Tuesday 10th February 2015

There have been suggestions today, based on a paper published in the journal  Open Heart, that dietary guidelines published in 1977 and 1983 in the UK and USA, recommending a reduction in dietary fat and saturated fatty acids intake, were not supported by available evidence from randomised controlled trials. More recent reviews of the scientific evidence on fat intake and risk of cardiovascular disease, however, have led to similar recommendations in dietary guidelines around the world (e.g. in the UK, US, Canada, Australia and Europe).

Since the 1980s the UK population has significantly reduced its intake of total fat and saturated fatty acids. During this period mortality from heart disease has dramatically reduced and this decline has been attributed to both dietary and lifestyle changes as well as improvements in evidence based treatments

The current UK guidelines, introduced by the Department of Health in 1984, state that, on average, the adult UK population should get less than 35% of their dietary energy from total fat and less than 11% from saturated fatty acids.

The UK recommendations set in 1984 were reviewed by the Committee on Medical Aspects of Food Policy (COMA) in the 1990s for the publication Nutritional Aspects of Cardiovascular Disease (DH 1994). Following a review of the scientific evidence considered in the 1984 report, alongside newer studies, COMA concluded that numerous studies had confirmed the cholesterol raising effect of diets high in saturated fatty acids (although not all saturated fatty acids raised cholesterol to the same extent), and recommended continuation of the population recommendations regarding fat and saturated fatty acids (i.e. <35% and 11% of dietary energy respectively).

A key review citied within this report identified more than 70 clinical studies looking at the effects of dietary fat on blood cholesterol and concluded that intakes of saturated fatty acids increased blood cholesterol and that polyunsaturated fatty acids appeared to lower it (Hegstead et al. 1993).

The WHO published its report Diet Nutrition and the Prevention of Chronic Disease in 2003 (WHO 2003). It again reviewed the evidence for a link between saturated fatty acids and cholesterol. The report stated “The relationship between dietary fats and CVD (cardiovascular disease) has been extensively investigated, with strong and consistent associations emerging from a wide body of evidence accrued from animal experiments, as well as observational studies, clinical trials and metabolic studies in diverse human populations. Saturated fatty acids raise total and low-density lipoprotein (LDL) cholesterol but individual fatty acids within this group have different effects”. The report highlighted that replacement of saturated fatty acids with polyunsaturated fatty acids appeared to have the most beneficial effect on heart disease risk. This report reiterated the previous WHO recommendation of 30% of energy from total fat and no more than 10% of energy intake from saturated fatty acids. It highlighted that the most effective replacement for saturated fatty acids for reducing coronary heart disease risk is polyunsaturated fatty acids.

In 2010, FAO published the paper Fats and Fatty Acids in Human Nutrition (FAO 2010). The strength of evidence was reviewed and evaluated during the expert consultation to arrive at recommendations and conclusions and to establish requirement levels. Only evidence that was defined as ‘convincing’ or ‘probable’ was used to formulate recommendations. On saturated fatty acids they reported there was convincing evidence that replacing saturated fatty acids (specifically those with a chain length between 12-16) with polyunsaturated fatty acids or with monounsaturated fatty acids decreases LDL-cholesterol concentration and the total/HDL cholesterol ratio. This was also the conclusion of the European Food Safety Authority (EFSA), which reviewed the evidence when setting the Dietary Reference Value for fat in 2010 (EFSA 2010). They stated that “There is a positive, dose-dependent relationship between the intake of a mixture of saturated fatty acids and blood low density lipoprotein (LDL) cholesterol concentrations, when compared to carbohydrates. There is also evidence from dietary intervention studies that decreasing the intakes of products rich in saturated fatty acids by replacement with products rich in n-6 polyunsaturated fatty acids (without changing total fat intake) decreased the number of cardiovascular events.”

Overall, the UK and major international health organisations have concluded, based on reviews of available evidence that there is a link between saturated fat intakes, blood cholesterol and cardiovascular risk and that dietary saturated fatty acids should be reduced, replacing them with unsaturated fatty acids.

It is worth noting that the levels of total dietary fat recommended by the UK and most other national and international bodies equate to a moderate fat diet rather than a low fat diet.
However, in the context of rising obesity levels in most countries around the world, the fact that fat is the most energy dense nutrient (at 9kcal per gram compared to alcohol at 7kcal per gram and protein and carbohydrate at 4kcal per gram) means that reducing high fat foods in the diet is generally a component of most strategies for weight loss.

Cardiovascular disease (CVD) is a multifactorial condition and no one factor alone determines risk of developing CVD. To reduce risk a range of modifiable risk factors including aspects of the diet, smoking, physical activity and body weight should be addressed. However, there is a substantial body of evidence that supports the benefit of replacing saturated with unsaturated fatty acids, which has grown since the recommendations in question were first introduced.

Diet and disease relationships are challenging to study because they require people to make sustained and consistent changes to their diet that can be measured accurately and assessed over decades to monitor their effects. As such it is important that evidence on diet and health is regularly monitored and re-evaluated to ensure that policy remains relevant. BNF is in the process of updating its CVD Task Force Report, which was first published in 2005. The final report will review the latest evidence on the strength of the associations between diet and a range of factors, including more novel risk factors, in relation to CVD risk.

References:
Department of Health (1984). Diet and cardiovascular disease. Committee on Medical Aspects of Food Policy. Report of the panel on diet in relation to cardiovascular disease. HMSO, London.

Department of Health (1994). Nutritional aspects of cardiovascular disease. Report of the cardiovascular review group, Committee on Medical Aspects of Food Policy. HMSO, London.

EFSA (2010). Scientific Opinion on Dietary Reference Values for fats, including saturated fatty acids, polyunsaturated fatty acids, monounsaturated fatty acids, trans fatty acids, and cholesterol. Available at http://www.efsa.europa.eu/en/efsajournal/pub/1461.htm (accessed 9th February 2015)

Hegsted DM, Ausman LM, Johnson JA et al. (1993). Dietary fat and serum lipids: an evaluation of the experimental data. American Journal of Clinical Nutrition. 57:875-83

WHO (2003). Diet, nutrition and the prevention of chronic disease. Available at http://libdoc.who.int/trs/WHO_TRS_916.pdf (accessed 9th February 2015)

WHO global strategy on diet, physical activity and health. Diet: http://www.who.int/dietphysicalactivity/diet/en/ (accessed 9th February 2015)

FAO (2010). Fats and fatty acids: a report of an expert consultation. Available at http://www.fao.org/docrep/013/i1953e/i1953e00.pdf (accessed 9th February 2015)