The Prospective Urban Rural Epidemiology (PURE) study has suggested that a high-carb diet is significantly less healthy than a diet high in fat, with the findings directly challenging current dietary guidelines, placing their validity on shaky ground.
Existing dietary guidelines in the UK recommend eating large amounts of starchy foods, whilst limiting dietary fat. They also advise decreasing dietary fat, particularly saturated fat, with the argument that they are associated with increased LDL cholesterol and cardiovascular disease. The PURE study suggests that this view may be incorrect, as neither total fat nor saturated fat were associated with increased risk of cardiovascular disease. Furthermore, the results indicate that replacing dietary fat with carbohydrate, as current guidelines suggest, could be a fatal mistake and that the opposite substitution may in fact be more favourable.
The study recorded dietary information from over 135,000 individuals, across 18 countries and 5 continents of varying affluence, with a median follow-up period of 7.4 years. This makes it one of the largest comprehensive studies of diet and health to date.
Results of the study
The study recorded total mortality, which is the simplest measure of general health, alongside a range of cardiovascular events such as heart disease and stroke. Total mortality rates were found to be significantly higher in people who ate a diet higher in carbohydrate and/or lower in fat, on average.
The group with the highest proportion of their calories coming from carbohydrate had a 28 per cent higher total mortality than the group with the lowest. Meanwhile, the group with the highest proportion of calories from fat had a 23 per cent lower total mortality than the group with the lowest. Increased intake of each of the three types of fat (saturated, monounsaturated and polyunsaturated) was associated with lower total mortality.
It is important to note that, whilst low-carbohydrate does not always mean high-fat, none of the diet groups in this study were particularly high in protein (the highest group consumption of protein by calories was 19.7 per cent), therefore the main trade-offs were between carbohydrate and fat.
Total fat, saturated and unsaturated fat intakes were not associated with cardiovascular disease, myocardial infarction or mortality. In fact, the group with the highest proportional saturated fat intake had a 21 per cent lower risk of stroke than the group with the lowest.
Cholesterol is poorly understood
Current dietary guidelines are based on a number of presumed causal relationships regarding cholesterol, and much like carbohydrate, it appears to be poorly understood. These relationships are between dietary saturated fat and LDL cholesterol levels, and then between LDL cholesterol and cardiovascular disease.
The authors note that, although dietary carbohydrate is often associated with lower LDL cholesterol, it is also linked with negative increases in other biomarkers that are better predictors of cardiovascular disease, such as triglycerides, total-to-HDL cholesterol ratio and ApoB-to-ApoA1 ratio.
Limitations of the study
The study did not differentiate between simple refined carbohydrates and complex unrefined carbohydrates, which is likely an important factor in the results.
Some of the highest carbohydrate intakes tended to be in countries of lower incomen, such as Bangladesh, where poverty is also likely to play a role in mortality.
The group with the lowest carbohydrate intake still received 46.4 per cent of their calories from carbohydrates, and the group with the highest total fat intake received 35.3 per cent of their calories from fat. Whilst these groups are shown to be the most favourable in the study, none of the groups would be categorised as ‘low-carbohydrate’.
As with all associational nutrition studies, the results do not prove a causal link between macronutrients and the observed effects, they merely display trends in a large group of people. The food frequency questionnaires used to assess diet are also a potential source of error.
Time to rethink low-fat dietary guidelines?
The results of the PURE study are consistent with the findings of randomised controlled trials and meta-analyses of similar observational studies, adding to the case against current dietary guidelines. The authors conclude that “global dietary guidelines should be reconsidered in light of these findings.”