generates results that fit with their conception of the problem sufficient to change their advice will also declare that a large multi-centre, multi country study is ‘interesting but more research is required’.
I would advise anyone who wants to improve their health, and is happy to leave the disagreements to those with a stake in the controversy, to follow the Mediterranean diet. It is well-researched, has withstood the scrutiny of randomised-control trials, proved its worth in the management of cardiovascular disease, obesity and as a possible adjunct to cancer prevention, and is very easy to follow.