Consumption of unsaturated fats is healthier

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Consuming higher amounts of unsaturated fats was associated with lower mortality, according to a large study from Harvard TH Chan School of Public Health.

In a large study population followed for more than three decades, researchers found that higher consumption of saturated and trans fats was linked with higher mortality compared with the same number of calories from carbohydrates. Most importantly, replacing saturated fats with unsaturated fats conferred substantial health benefits.

In a large study population followed for more than three decades, researchers found that higher consumption of saturated and trans fats was linked with higher mortality compared with the same number of calories from carbohydrates. Most importantly, replacing saturated fats with unsaturated fats conferred substantial health benefits.

This study provides further support for the 2015-2020 Dietary Guidelines for Americans that emphasise the types of fat rather than total amount of fat in the diet.

The study is the most detailed and powerful examination to date on how dietary fats impact health. It suggests that replacing saturated fats like butter, lard, and fat in red meat with unsaturated fats from plant-based foods – like olive oil, canola oil, and soybean oil – can confer substantial health benefits and should continue to be a key message in dietary recommendations.

“There has been widespread confusion in the biomedical community and the general public in the last couple of years about the health effects of specific types of fat in the diet,” said Dong Wang, a doctoral candidate, SD ’16, in the departments of nutrition and epidemiology at Harvard Chan School and lead author of the study. “This study documents important benefits of unsaturated fats, especially when they replace saturated and trans fats.”

The study included 126,233 participants from two large long-term studies – the Nurses’ Health Study and the Health Professionals Follow-Up Study – who answered survey questions every 2-4 years about their diet, lifestyle, and health for up to 32 years. During the follow-up, 33,304 deaths were documented. Researchers from Harvard Chan School and Brigham and Women’s Hospital examined the relationship between types of fats in the participants’ diets and overall deaths among the group during the study period, as well as deaths due to cardiovascular disease (CVD), cancer, neurodegenerative disease, and respiratory disease.

Different types of dietary fat had different associations with mortality, the researchers found. Trans fats – on their way to being largely phased out of food – had the most significant adverse impact on health. Every 2% higher intake of trans fat was associated with a 16% higher chance of premature death during the study period. Higher consumption of saturated fats was also linked with greater mortality risk. When compared with the same number of calories from carbohydrate, every 5% increase in saturated fat intake was associated with an 8% higher risk of overall mortality.

Conversely, intake of high amounts of unsaturated fats – both polyunsaturated and monounsaturated – was associated with between 11% and 19% lower overall mortality compared with the same number of calories from carbohydrates. Among the polyunsaturated fats, both omega-6, found in most plant oils, and omega-3 fatty acids, found in fish and soy and canola oils, were associated with lower risk of premature death.

The health effects of specific types of fats depended on what people were replacing them with, the researchers found. For example, people who replaced saturated fats with unsaturated fats – especially polyunsaturated fats – had significantly lower risk of death overall during the study period, as well as lower risk of death from CVD, cancer, neurodegenerative disease, and respiratory disease, compared with those who maintained high intakes of saturated fats. The findings for cardiovascular disease are consistent with many earlier studies showing reduced total and LDL (“bad”) cholesterol when unsaturated fats replace trans or saturated fats.

People who replaced saturated fats with carbohydrates had only slightly lower mortality risk. In addition, replacing total fat with carbohydrates was associated with modestly higher mortality. This was not surprising, the authors said, because carbohydrates in the American diet tend to be primarily refined starch and sugar, which have a similar influence on mortality risk as saturated fats.

“Our study shows the importance of eliminating trans fat and replacing saturated fat with unsaturated fats, including both omega-6 and omega-3 polyunsaturated fatty acids. In practice, this can be achieved by replacing animal fats with a variety of liquid vegetable oils,” said senior author Frank Hu, professor of nutrition and epidemiology at Harvard Chan School and professor of medicine at Harvard Medical School.

Abstract
Importance: Previous studies have shown distinct associations between specific dietary fat and cardiovascular disease. However, evidence on specific dietary fat and mortality remains limited and inconsistent.
Objective: To examine the associations of specific dietary fats with total and cause-specific mortality in 2 large ongoing cohort studies.
Design, Setting, and Participants: This cohort study investigated 83 349 women from the Nurses’ Health Study (July 1, 1980, to June 30, 2012) and 42 884 men from the Health Professionals Follow-up Study (February 1, 1986, to January 31, 2012) who were free of cardiovascular disease, cancer, and types 1 and 2 diabetes at baseline. Dietary fat intake was assessed at baseline and updated every 2 to 4 years. Information on mortality was obtained from systematic searches of the vital records of states and the National Death Index, supplemented by reports from family members or postal authorities. Data were analyzed from September 18, 2014, to March 27, 2016.
Main Outcomes and Measures: Total and cause-specific mortality.
Results: During 3 439 954 person-years of follow-up, 33 304 deaths were documented. After adjustment for known and suspected risk factors, dietary total fat compared with total carbohydrates was inversely associated with total mortality (hazard ratio [HR] comparing extreme quintiles, 0.84; 95% CI, 0.81-0.88; P < .001 for trend). The HRs of total mortality comparing extreme quintiles of specific dietary fats were 1.08 (95% CI, 1.03-1.14) for saturated fat, 0.81 (95% CI, 0.78-0.84) for polyunsaturated fatty acid (PUFA), 0.89 (95% CI, 0.84-0.94) for monounsaturated fatty acid (MUFA), and 1.13 (95% CI, 1.07-1.18) for trans-fat (P < .001 for trend for all). Replacing 5% of energy from saturated fats with equivalent energy from PUFA and MUFA was associated with estimated reductions in total mortality of 27% (HR, 0.73; 95% CI, 0.70-0.77) and 13% (HR, 0.87; 95% CI, 0.82-0.93), respectively. The HR for total mortality comparing extreme quintiles of ω-6 PUFA intake was 0.85 (95% CI, 0.81-0.89; P < .001 for trend). Intake of ω-6 PUFA, especially linoleic acid, was inversely associated with mortality owing to most major causes, whereas marine ω-3 PUFA intake was associated with a modestly lower total mortality (HR comparing extreme quintiles, 0.96; 95% CI, 0.93-1.00; P = .002 for trend).
Conclusions and Relevance: Different types of dietary fats have divergent associations with total and cause-specific mortality. These findings support current dietary recommendations to replace saturated fat and trans-fat with unsaturated fats.

Authors
Dong D Wang; Yanping Li; Stephanie E Chiuve; Meir J Stampfer; JoAnn E Manson; Eric B Rimm; Walter C Willett; Frank B Hu

Harvard TH Chan School of Public Health
JAMA Internal Medicine abstract


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