At the end of August this year (2017), the results of an observational study called the PURE Study (Perspective Urban Rural Epidemiological) were published in a medical journal called The Lancet (1). You may have heard talk about the study on the radio or noticed headlines declaring that high carbohydrate diets lead to decreased longevity. The purpose of the PURE Study was to examine the effect of food choices on the risk of death from cardiovascular disease, stroke and non-cardiovascular disease. The study looked at the eating habits of about 135,000 people from eighteen countries (Canada, Sweden and the United Arab Emirates (higher-income countries); Argentina, Brazil, China, Chile, Colombia, Iran, Malaysia, occupied Palestinian territory, Poland, South Africa and Turkey (middle-income countries); and Bangladesh, India, Pakistan and Zimbabwe (lower-income countries)). Participants were asked to complete a single food-frequency questionnaire at the start of the study. At three different times during the study their state of health was assessed. The collected data was then analyzed and adjusted for confounding factors.
A NOTE ON CONFOUNDING: Confounding is distortion of results by an additional variable that may affect a study endpoint. In the PURE Study, income level is a definite confounding factor. Poverty has a major impact on food choices. The effects on diet by low income in a study such as this are extremely challenging to separate out. Confounding factors can mask an actual association or, more commonly, falsely show an apparent association when no real association exists. (2)
PURE STUDY CONCLUSIONS
At the end of the study period, the conclusions of the authors include the following;
- Though health outcomes across all the countries improved and mortality decreased with higher vegetable, fruit and legume intake, after data adjustment this benefit appeared to peak at only three servings of vegetables, fruit and legumes daily.
- Eating a diet consisting of more than 60% of energy from carbohydrates is associated with increased risk of death.
- Eating a diet consisting of more than 30% of energy from fats and containing a low amount of high carbohydrate foods is associated with improved longevity.
QUESTIONING THESE CONCLUSIONS
A close look into the details of this study reveals a multitude of problems. Design and analysis flaws inevitably lead to mistaken results. In the following paragraphs I hope to illustrate why this study’s conclusions are suspect.
PROBLEMS WITH THE STUDY DESIGN
- Food questionnaires are notoriously inaccurate. People have difficulty just remembering what they have eaten in their most recent meals let alone what their food choices have been over the previous week or month. Additionally, food choice is a very personal matter and one that is often tangled up with guilt about unhealthy food preferences. Many people will not admit even to themselves the extent of their poor eating habits.
- The Chinese data (almost one-third of the total study population) shows a large discrepancy in fat intake as compared with similar questionnaires used in other Chinese studies. Average fat intake in the PURE study was about 17.7% of total daily calories yet other surveys have found an average intake of about 30%. This calls into question the reliability of the dietary intake data. (3)
- There was no follow-up food questionnaire over the study period so that any dietary changes that might have occurred were completely missed.
- All carbohydrates were lumped together so that there was no separation between highly nutritious unprocessed plant-sourced carbohydrates and carbohydrates from processed foods and added sugar. The PURE Study looked only at macronutrients (carbohydrates, proteins and fats) with no differentiation between different carbohydrate types. This completely obscures the effect of different carbohydrate sources on the human body.
To illustrate this, consider the following two meal examples.
The first is a healthy plant-based stew consisting of chick peas, broccoli, peppers, mushrooms and sweet potatoes in a cashew-based sauce served over brown rice and accompanied by green tea.
The second is a lasagna made up of layers of white pasta, meat sauce and cheese served with a sugar sweetened carbonated beverage.
Both of these meals contain approximately 60% carbohydrate, 20% protein and 20% fat. However their effect on the body is markedly different. The first meal is anti-inflammatory and high in fiber and other nutrients such as antioxidants, vitamins and minerals while the second meal contains almost no fiber or other nutrients and promotes inflammation in the body.
It is obvious that the type of carbohydrate eaten is of utmost importance when it comes to food and unfortunately this important variable was not included in the study.
PROBLEMS IN THE DATA ANALYSIS
- Participants eating the highest levels of vegetables, fruits and legumes had the lowest mortality and also the lowest levels of heart disease and stroke. 8% of the people with the lowest vegetable, fruit and legume intake died during the study compared with only 3% of the people with the highest vegetable, fruit and legume intake. This result was analyzed out of the data.
- Participants with the lowest intake of vegetables, fruits and legumes also had the lowest intake of total calories. In fact, participants with the highest intake of vegetables, fruits and legumes consumed almost double the calories obtained by participants with the lowest vegetable, fruit and legume intake. Country specific results reveal that those countries with the lowest vegetable, fruit and legume intake are the poorest (Malaysia, Pakistan and Zimbabwe). Higher rates of mortality in poorer populations is related to many variables other than just food choices; factors such as lack of medical care, poor access to education, higher rates of smoking and inability to afford high quality protein-containing food. (3)
- The diet of participants from low and middle income countries was very high in refined carbohydrates such as white rice and white bread. People in these countries also have the least access to medical care and the highest risk of dying from infectious diseases. (3)
- Countries with the lowest intake of fat also had the lowest intake of protein, once again suggesting an inadequate diet due to low income. This was not taken into account in this study. (3)
- Participants with the highest fat intake tended to be of higher income with a high level of medical care and a lower risk of non-cardiovascular related death such as death from infectious diseases.
- The level of dietary saturated fat across the countries included in this study ranged from 6% to 11% of total calories, a level approaching the current recommended level and one that is significantly lower than that of North America and Europe. To suggest that increasing dietary saturated fat would be healthy for populations already consuming higher levels than those found in this study makes no sense at all.
THE STUDY CONCLUSIONS ARE PUZZLING AT THE VERY LEAST
Taking into consideration all the results of this study, it is difficult to understand the conclusions of the authors. They were fully aware of the limitations of this study and in fact listed many of these limitations in their published papers. However this did not prevent them from placing the blame for increased mortality squarely on the macronutrient known as “carbohydrates”.
Wholescale blame for worse health outcomes on “carbohydrates” is completely irrational. As we have observed, the term “carbohydrate” encompasses a huge variety of different food types. A more reasonable conclusion for this study would be that people of low income who cannot afford to buy sufficient healthy food tend to eat diets high in refined carbohydrates and have less access to education and decent medical care and consequently get sick and die more often than people of higher income who have access to a sufficient quantity and quality of calories as well as good medical care.
The PURE Study is an observational study, a type of scientific study that can only result in associations between factors such as dietary choices and health outcomes. Observational studies do not prove that any particular variable is the cause of an outcome. It takes interventional studies, those that are randomized, standardized and controlled, to demonstrate cause and effect. It is important to remember that the scientific community has amassed an abundance of well-designed interventional trials with robust results that provide overwhelming evidence that both high fat intake and high intake of REFINED carbohydrate foods increase the risk of cardiovascular disease and increase mortality while high intake of WHOLE carbohydrate foods (starchy and non-starchy vegetables, fruits, legumes and whole grains) increase longevity and decrease the risk of many chronic conditions including cardiovascular disease. (4,5,6,7,8,9) The PURE Study cannot refute the solid evidence that we already have.
Sadly the PURE Study is not a breakthrough. It is a breakdown in study design and analysis. Added to that is the propensity of mainstream media to jump on the good news bandwagon, proclaiming news that they know people will want to hear before looking with any depth into the details. We must not blindly accept the questionable results and sensational headlines produced by any particular study. We already know the direction to take for good health. Simply eat a diet centered on whole foods – vegetables, fruits, legumes, nuts, seeds and whole grains.
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. The Lancet. 2017 Aug 28; S0140-6736(17)32252-3
2 Skelly, A.C., Dettori, J.R., and Brodt, E.D. Assessing bias: the importance of considering confounding. Evid Based Spine Care J. 2012 Feb; 3(1): 9–12
4 Micha, R., Peñalvo, J.L., Cudhea, F., Imamura, F., Rehm, C.D., and Mozaffarian, D. Association Between Dietary Factors and Mortality From Heart Disease, Stroke, and Type 2 Diabetes in the United States. JAMA. 2017 Mar 7; 317(9):912-924
5 Anand,S.S., Hawkes, C., de Souza,R.J., Mente,A. et al. Food Consumption and its impact on Cardiovascular Disease: Importance of Solutions focused on the globalized food system. A Report from the Workshop convened by the World Heart Federation . J Am Coll Cardiol. 2015 Oct 6; 66(14): 1590–1614.
6 Jousilahti, P., Laatikainen, T., Salomaa, V., et al. 40-Year CHD Mortality Trends and the Role of Risk Factors in Mortality Decline: The North Karelia Project Experience. Glob Heart. 2016 Jun; 11(2):207-12
7 Gardner, C.D., Coulston, A., Chatterjee, L., Rigby, A. et al. The Effect of a Plant-Based Diet on Plasma Lipids in Hypercholesterolemic Adults: A Randomized Trial. Ann Intern Med. 2005;142(9):725-733.
8 Knowler, W.C. et al. Reduction in the incidence of type-2 diabetes with lifestyle intervention or metformin. N Engl J Med 2001 Feb 7; 346(6): 393-403
9 Barnard, R.J. Effects of Life-style Modification on Serum Lipids. Arch Intern Med. 1991; 151(7):1389-1394.
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