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.
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).
