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Study questions role of saturated fats in main killer diseases

But industrially produced trans fats linked to higher risk of death due to heart disease

Mark Gould

Wednesday, 12 August 2015

A major review and meta-analysis of evidence on the health implications of dietary fats has failed to find a link between food containing saturated fats, such as eggs, chocolate and cream, and an increased risk of dying from heart disease, stroke or type-2 diabetes.

The study* nevertheless did find that industrially-produced “trans fats” made from hydrogenated oils, and once used in margarine, snack foods and packaged baked foods such as some cakes and crisps, are linked with a greater risk of death from coronary heart disease.

The latest findings, published in The BMJ, appear to support the growing realisation that the prevailing health advice for the past half century to cut down on foods that are rich in saturated fats such as butter and cheese may have been misguided.

The study, carried out in Canada by Russell de Souza of McMaster University in Hamilton, Ontario, and colleagues found no association between saturated fats and ill health, but did find a link with the consumption of foods containing trans fats, such as margarine.

"Saturated fats are not associated with all-cause mortality, CVD, CHD, ischaemic stroke, or type 2 diabetes, but the evidence is heterogeneous with methodological limitations. Trans fats are associated with all-cause mortality, total CHD, and CHD mortality, probably because of higher levels of intake of industrial trans fats than ruminant trans fats. Dietary guidelines must carefully consider the health effects of recommendations for alternative macronutrients to replace trans fats and saturated fats," they concluded.

The authors go on to say that their systematic review and meta-analysis of evidence "from large generally well designed observational studies does not support a robust association of saturated fats with all-cause mortality, CHD, CHD mortality, ischaemic stroke, or diabetes in healthy individuals"; but the choice of comparison nutrient (such as refined or high quality carbohydrate) must be carefully considered.

They say few observational studies have modelled the effect of replacing saturated or trans fats with other nutrients. In large prospective studies, when polyunsaturated fats replace saturated fats, risk of CHD is reduced but not when MUFA (monounsaturated fatty acids) or carbohydrate is the replacement choice. Higher, compared with lower, intakes of trans fats are associated with a 20-30% increased risk of all-cause mortality, CHD and CHD mortality, regardless of choice of replacement nutrient, but associations with type 2 diabetes and stroke are unclear. The association seems to be most consistently driven by industrially produced trans fats, probably because of their higher intakes among participants during the follow-up periods of the included studies. Dietary guidelines for saturated and trans fatty acids must carefully consider the effect of replacement nutrients.

The researchers say several questions could not be answered by the review. "First, do different sources (for example, animal v plant) and chain lengths (odd v even) of saturated fat have different effects on health, particularly with respect to risk of diabetes? The current evidence reviewed suggests that dairy fats, specifically odd chained saturated fatty acids, might be protective against type 2 diabetes; but, apart from recommendations for broad sources of fatty acids (such as dairy v plant v animal flesh), it is not feasible to separate different types of saturated fats with respect to food choices because the foods contain a combination of several saturated fats."

* Russell J de Souza, et al. Intake of saturated and trans unsaturated fatty acids and risk of all cause mortality, cardiovascular disease, and type 2 diabetes: systematic review and meta-analysis of observational studies. BMJ 2015;351:h3978. doi: 10.1136/bmj.h3978

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