For August 2016, BNF looks at a published study that explores the association of specific dietary fats with total and specific cause mortality.

 

Association of Specific Dietary Fats With Total and Cause-Specific Mortality.

Wang DD, Li Y, Chiuve SE et al.  JAMA Internal Medicine. 176(8):1134-45 doi: 10.1001/jamainternmed.2016.2417.

Background

Official dietary guidelines, including from the UK’s Department of Health, the World Health Organization and the US Food and Drug Administration, have remained consistent regarding the advice to reduce saturated fat via replacement with small amounts of monounsaturated and polyunsaturated fats where possible.  However this advice has been questioned by some researchers recently, and consequent media headlines have created some confusion about the place of saturated fat, or indeed fat per se, in a healthy, balanced diet. The Scientific Advisory Committee on Nutrition is currently reviewing the totality of evidence on saturated fat and health but, in the meantime, it is important to consider evidence from well conducted observational studies and randomised controlled studies (RCTs) to provide clear and consistent advice on this topic for the general public.

Summary

This paper from Wang et al. reports on observational research that included over 125 000 US participants, followed up for around 30 years. Higher intakes of saturated fatty acids and trans fatty acids were associated with an increased risk of mortality (from all causes), whereas higher intakes of polyunsaturated and monounsaturated fatty acids were associated with a lower risk.

Replacing 5% of energy from saturated fatty acids with equivalent energy from polyunsaturated and monounsaturated fatty acids was associated with lower mortality.

These findings suggest that replacement of saturated fats with unsaturated fats can confer health benefits, which supports current dietary recommendations.

Study objective

The authors sought to examine the associations of specific dietary fatty acids with total and cause-specific mortality in 2 large ongoing cohort studies.

Study design

This study combines data from two large US cohort studies with a combined total of 126,236 participants. the Nurses' Health Study (83,349 women, starting 1980) and the Health Professionals’ Follow-Up Study (42,884 men, starting 1986), up to 2012. Diets of the participants were assessed every 4 years, using food-frequency questionnaires.

Researchers looked at the effect of specific dietary fats compared to carbohydrates, including saturated, polyunsaturated, monounsaturated and trans fatty acids, on mortality from any cause or from specific causes. They also calculated the effect of swapping 5% of dietary energy intake from saturated fats or trans fats with polyunsaturated or monounsaturated fats. The results were adjusted for confounding risk factors, including age, ethnicity, body mass index (BMI), physical activity, smoking, alcohol consumption, vitamin supplement use and family medical history.

Key results

•33 304 deaths were documented.

•After adjustment for other potential risk factors, dietary total fat, compared with total carbohydrates, was inversely associated with total mortality. But not all of the fats had equal associations.

•Comparing extreme quintiles (lowest to highest intake), people who ate more saturated and trans fats were at greater risk of dying during the study, and those who ate more polyunsaturated fats or monounsaturated fats were at lower risk.

•The study researchers also reported that replacing 5% of energy from saturated fats with equivalent energy from polyunsaturated and monounsaturated fats was associated with reductions in total mortality,

 

Association of specific dietary fats with total mortality

(Adjusted Hazard Ratio comparing highest to lowest quintiles of fat intakes, 95% Confidence interval)

Type of fat Association with total mortality Estimated effect
Saturated fat 1.08 (95%CI, 1.03-1.14) 8% more likely to have died
Trans fats 1.13 (95%CI, 1.07-1.18) 13% more likely to have died
PUFA 0.81 (95%CI, 0.78-0.84) 19% less likely to have died
MUFA 0.89 (95%CI, 0.84-0.94) 11% less likely to have died

 PUFA: polyunsaturated fatty acids; MUFA: monounsaturated fatty

 

Estimated reductions in total mortality and cardiovascular disease mortality replacing 5% of energy from saturated fats with equivalent energy from PUFA and MUFA

Replacement of 5% SFA with

total mortality

HR (95% CI)

Estimated reduction total mortality

CVD mortality

HR (95% CI)

Estimated reduction CVD mortality
UFA 0.78 (0.75-0.82) 22% 0.80 (0.73-0.88) 20%
PUFA 0.73; (0.70-0.77) 27% 0.72 (0.65-0.80) 28%
MUFA 0.87; 0.82-0.93), 13% 0.96 (0.84-1.09) NS

HR: hazard ratio; NS: non significant; UFA: unsaturated fatty acids; PUFA: polyunsaturated fatty acids; MUFA: monounsaturated fatty

 

Authors’ Conclusions

Different types of dietary fats have divergent associations with mortality.

Limitations

This was a large observational study from a well-regarded research group from the Harvard School of Public Health and Brigham and Women's Hospital, with a continual assessment of dietary intakes and adjustment of results for potential confounding risk factors.

The design of the study does not allow for inference about causation, so it cannot be interpreted as evidence of ‘cause and effect’. In addition, as will all observational data, the authors could not rule out confounding by unmeasured factors.

Limitations of the study also include reliance on food frequency questionnaires, which can be inaccurate, and reverse causation might have explained some of the findings i.e. people with poor health might have changed their habitual diets. However, people with known major chronic diseases at baseline were excluded, and those concerned about a serious illness might adopt a diet generally perceived to be healthier, which would not explain the findings.

Wider research context

The results of this study are in line with a recent Cochrane review where reduction (27%) in cardiovascular events was observed where polyunsaturated fatty acids replaced saturated fatty acids in the diet.

Relevance

These findings support the widely held view that high intakes of trans and saturated fatty acids in the diet are adverse to health. UK average intake of trans fatty acids is below the current dietary recommendation and at the lowest quintile levels reported in this study. However, average

intake of saturated fatty acids remains higher than recommended (no more than 11% food energy).

The findings support advice to replace saturated fatty acids with unsaturated fatty acids where possible. Consideration needs to be given to translating the messages from such scientific research into practical messages for consumers, so that appropriate food choices are facilitated.