PERSPECTIVES WITH DR ALAN BARCLAY
PURE BUT NOT SO SIMPLE
Most nutrition experts have been recommending that we enjoy traditional healthy eating patterns like the Mediterranean and Okinawan diets for many years now, rather than focusing on single nutrients, ingredients or food groups. After all, we eat foods, not nutrients, and the one-nutrient-at-a-time approach is fraught with unintended consequences as nutrition scientists such as Dr David Katz have enumerated very clearly on numerous occasions. However, the old fat versus carbohydrate debate still seems to attract media attention and the recent publication of the results of the PURE (Prospective Urban Rural Epidemiology) study are another example of hype over serious dietary substance.
The PURE study followed over 135,000 people living in 18 countries (three high-income (Canada, Sweden, and United Arab Emirates), 11 middle-income (Argentina, Brazil, Chile, China, Colombia, Iran, Malaysia, occupied Palestinian territory, Poland, South Africa, and Turkey) and four low-income countries (Bangladesh, India, Pakistan, and Zimbabwe) for over 7 years and found that death rates were highest in those who reported having the highest carbohydrate intakes, and conversely were lower in those with higher fat intakes. “Global dietary guidelines should be reconsidered in light of these findings,” they proclaim.
While the PURE study may sound impressive, like all observational studies, it can only show associations (like the Methodist minister and Cuban rum story). It also has a number of significant limitations, including the fact that the associations were only observed in the extreme levels of consumption (43% and 78% of energy from carbohydrates and 11% and 38% of energy from fats), and that diabetes diagnosis was self-reported (so we don’t know how many people really had diabetes). Many people in the low-income countries may have had diabetes but didn’t know it. This would significantly confound the results. However, one of the most significant limitations is how they estimated people’s food and nutrient intakes.
At the very beginning of the study (seven years in the past), a food frequency questionnaire was used to assess people’s food intakes. That was the only time people were asked what they ate. Food frequency questionnaires ask you to recall all the foods and drinks you consumed over the previous 12 months – a difficult task for most of us at the best of times (what did you eat last week?). These questionnaires also have to be carefully designed to reflect the food preferences of the people being studied – it’s not wise to use a questionnaire designed for one country in a different country, as food preferences and the food supply are usually very different. And finally, food frequency questionnaires need to be validated to see how well they measure actual food and nutrient intakes. There are many different ways of doing this. Overall, it’s highly unlikely that the protein, fat and carbohydrate estimates used in the PURE study are very accurate, which of course has profound implications for the results and their interpretation.
Finally, the study looked at the different kinds of fat (saturated, mono and polyunsaturated) but for some reason was not able to look at carbohydrate quality – not even examining the effect of dietary fibre, let alone refined carbohydrates (both starches and sugars), glycemic index or load. Like fats, all carbohydrates are of course not the same, and it is not very useful to lump them all together.
Despite all these significant limitations, and taking the study’s results at face value, we must consider how relevant they are in comparison to what the average person is eating today. In Australia, for example, our most recent national nutrition survey determined that the average adult consumed 43.5% of energy from total carbohydrate and 30.9% from fat. The nutrient reference values that underpin Australia’s dietary guidelines recommend that Australians consume 45-65% of energy from carbohydrates from carbohydrates and 20-35% of energy from fats. These ranges are very similar to what are recommended in the PURE study – our dietary guidelines therefore do not need updating based on this. We are already eating the minimum amount of carbohydrate and close to the upper end of the recommended range for fat. We therefore need to be eating better quality (minimally refined, high fibre, low GI) carbohydrates, not less, and similarly we need to be eating more poly and mono-unsaturated fat, not more saturated fat.
This is all very academic. We eat foods not nutrients. Most people don’t know what percent of energy they get from protein, fat or carbohydrate. Patterns of eating are much more useful, which is what most modern dietary guidelines focus on: recommending that we eat mostly “good carbs” like fruits, vegetables, legumes, wholegrains, milk and yoghurt and save refined carbohydrates like sugar-sweetened beverages, confectionery, savoury starchy snacks (e.g., chips, crisps), etc for special occasions. Keep it relevant. Keep it simple.
- Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): a prospective cohort study.
Alan Barclay, PhD is a consultant dietitian. He worked for Diabetes Australia (NSW) from 1998–2014 . He is author/co-author of more than 30 scientific publications, and author/co-author of The good Carbs Cookbook (Murdoch Books), Reversing Diabetes (Murdoch Books), The Low GI Diet: Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment, New York).