PURE Study: Killer Carbs or Poor Living Conditions?

Have you heard about the “dangers” of carbohydrates and the “benefits” of fat and protein? A new study may point to these misleading conclusions. We analyzed the findings and found the data tell a different story.

What is the PURE Study?

The controversy stems mostly from a new paper published about the Prospective Urban Rural Epidemiological (PURE) study.¹ In this study, investigators looked at the relationship between nutrition and disease in 18 countries, focusing on understudied regions. More than 135,000 participants were enrolled between 2003-2013, with a median follow-up of 7.4 years.

The Controversial Findings

Investigators found that people whose diets were highest in carbohydrate (about 74-81 percent of energy intake) had a 28 percent greater risk of dying, mainly from noncardiovascular causes, compared to those who ate the least carbohydrate (about 43-49 percent of energy intake).

They also found that those eating the most fat (35 percent of energy) and saturated fat (13 percent of energy) had a lower risk of dying, again mainly from noncardiovascular causes. Based on these findings, the researchers called for the “reconsideration” of global dietary guidelines. Specifically, they suggested lowering carbohydrate intake to 50-55 percent calories, increasing fat to 35 percent of calories, and removing limits on saturated fat.

Problems with the PURE Study

Unfortunately, the PURE study suffered from methodological issues that rendered these sweeping conclusions meaningless.

High carbohydrate intake was a red flag for poverty. Unfortunately, the authors missed it. Even though they admitted that “most participants from low- and middle-income countries consumed a very high carbohydrate diet (at least 60 percent of energy), especially from refined sources…,” they did not make the connection.

Poor living conditions kill. Poverty is linked to not only low food access, but poor living conditions. These include smoke exposure from poorly ventilated stoves and lamps (used by one-third of the world’s population), contaminated water and air, and a lack of both preventive and therapeutic medical care.^2,3 All of these greatly increase one’s risk of dying. To illustrate the massive impact of poverty, a study of more than 217,000 people in India found that individuals with the fewest assets had a 294 percent higher risk of dying than those with the most assets.⁴

…And researchers did not fully control for living conditions. Investigators did try to account for poverty via education and even household income. However, statistical tools couldn’t capture the vastly different living conditions between groups. To illustrate: An educated person living in a polluted, poor city may be in greater danger than a less-educated person in a clean city with guaranteed medical access.

They mistook “wealth” foods for health foods. In many places around the world, only the wealthy can afford foods high in fat and saturated fat, like animal products. As a result, these nutrients can act as a marker for lifesaving affluence. Indeed, researchers found that people in the wealthiest regions—Europe, North America, and the Middle East—consumed more total fat and saturated fat than people in lower-income areas. Yet investigators didn’t adequately control for this. Instead, they concluded high-fat foods must be good for you, while in fact being well-off is good for you.

Researchers did not distinguish between healthy and refined carbohydrates. Despite breaking fat down into specific types, PURE study investigators considered all carbohydrates equal, from soda to sweet potatoes. Yet decades of science tells us this simply isn’t true—the body handles lentils differently than lollipops.

Researchers did not report added fats used in recipes. Researchers listed top sources of carbohydrates, protein, and fat for each country in published supplementary material. In several poor countries, the main source of carbohydrate consumed was also a top source of protein and fat, for example white rice in Bangladesh. As these sources of refined carbs are very low in fat, this implies that fats have been added in the preparation of these foods. That makes it very difficult to separate the impact of fat versus carbohydrate on health outcomes.

Other findings from the same study contradicted their conclusions. Perhaps unsurprisingly, the PURE study also showed that eating healthful carbohydrate-rich foods—like fruits, vegetables, and legumes—was linked to a lower risk of death.⁵ This further implies that high carbohydrate (especially refined carbohydrate) intake was a sign of poverty and nothing more.

A one-time survey was used to measure what people ate…for the next seven years. Diets are rapidly changing around the world. It seems unlikely that a single questionnaire accurately captured what people ate, year after year. Yet that’s the only way researchers measured people’s diets.

The Correct Conclusion

Poor living conditions kill, not carbohydrates. As always, many studies show that vegan diets—which are high in complex carbohydrate and typically low in fat—are linked to a lower risk of many of our top killers, like heart disease and diabetes.⁶

The key to improving global health lies not in eating more fat and protein, but in alleviating poverty, ensuring access to medical care, and eating healthful carbohydrates like fruits, vegetables, whole grains, and legumes. All of these help decrease the risk of dying before our time, regardless of the cause.^5-7

References

1. Dehghan M, Mente A, Zhang X, et al. Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): a prospective cohort study. Lancet. 2017;pii: S0140-6736(17)32252-3.

2. Wagstaff A. Poverty and health sector inequalities. Bull World Health Organ. 2002;80:97-105.

3. Gordon SB, Bruce NG, Grigg J, et al. Respiratory risks from household air pollution in low and middle income countries. Lancet Respir Med. 2014;2:823-860.

4. Po JYT, Subramanian SV. Mortality burden and socioeconomic status in India. PLOS One. 2011;6:e16844.

5. Miller V, Mente A, Dehghan M, et al. Fruit, vegetable, and legume intake, and cardiovascular disease and deaths in 18 countries (PURE): a prospective cohort study. The Lancet. 2017;pii: S0140-6736(17)32253-5.

6. Kahleova H, Levin S, Barnard N. Cardio-metabolic effects of plant-based diets. Nutrients. 2017;9:848.

7. GBD 2015 Risk Factors Collaborators. Global, regional, and national comparative risk assessment of 79 behavioural, environmental and occupational, and metabolic risks or clusters of risks, 1990-2015: a systematic analysis for the Global Burden of Disease Study 2015. Lancet. 2016;388