Consuming higher amounts of unsaturated fats was associated with lower mortality, according to a study from researchers at Harvard T.H. Chan School of Public Health. The findings also showed that higher consumption of saturated and trans fats was linked with higher mortality and that replacing saturated fats with unsaturated fats conferred substantial health benefits. In addition, replacing total fat with carbohydrates was associated with modestly higher mortality.
We talked to lead author Dong Wang, a research fellow in the Department Nutrition, to take a closer look at the study.
1. Existing research has shown distinct associations between dietary fat and cardiovascular disease, however this study goes well beyond just one outcome. Tell us a bit more about the study design and what you set out to investigate.
Our objective was to examine the associations of specific dietary fats with both total and cause-specific mortality. We investigated 126,233 participants from two prospective cohort studies, the Nurses’ Health Study and Health Professionals Follow-Up Study. These men and women were free from cardiovascular disease, cancer, and diabetes at baseline when they joined the cohort, and dietary fat intake was assessed initially and updated every two to four years using validated food frequency questionnaires. Because of this very large sample size, combined with repeated and validated measurements of diet and lifestyle over an extended period of follow-up (up to 32 years), this study is by far the most detailed and powerful examination of this topic.
2. The study concludes that different types of dietary fat have “divergent effects on mortality.” What are the key findings on trans, saturated, and unsaturated fats?
Our study found that higher intake of saturated fat (found in foods like butter, lard, and red meat) and especially trans fat (predominantly from partially hydrogenated vegetable oil), was associated with greater risk of mortality when compared with the same number of calories from carbohydrate. When compared with carbohydrates, every 5% increase of total calories from saturated fat was associated with an 8% higher risk of overall mortality, and every 2% higher intake of trans fat was associated with a 16% higher risk of overall mortality.
Alternatively, higher intake of unsaturated fats compared to carbohydrates—including both polyunsaturated fats and monounsaturated fats from primarily plant-based foods—was associated with between 11% and 19% lower overall mortality for an increase in 5% of total calories. Polyunsaturated fats, which include both omega-6 fatty acids (found in most plant oils, nuts, and seeds), and omega-3 fatty acids (found in high concentrations in walnuts, flax seeds, soybean oil, and fish), were associated with lower risk of premature death.
3. Overall, these findings are consistent with the dietary recommendations to eliminate trans fat and to replace foods high in saturated fat with sources of unsaturated fat. Why is this particular replacement so beneficial?
Indeed, trans fats had the most significant negative impact on health. Fortunately, trans fats are less and less accessible due to the recent reductions of partially hydrogenated oils from our processed food supply.
When it comes to saturated fat, what you replace it with matters. People who replaced saturated fat in their diets with unsaturated fats—especially polyunsaturated fats—had a far lower risk of death from any cause, as well as death due to cardiovascular disease, cancer, neurodegenerative disease, and respiratory disease. Those who replaced saturated fat with carbohydrates only slightly lowered mortality risk. This is most likely because carbohydrates in typical American diets are high in refined starch and sugar, which have a similar influence on mortality risk as saturated fats.
4. The finding for cardiovascular disease is also consistent with existing guidelines based on evidence of reduced total and LDL cholesterol when unsaturated fats replace trans or saturated fats. Yet debate persists surrounding the health benefits of limiting saturated fat or butter. From where does this controversy stem, and how does your research help clear-up the confusion?
The recent widespread confusion about the health effects of specific types of dietary fat is in part caused by a misleading 2014 Annals of Internal Medicine paper, which concluded there is no evidence supporting the longstanding recommendation to limit saturated fat consumption and replace it with unsaturated fats. The authors of this paper employed a meta-analysis (a statistical analysis that summarizes data from many different studies), and therefore could not look at specific macronutrient comparisons with saturated fat. In addition, this paper also had errors in data extraction, omitted important studies, and only examined coronary heart disease.
In addition, a recently published BMJ paper based on 1960’s data from the Minnesota Coronary Experiment suggested that replacing saturated fat with vegetable oils rich in linoleic acid was not protective against death from coronary heart disease or all causes, which appeared to challenge current guidelines and added further confusion. However, this study was of very short duration, extremely low in follow-up rate, and the intervention likely reduced intake of important omega-3 polyunsaturated fatty acids. The extreme intervention diet was also never consumed by any appreciable number of Americans and likely confounded by intake of trans fat.
Finally, a new systematic review and meta-analysis looking at the association of butter consumption (a concentrated source of saturated fat) with chronic disease and all-cause mortality resulted in headlines touting “butter is back,” even though the findings were predominantly neutral, and the authors pointed out that unsaturated fats were found to be a better choice than butter.
Our research provides a much more comprehensive examination because we were able to look at specific comparisons between different types of fat or carbohydrate. We also examined a much broader range of outcomes, including total mortality and mortality due to cardiovascular disease, cancer, neurodegenerative disease and respiratory disease. To our knowledge, this is by far the most detailed and powerful examination of these important issues, and is supportive of the strong evidence base of current dietary recommendations that emphasize limiting saturated fat, eliminating trans fat, and replacing these with plant-based oils and other foods high in unsaturated fat.
5. So in terms of disease-specific mortality in relation to dietary fat intake, what correlations did you find?
- Higher trans fat intake was associated with significantly higher risk of neurodegenerative and respiratory disease mortality.
- Higher saturated fat intake was associated with a substantial increase in mortality due to respiratory disease.
- Higher intakes of monounsaturated and polyunsaturated fats were associated with lower risk of neurodegenerative and respiratory disease mortality.
- Intake of omega-3 polyunsaturated fatty acids, primarily alpha-linolenic acid (found in vegetable oils and nuts—especially walnuts—flax seeds, flaxseed oil, and leafy vegetables), was associated with lower risk of death due to neurodegenerative disease.
- Intake of marine-based omega-3 polyunsaturated fatty acids (mainly from fish) was associated with lower respiratory disease mortality and sudden cardiac death.
- Intake of omega-6 polyunsaturated fatty acids (found in vegetable oils, nuts, and seeds) was associated with lower risk of total death and death due to cardiovascular disease, cancer, and respiratory disease.
6. Based on this study, what are some simple steps consumers can take to improve their diet?
Overall, when it comes to dietary fat, what matters most is the type of fat you eat. This study documents the important benefits of unsaturated fats, especially when they replace saturated and trans fats. In practice this can be cutting back on red meat to incorporate more fish, nuts, and seeds, and replacing butter when cooking with a variety of liquid vegetable oils, such as olive, canola, and sunflower oil. It’s also important to limit refined grains, potatoes, and added sugar.