“Low-fat diet could kill you,” . The origin of this surprising message was the publication of the innocuous-sounding Prospective Urban Rural Epidemiology (PURE) study.
Its results ““, said the Daily Mail. Many low-carb, high-fat campaigners likewise saw it as public vindication of their non-mainstream mantra. , said that it was time for “a complete U-turn” in the UK’s approach to official advice on what to eat.
Big claims, but did the results justify them? . Individuals were surveyed using questionnaires, and usually followed up for around seven years, with associations looked for between what they ate and cardiovascular disease and death.
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It is an important study, constituting a huge international undertaking and the first time these associations have been investigated at the same time across high, middle and low-income countries. But in a desire for headline-grabbing novelty, important points were lost.
Weighing the evidence
Did PURE show that a low-fat, high-carb diet is deadly? It did show a significant increase in mortality among those on diets very high in carbohydrates, although interestingly, no associated increase in heart disease. But PURE comes with all the limitations of observational studies, in which cause and effect are often impossible to establish.
With the inclusion of such a diversity of countries, there may also be a few new limitations.
Key among those is that in many of the populations covered, low-fat, high-carbohydrate diets are not a healthy-eating thing or a result of following official dietary guidelines. Instead, the highest carbohydrate intakes, often seen when most calories come from rice – usually white rice – are strongly associated with poverty and food insecurity. Harvard nutritionists call this a , which is also high in salt and low in animal produce and vegetable oils.
In such cases, many other factors, such as micronutrient deficiency, reduced access to healthcare and poor sanitation, are also likely to raise mortality. These are incredibly difficult to adjust for. Increased fat intake usually represents higher meat consumption, and in areas where malnutrition can be a problem, it is hardly surprising that access to such nutrient-rich foods showed an association with reduced mortality.
Health learnings
Highly limited, observational data such as that in the PURE study is important when it comes to learning more about how food affects health – and it will be a useful source of hypotheses and further experimentation. But in isolation, it should not be used to rewrite dietary guidelines, and for the study authors to suggest doing so is overblown.
In reality, the highest average national levels of dietary fat observed in the study were lower than those typical in the US and UK, and not far off government recommendations in those two countries. In this context, interpreting it as a call for us all to increase our consumption of fat is irresponsible, perhaps even dangerous.
For now, what PURE really identifies is that those of us with plentiful access to diverse foods are less likely to die. Although it provides no evidence that we need to increase the amount of fat in our privileged Western diets, perhaps it should be a call to help improve the lives of those that have much less choice.
