Reference
Dhghan 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;S0140-6736(17):1-13.
Objective
The present study investigates whether diets high in fats or diets high in carbohydrates are associated with an increase of both cardiovascular disease (CVD) and total mortality.
Design
Epidemiological cohort study
Participants
Participants included 135,335 adults aged 35 to 70, enrolled between January 1, 2003 and March 31, 2013 and followed for a median of 7.4 years. Because the study aimed to include populations that varied by traditional diets and socioeconomic factors, participants were selected from 18 countries (from 5 continents): Canada, Switzerland, United Arab Emirates, Argentina, Brazil, Chile, Cuba, Columbia, Iran, Malaysia, occupied Palestinian territory, Poland, South Africa, Turkey, Bangladesh, India, Pakistan, and Zimbabwe.
Authors of the study excluded participants whose follow-up information was not available or had preexisting CVD.
Study Parameters Assessed
Standardized questionnaires were used to collect information about demographic factors, socioeconomic status (education, income, and employment), lifestyle (smoking, alcohol intake, and physical activity), health history, and medication use.
For decades, the conventional medical community has recommended that people consume a low-fat, high-carbohydrate diet to prevent CVD and early death.
Participants’ regular food intake was recorded using country-specific or region-specific validated food frequency questionnaires (FFQs) at baseline and included several 24-hour diet recalls in 60 to 250 individuals from each country as a reference to validate FFQs.
Primary Outcome Measures
The primary outcomes were total mortality and major cardiovascular events (fatal CVD, nonfatal myocardial infarction [MI], stroke, and heart failure). Secondary outcomes were all MIs, stroke, CVD mortality, and non-CVD mortality.
Key Findings
During follow-up, investigators documented 5,796 deaths and 4,784 major CVD events.
The highest quintile of carbohydrate intake (>60% of total calories) was associated with a higher risk of total mortality when compared to the lowest quintile of carbohydrate intake (hazard ratio [HR]: 1.28; P=0.0001). Carbohydrate intake was not associated with an increased risk of CVD or mortality.
The highest quintile of total fat intake (>35% of total calories) was correlated with a 23% lower total mortality versus the lowest quintile (P=0.0001). In addition, the individual types of fats consumed were also associated with lower risk of total mortality. For example, diets containing 10% to 15% saturated fat correlated with a 14% lower risk of total mortality vs those containing the least amount of saturated fat (P=0.0088). Diets highest in polyunsaturated and monounsaturated fats were also associated with a 20% (P=0.0001) and 19% (P=0.001) lower risk of total mortality vs the lowest quintiles of intake.
Higher saturated fat content was associated with lower risk of stroke. However, fat intake (total, saturated, or unsaturated) was not significantly associated with risk of MI or CVD mortality.
When 5% of the caloric intake from carbohydrates was replaced with polyunsaturated fats, an 11% reduction of total mortality and a 16% reduction of noncardiovascular mortality risk was observed. Replacement of carbohydrates with saturated fats was associated with 20% lower risk of stroke, while polyunsaturated and monounsaturated fat intake did not appear to affect risk of stroke.
Practice Implications
Cardiovascular disease has become a global epidemic. For decades, the conventional medical community has recommended that people consume a low-fat, high-carbohydrate diet to prevent CVD and early death. This idea came about primarily from the 1980 study by Ancel Keys, who investigated the diets and rates of CVD in 7 countries. His data suggested that elevated blood cholesterol levels and intake of dietary saturated fats is a major factor in the occurrence of coronary heart disease and risk of stroke.1 These findings assumed that elevated cholesterol levels and dietary saturated fat intake were the causative factors in heart disease risk, without taking into account other parameters influenced by saturated fat intake, such as high density lipoprotein (HDL) cholesterol levels and HDL to total cholesterol ratio, which when elevated has been shown to protect against heart disease.2
The present study is inspired by more recent meta-analyses of randomized trials and prospective cohort studies that showed either no association or even a lower risk of total mortality and CVD events with higher saturated fat consumption.3 In addition, various socioeconomic factors were considered with the assumption that countries in Europe and North America may have access to more dietary fat and increased overall caloric intake compared with those regions with less access to expensive, fatty foods and who may experience undernutrition as opposed to overnutrition. Most observational studies linking high saturated fat intake and total mortality have been conducted in more affluent countries whose saturated fat intake may range from 7% to 15% of total calories,4 and one of the authors’ goals in the present study was to determine if these results may be extrapolated to regions of the world where food is scarcer and diets comprise mostly less expensive carbohydrates.
Consistent with these more recent studies, the authors determined that across the board, participants who consumed more fat, including more saturated fat, and fewer carbohydrates experienced less incidence of total mortality, while no increase in major CVD was observed. Not surprisingly, higher intake of polyunsaturated and monounsaturated fats was also associated with a lower risk of all-cause mortality and was not linked to an increase in CVD events nor mortality. These results were consistent between Asian and non-Asian countries, notable because it is often cited that people in Asia live longer than those in other parts of the world as the result of their low-fat, high-carbohydrate diets.5
The current recommendation to limit total fat intake to less than 30% and saturated fat to less than 10% of total calories consumed is not supported by the findings of this study, and individuals who consume a high-carbohydrate diet may in fact benefit from replacing some of these carbs with fats. Lowest mortality rates were seen when total carbohydrate intake was below 60% of total calories consumed; however, diets consisting of less than 50% of the daily total were not associated with increased benefit. In addition, 35% of daily caloric intake from fats, of which 10% to 15% (and not less than 7%) should be saturated, appears to be the optimal range, according to these findings.
One of the main limitations of this study is that the authors did not distinguish between the sources or types of carbohydrates consumed. We are not able to determine if the participants were consuming refined white flour, whole, unprocessed grains, starchy root vegetables, or what combination of these. Fruits and non-starchy vegetables were undoubtedly included under the “carbohydrate” category, but there is no way to know which and how much of these were consumed by each population studied.
(For more on the health effects of carbohydrates, see "Bread and the Microbiome: A Personal Matter" in this month's issue.)
Diets containing a high quantity of refined grains and foods with a high glycemic index are associated with an increased risk of CVD,6 diabetes,7 and several types of cancer.8 Conversely, diets high in fiber, as found in unprocessed grains, fruits, and vegetables, are correlated with a decreased risk of heart disease, cancer, and diabetes.9
In addition, it is well-established that diets high in the antioxidants found in many fruits and vegetables are protective against many health conditions, including CVD.10,11 This is particularly important in the presence of lipids that may accumulate in the blood vessels, as these phytonutrients prevent lipid peroxidation and endothelial inflammation, major factors in the development of CVD.12
Without having information regarding the source of the carbohydrates in participants’ diets, it is difficult to conclude exactly which factors contribute to the results of the present study. Taken all together, however, the current evidence suggests that dietary fat, even saturated fat, is not the enemy that conventional medicine has been touting for so many years, and a reduction of total carbohydrates is likely beneficial for most people.
