Much publicised, and criticised, studies in the Lancet on association of fats, carbohydrate and vegetable intake with cardiovascular disease and mortality
These two papers in the journal The Lancet report on the initial findings of the Prospective Urban Rural Epidemiology (PURE) study. This large population-based study found that a diet that includes a moderate intake of fat and fruits and vegetables, and in which less than 60% of energy comes from carbohydrates, is associated with lower risk of death. The authors call for a reconsideration of global dietary recommendations in light of their results.
The key findings of the papers were:
- Higher intakes of fats and animal protein were each associated with lower mortality (but note that these intakes were not at levels higher than those consistent with UK dietary recommendations)
- High carbohydrate intake was associated with increased mortality. Note that ‘high’ in this case was at levels above 60% of energy intakes - higher than the standard UK intake although typical of levels consumed by very poor people consuming subsistence diets in low income countries.
- Benefits of fruit, vegetable and legume consumption appear to be at a maximum for both non-cardiovascular mortality and total mortality at three to four servings per day (equivalent to 375–500 g/day).
The study involved 135,000 participants across 18 countries, with a focus on the Middle East, South America, Africa, and South Asia. Dietary intake was only measured once, at the start of participation. Food consumption was assessed with a food-frequency questionnaire (FFQ) at baseline in which people were asked: “during the past year, on average, how often have you consumed the following foods or drinks” for a list of between 95 to 250 foods. Certain health outcomes of the participants were then followed for an average of 7.4 years and these formed the basis for the associations measured.
While most previous large scale nutritional epidemiology studies have been based in high income countries in Europe and North America, in this study around 90% of the participants came from low to middle income countries. The attention from epidemiologists to these populations is laudable. However, a main issue is that results are generated from combining data across the globally poor to the very affluent, who will have very different nutritional deficiencies or surpluses.
The papers and their conclusions have received widespread media attention. Much of the reporting has been criticised by scientists as unhelpfully distorting the findings (see some headlines below). In addition, scientists have found fault with the authors of the PURE papers for drawing conclusions that their own data does not fully suggest.
Criticisms on the methodology of the papers range widely, with comments on flawed study design, data collection and statistical methods.
A response by Susan Jebb, Professor of diet and population health at the University of Oxford:
This paper considers the relationship between diet and health outcomes for predominantly low and middle income countries (15 out of 18 countries studied) where the pattern of disease is very different from that observed in the UK.
It found that a high proportion of carbohydrate in the diet (more than about 60% of energy) was associated with higher death rates. Most of the current debate about diet and health has focused on cardiovascular mortality, but there were no significant associations between carbohydrate intake and major cardiovascular diseases. The apparent excess mortality among those consuming high carbohydrate diets was from non-cardiovascular deaths and is unexplained.
Only 11% of participants are from Europe or North America and the relevance of this data for UK dietary recommendations is limited. The background diet of most of the countries in this analysis is very different from the UK. For example, here only the highest quintile (top 20%) of dietary fat intake reaches the average intake for the UK and the lowest quintile (bottom 20%) of carbohydrate is close to the UK average. There are many other non-diet related factors which contribute to differences in ill-health and the causes of death. It is quite possible that the higher mortality observed in this study in groups consuming a high proportion of energy from carbohydrate and less from fat, reflects differences in socio-economic status that cannot be adequately removed from the statistical analysis of the relationship between diet and health outcomes.
In their press release the authors remark “The best diets will include a balance of carbohydrates and fats – approximately 50-55% carbohydrates and around 35% total fat, including both saturated and unsaturated fats.” This is a thumbs-up for UK recommendations which advise up to 35% energy from fat and an average of 50% energy from carbohydrate (of which only 5% should be sugar).
Selected quotes from the blog by David Katz:
‘Those countries with the lowest intake of dietary fat also had the lowest intake of protein, suggesting these were people with food insecurity, having trouble obtaining adequate food intake, or dietary variety.’
‘The researchers examined the replacement of carbohydrate as percentage of calories, with fat as a percentage of calories, but did not report variation in total calories, or the degree to which very high intake of carbohydrate as a percent of that total correlated with very low calorie intake overall, and malnutrition. Looking across the several papers, it is apparent that correlation is strong.’
‘The findings actually suggest that intake of carbohydrate as a percent of total calories was highest (e.g., a diet of white rice and little else) where there was the most poverty, the least access to medical care, and the greatest risk of dying of trauma, infectious diseases, and so on. […] To be quite clear about it, there was no adjustment for, or even mention of, access to a hospital or medical care in the PURE papers.’
‘On the basis of all of the details in these published papers, the conclusion, and attendant headlines, might have been: “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.”’
‘Saturated fat intake ranged across the countries studied from about 6% of calories to a high of about 11% of calories, again all lower than average levels in the U.S. and much of Europe, and actually very close to recommended levels. Headlines encouraging populations that already eat more saturated fat than this to add even more are not merely unjustified by anything in the study, they are egregiously irresponsible.’
‘What is odd in this case is the publication of an observational study to refute the findings of many intervention trials, including randomized controlled trials. As a rule, observational studies are used to generate hypotheses, and intervention trials- especially RCTs- are used to test those hypotheses. Observational studies come first, and only suggest associations; intervention studies come after to confirm or refute.’
Examples of media headlines:
Forbes: Cut down on carbs and have some fats (and legumes), new studies suggest
Indian Express: Eating low fat, high carbohydrate foods may kill you
Reuters’ Business Insider: Study challenges conventional wisdom on fats, fruits and vegetables
Huffington Post: Low-fat diet could be increasing your risk of early death, major study claims
Telegraph: Low-fat diet could kill you, major study shows
The Week: Stunning new study suggests low-fat diets are seriously increasing people's risk of death
The relationship between macronutrients and cardiovascular disease and mortality is controversial. Most available data are from European and North American populations where nutrition excess is more likely, so their applicability to other populations is unclear.
The Prospective Urban Rural Epidemiology (PURE) study is a large, epidemiological cohort study of individuals aged 35–70 years (enrolled between Jan 1, 2003, and March 31, 2013) in 18 countries with a median follow-up of 7·4 years (IQR 5·3–9·3). Dietary intake of 135 335 individuals was recorded using validated food frequency questionnaires. The primary outcomes were total mortality and major cardiovascular events (fatal cardiovascular disease, non-fatal myocardial infarction, stroke, and heart failure). Secondary outcomes were all myocardial infarctions, stroke, cardiovascular disease mortality, and non-cardiovascular disease mortality. Participants were categorised into quintiles of nutrient intake (carbohydrate, fats, and protein) based on percentage of energy provided by nutrients. We assessed the associations between consumption of carbohydrate, total fat, and each type of fat with cardiovascular disease and total mortality. We calculated hazard ratios (HRs) using a multivariable Cox frailty model with random intercepts to account for centre clustering.
During follow-up, we documented 5796 deaths and 4784 major cardiovascular disease events. Higher carbohydrate intake was associated with an increased risk of total mortality (highest [quintile 5] vs lowest quintile [quintile 1] category, HR 1·28 [95% CI 1·12–1·46], ptrend=0·0001) but not with the risk of cardiovascular disease or cardiovascular disease mortality. Intake of total fat and each type of fat was associated with lower risk of total mortality (quintile 5 vs quintile 1, total fat: HR 0·77 [95% CI 0·67–0·87], ptrend<0·0001; saturated fat, HR 0·86 [0·76–0·99], ptrend=0·0088; monounsaturated fat: HR 0·81 [0·71–0·92], ptrend<0·0001; and polyunsaturated fat: HR 0·80 [0·71–0·89], ptrend<0·0001). Higher saturated fat intake was associated with lower risk of stroke (quintile 5 vs quintile 1, HR 0·79 [95% CI 0·64–0·98], ptrend=0·0498). Total fat and saturated and unsaturated fats were not significantly associated with risk of myocardial infarction or cardiovascular disease mortality.
High carbohydrate intake was associated with higher risk of total mortality, whereas total fat and individual types of fat were related to lower total mortality. Total fat and types of fat were not associated with cardiovascular disease, myocardial infarction, or cardiovascular disease mortality, whereas saturated fat had an inverse association with stroke. Global dietary guidelines should be reconsidered in light of these findings.
Dehghan, M., Mente, A., Zhang, X., Swaminathan, S., Li, W., Mohan, V., ... & Amma, L. I. (2017). Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): a prospective cohort study. The Lancet.
Miller, V., Mente, A., Dehghan, M., Rangarajan, S., Zhang, X., Swaminathan, S., ... & Bangdiwala, S. I. (2017). Fruit, vegetable, and legume intake, and cardiovascular disease and deaths in 18 countries (PURE): a prospective cohort study. The Lancet.
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While some of the food system challenges facing humanity are local, in an interconnected world, adopting a global perspective is essential. Many environmental issues, such as climate change, need supranational commitments and action to be addressed effectively. Due to ever increasing global trade flows, prices of commodities are connected through space; a drought in Romania may thus increase the price of wheat in Zimbabwe.