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Veronique Gingras, MSc, PhD

In August and September 2017, three nutrition studies derived from the PURE study were published in the Lancet and an old, endless – and somewhat tiresome – debate resurfaced: is a low-carbohydrate or a low-fat diet better for your health? These publications received tremendous attention with headlines such as: Study challenges conventional wisdom on fats, fruits and vegetables or PURE shakes up nutritional field: finds high fat intake beneficial. However, many of the headlines either exaggerated or misrepresented the findings. The media coverage that followed highlights how important it is to be cautious when we interpret and present results.
What is the PURE (Prospective Urban Rural Epidemiology) study? Why did it draw so much attention? PURE is a very large epidemiological study (more than 135,000 participants) including men and women 35 to 75 years old from 18 countries across five continents. One of the main findings, which probably received the most media coverage, is that a high carbohydrate intake was associated with high total mortality, and a high fat diet was associated with low total mortality (The Lancet, 2017).

However, according to David L. Katz in his critical analysis of the PURE nutritional results, one conclusion should have emerged instead: “very poor people with barely anything to eat get sick and die more often than affluent people with access to both ample diets and hospitals.” Studying the relationship between diet and health outcomes in developing countries (the main objective of the PURE study) was a key aspect of the study; yet none of the media coverage focused on this aspect.   

I will not do an in-depth analysis of why some of the PURE study conclusions or their presentation in the media was flawed. Several authors have done so already (e.g. Science Media Centre, The Nutrition Source, Thinking Nutrition). Yet, here are some examples of the key arguments:

  • The nutritional data was collected only once, when participants were enrolled in the study, and associated with outcomes approximately 7 years later, without accounting for any change in diet over time.
  • The impact of a high carbohydrate or a high fat diet on total mortality was mostly related to non-cardiovascular diseases (including, for example, infectious disease), which cannot be explained by nutrition alone.
  • All carbohydrates were pooled together, independent of the source; no distinction was made as to whether the carbohydrates came from fruits, pastries, sugar-sweetened beverages or whole-grains.

The sensationalist way results were presented was troubling. It is not the first time that nutritional findings are presented as “shocking news” and “revolutionary”, and this is often misleading. Research in nutrition is notoriously difficult and the reason is simple; eating is complex. We eat food, not nutrients. We eat meals, not single food items. And, these vary daily, weekly and seasonally. How can we expect to single out one nutrient or one food item as being the cause for cardiovascular disease or death?

Studies usually come with sections discussing their strengths and limitations, which should be considered when interpreting their data. Readers should remain cautious when the results of a single study are presented as revolutionizing nutrition knowledge. Many uncertainties remain in the field of nutrition research; the PURE study will likely bring new insights, but is unlikely to lead to a major change in nutrition recommendations.  As Yoni Freedhoff said about science reporting, “I'd recommend you ignore any article that suggests a single food confers either miraculous or devastating health effects”.


 


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