Friday November 25 2016
Saturated fats: do they deserve their bad reputation?
"Swapping butter and meat for olive oil and fish does cut the risk of heart disease," The Times reports.
The headline is prompted by the findings from a US study involving data from over 100,000 men and women, followed for more than 20 years. The results showed that consumption of different types of saturated fats was associated with an increased risk of coronary heart disease.
The researchers also found that replacing just 1% of energy consumed in the form of saturated fats with polyunsaturated fats, monounsaturated fats, wholegrain carbohydrates or plant proteins, led to a 5-8% decreased risk of coronary heart disease.
The debate regarding the risks of "sat fats" continues.
A report we discussed in May this year argued that the current UK guidelines on saturated fats were flawed as there was no proven link between saturated fat consumption and heart disease. But critics attacked the report for lacking independent peer review. The British Heart Foundation said it did not offer enough evidence to "take it seriously".
Current guidelines recommend that men eat no more than 30g of saturated fat a day and women no more than 20g of saturated fat, and this latest research appears to support current guidelines.
Where did the story come from?
The study was carried out by researchers from Harvard T.H. Chan School of Public Health, US and the Unilever Research & Development institute in the Netherlands. It was funded by the National Institutes of Health and the National Heart, Lung and Blood Institute and supported by Unilever. The study was published in the peer-reviewed British Medical Journal on an open-access basis, so it is free to read online.
One author declares they are supported by a grant from Unilever Research & Development and three other authors are employees of Unilever Research & Development. Unilever is a producer of food consumer products and as such there may be a conflict of interest.
Generally the UK’s media reported the story accurately.
However, the Daily Mail suggests that the fats identified in the study as increasing risk of coronary heart disease "be replaced in diets by other food like carbohydrates".
This may be misleading, as food products perceived by the public as carbohydrates may also contain ingredients such as butter that are high in saturated fats. The study only looked at wholegrain carbohydrates as a replacement for these fats.
What kind of research was this?
This was a longitudinal cohort study, which recruited male and female health professionals and followed them for over 20 years to assess how proportions of saturated fatty acids in the diet might affect risk of coronary heart disease later on.
This type of study is useful for suggesting links between factors but cannot prove that one factor – saturated fat intake – causes another – coronary heart disease.
The researchers tried to control for confounding factors, but there may be unmeasured factors, such as stress, that affect risk of coronary heart disease.
What did the research involve?
Researchers used data from the Nurses' Health Study, which included 73,147 female nurses and a cohort of 42,635 men from the Health Professionals Follow-up Study.
Information was collected at study baseline (1984 in the Nurses' Study and 1986 in the Health Professionals Study) on medical history, lifestyle, potential risk factors and disease diagnosis.
Participants also completed a food frequency questionnaire at baseline and then every four years until 2010, in which participants were asked how often they consumed specific foods in the previous year, ranging from "never" to "at least six per day". Cumulative averages of food intake were calculated from all dietary questionnaires completed in the follow-up.
Saturated fatty acids were distinguished by the length of their carbon chain. The number on the left indicates the number of carbon atoms and the number on the right the number of double bonds (saturated fatty acids do not have any double bonds). Therefore lauric acid (12:0) has 12 carbon atoms with no double bonds.
The major fatty acids included in the analysis were:
- lauric acid (12:0), found in high quantities in coconut and palm kernel oils
- myristic acid (14:0), found in cheese, butter, fresh and dried coconut and coconut oil
- palmistic acid (16:0) found in palm oil, palm kernel oil, coconut oil and often in chocolate
- stearic acid (18:0) found in butter, milk, meat/poultry/fish, lard, grain products and cocoa butter
Age-adjusted intake of individual saturated fatty acids was calculated and risk of non-fatal and fatal coronary heart disease was determined. The researchers adjusted their results to take into account the following possible confounding factors:
- family history of myocardial infarction (heart attack)
- body mass index
- cigarette smoking
- alcohol intake
- physical activity
- multivitamin use
- menopausal status
- postmenopausal hormone use
- current aspirin use
- baseline hypertension
- baseline hypercholesterolemia
- total energy intake
What were the basic results?
All participants were free of chronic illness at the beginning of the study. During the follow-up period, 7,035 cases of coronary heart disease were identified (4,348 were non-fatal; 2,687 were fatal).
Higher consumption of one type of fatty acid was associated with higher consumption of all fatty acids analysed.
Comparing groups with the highest and lowest intake of individual saturated fat intakes, there was an increased risk of coronary heart disease of:
- 13% (hazard ratio (HR) 1.13, 95% confidence interval (CI) 1.05 to 1.22) for 14:0 chain
- 18% (HR 1.18, 96% CI 1.09 to 1.27) for 16:0 chain
- 18% (HR 1.18, 95% CI 1.09 to 1.28) for 18:0 chain
- 18% (HR 1.18, 95% CI 1.09 to 1.28) for 12:0 to 18:0 chains combined
Replacement of 1% of energy intake from the 12:0 to 18:0 chain fats led to a reduced coronary heart disease risk of:
- 8% (HR 1.08, 95% CI 0.89 to 0.96) when replaced by polyunsaturated fat
- 6% (HR 1.06, 95% CI 0.91 to 0.97) when replaced by wholegrain carbohydrates
- 7% (HR 1.07, 95% CI 0.89 to 0.97) when replaced by plant proteins
- There was no significant decrease when replaced by monounsaturated fat (HR 1.05, 95% CI 0.90 to 1.01)
Participants who consumed higher proportions of saturated fatty acids were also more likely to be white, non-smokers, engage in less physical activity, less likely to take multivitamins and have a higher intake of total energy.
How did the researchers interpret the results?
The researchers concluded that "dietary replacement of 12:0-18:0 with more healthy macronutrients – such as polyunsaturated fat and wholegrain carbohydrates – was associated with a lower risk of coronary heart disease".
They further add that "owing to high correlations among individual saturated fatty acids (SFAs) in diet, these findings support the current dietary recommendations that focus on replacement of total saturated fat as an effective approach to preventing cardiovascular disease. The public health and clinical significance of modulating the content of individual SFAs in specific foods should be further evaluated".
This study shows an association between increased intake of individual saturated fats and increased risk of coronary heart disease.
It also shows a link between the replacement of these fatty acids with other types of fat, plant protein, or wholegrain carbohydrates and a reduction in coronary heart disease risk.
The strengths of this study are the large sample size and long follow-up period that looked at repeated measures such as diet, lifestyle and health outcomes.
It also provides clear support for dietary guidelines that recommend replacing dietary energy from saturated fats with polyunsaturated fats as well as wholegrain carbohydrates and plant source proteins.
However, there are a number of limitations to the study:
- Although the study adjusted for confounding variables, there may be other factors that were not accounted for. For example, stress and life events might be contributors to coronary heart disease, but were not measured.
- The analysis was based on self-reported dietary intake and therefore may be subject to recall bias.
- The study populations were comprised of health professionals who might have very similar lifestyles to one another; therefore the results may not be representative of other populations.
- Finally, most people did not just eat only one type of saturated fat, so it is hard to disentangle which have more association with coronary heart disease.
Also, the study did not consider other types of fatty acids, such as those found in dairy products, that may have beneficial effects.
There is ongoing controversy about how much of a threat saturated fats do actually pose to health.
Current UK government guidelines advise cutting down on all fats and replacing saturated fat with some unsaturated fat. Read more advice about fats