25 April 2017

BNF responds to the editorial in the British Journal of Sports Medicine: Saturated fat does not clog the arteries: coronary heart disease is a chronic inflammatory condition, the risk of which can be effectively reduced from healthy lifestyle interventions and argues that whilst a healthy dietary pattern and being physically active benefits heart health, suggesting we don’t need to worry about saturated fat does not!

Nutrition science – not black and white

Nutrition science, like all science, is complex and advice cannot always fit neatly into simple black and white messages. Perhaps nothing reflects this more than the current controversy around the direct relationship between saturated fat and prevalence of heart disease. This relationship has been explored widely in the scientific literature leading to media headlines debating the link over the last few years.  Debate and discussion in nutrition science is welcome. However editorials or opinion pieces based on selective reporting of studies and the over-simplification of the scientific literature that suggest there is no need to worry about saturated fat may confuse those trying to understand how some scientists and nutrition organisations can reach different conclusions. Such editorials may also discourage people from following evidence based dietary guidelines and taking effective cholesterol lowering medication.

The evidence on saturated fat and cardiovascular disease

Whilst dietary advice remains to reduce saturated fat, a small number of published systematic reviews have highlighted a lack of an association between intake of saturated fat and higher risk of cardiovascular mortality from prospective studies. However these types of studies have inherent limitations to investigate this specific relationship. For example the use of baseline dietary assessments as a means of classifying subjects’ diets over a period of 20–30 years, particularly with respect to dietary fat intakes which have changed dramatically over this period, is problematic.  And because of the many limitations, reviews can miss one of the key points - the important benefit of replacing saturated fat with unsaturated fats.

It is key, in this time of debate and controversy, that we understand the science behind the current dietary advice.  Most saturated fatty acids (SFA), with chain lengths between 12-16 carbons, increase low-density lipoprotein-cholesterol (LDL -C) levels compared with unsaturated fatty acids. Increased LDL-cholesterol is directly associated to an increased coronary heart disease risk, and decreasing LDL-cholesterol (e.g. with cholesterol lowering drugs) reduces heart disease and mortality. The evidence from large clinical trials demonstrates very clearly that lowering LDL cholesterol reduces our risk of death overall and from heart attacks and strokes, regardless of age. Dietary guidelines, which are determined following a review of all relevant evidence available at the time, therefore recommend limiting SFA intake.

Even the authors of peer reviewed studies with findings that may support a more controversial view would themselves openly acknowledge limitations in their research design. For example, in a review (highlighted in the editorial by Malhotra et al.) that suggested no association between LDL-cholesterol and heart disease, the researchers acknowledge that the lack of examination of statin use in studies could be a confounding factor, in that the people found to have the highest LDL cholesterol levels at the beginning of the study may have then been started on statins, and this could have dramatically cut their mortality risk.

The multifactorial nature of diet-disease relationships

The relationship between saturated fatty acids and heart health is more complex than the simple message of ‘saturated fatty acids are bad’. Whether or not the risk of heart disease is reduced with lower intake of saturated fat is dependent on what replaces saturated fatty acids in the diet. This is a point that often gets lost in the debate. There is strong and consistent evidence that replacing saturated fatty acids with unsaturated fatty acids reduces the risk of CVD events and coronary-related mortality, as illustrated in a recent Cochrane systematic review (Hooper et al. 2015).

The food matrix adds to the complexity of the relationship between dietary fat and health. Not only do individual saturated fatty acids have different effects on blood cholesterol, but not all foods containing a higher proportion of saturated fatty acids will have the same effect on health because of the presence of other health promoting constituents. The diet as a whole or the overall dietary pattern is crucial when it comes to looking at the relationship between nutrition and heart disease.

It’s also important to recognise the complexity of the development of heart disease. Atherosclerosis is characterized by a complex, multifactorial pathophysiology which is affected not just by blood cholesterol levels, but also by many other factors including systemic inflammation, blood pressure, endothelial function, and obesity. So blood cholesterol is a risk factor for heart disease amongst many others and we need to consider how individual foods or preferably dietary patterns may act on these multiple pathways.

Healthy dietary patterns 

A feature of the dietary guidelines in the UK, as well as other national and international guidelines, and  of recognised healthy dietary patterns such as the Mediterranean diet is partial replacement of foods high in saturated fat with those in which the fat is predominantly unsaturated (such as olive oil, rapeseed oil, nuts, seeds, avocados, oily fish). The suggestion in the editorial that we can eat more saturated fat whilst highlighting the health benefit of a Mediterranean diet is contradictory, because in fact this dietary pattern is low in saturated fat (typically provides about 9% of dietary energy). It is also important to clarify that dietary guidelines in the UK do not advocate a ‘low fat’ diet  typically defined as total fat of 10-30% of energy, but recommend total fat intake makes up to 35% of energy intake – a recommendation we are currently meeting. There is also official guidance to consider the quality of the fat we eat – replacing saturated fat – found in cakes, biscuits, pies, pastries, fatty cuts of meats and cream – with healthier unsaturated fats. There is also strong advice to consider the type or quality of carbohydrate we eat – to reduce  our intake of  added (or free) sugars and  to choose wholegrain or high fibre versions of carbohydrate-rich foods like brown rice, wholegrain pasta, wholegrain breakfast cereals like porridge oats or wholegrain breads.

Indeed there is a remarkable consistency in ‘healthy dietary patterns’ i.e. those diets explored in the scientific literature that are associated with lower risk of chronic diseases including heart disease. Such diets are rich in vegetables and fruits, nuts, whole grain cereals, fish, low-fat dairy products and vegetable oils, but low in refined cereals and sugar-rich or salt-rich products and fatty and processed meats. We should compare this wealth of evidence to the number of studies demonstrating any benefit of diets high in saturated fats to heart heath – these are strikingly absent.

Physical activity and cardiovascular disease risk

As well as a varied and balanced diet, physical activity is important for reducing risk of chronic disease, and this is recognised in government health guidelines. Walking is advantageous because of the ease in which it can be incorporated into everyday life – whether that includes walking more with family and friends, and/or incorporating walking in a working day. For many, it may be an easier behaviour change to implement than taking part in organised sport or using the gym. But whether walking is the best form of exercise for health is unclear – a recent study from the University of Glasgow suggested that although walking and cycling as modes of commuting both reduced risk of cardiovascular events, cycling was associated with a greater reduction (46% vs 27%). The important thing is that people increase their physical activity in a way that can easily be incorporated so that it is a long term lifestyle change,

Improving diet, physical activity and wellbeing are all recognised government public health objectives. In terms of dietary advice, a healthy diet isn’t just about one nutrient – even  sugar or fat – the whole dietary pattern is key. If by eating ‘real’ food ,the editorial is suggesting a diet that includes plenty of fruit and vegetables, wholegrains, some dairy, nuts, pulses, eggs, lean meat and oily fish, and is low in saturated fats, sugars and salts, we wholeheartedly agree!