The saturated fat bogey

Everyone knows that to be healthy we should consume less saturated fat. Well, the thing is, the best scientific evidence we have available indicates that dietary saturated fat does not increase the risk of heart disease or stroke.

There are certainly scientific studies that support the dangers of saturated fats, but there are also just as many (or more) that don’t, and even some that show they’re beneficial. The problem arises because the first studies to come out are like sign posts – they point to whether something is worth studying in more detail, but are not themselves definitive. The definitive health science studies can take decades to come to a conclusion, and in the meantime, dietary advice consolidates around the initial idea, which is usually a plausible one.

To be definitive, studies must be performed prospectively. That is, they must recruit people and follow them for what could be decades, monitoring their health and their diet to see what happens. In contrast, retrospective studies – taking a group of people with, say, heart disease, and trying to determine their history of saturated fat intake – is not as sound an approach. Retrospective studies are not definitive, but they are easier and quicker to perform and give an idea as to what should be done next – they don’t answer the question with certainty. As they are quicker to perform, they are the first ones to get published, and so they have the highest impact. Because they usually have some weakness in design, they reach a range of conclusions that can be cherry-picked as needed.

There are a myriad of confounds – exercise, smoking, occupation, family history to name a few – that must be factored in. So, to be definitive, even prospective studies must recruit tens to hundreds of thousands of people to be sure. They are extraordinarily time-consuming, difficult and expensive.

If that’s not exhausting enough, a single prospective study is not as convincing as a collection of them. Once enough have been completed, an analysis of their combined findings can be undertaken and a more confident conclusion is finally reached.

Which brings me to such a comprehensive analysis of dietary saturated fat that was recently performed (ref. 1). The analysis combined the results from 11 separate prospective studies (with a total of 347,747 persons) that followed the participants for up to 23 years. The analysis found that “there is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of coronary heart disease or cardiovascular disease”.

A 2013 review of 16 studies (11 prospective) of high-fat dairy consumption (ref. 2) reported that “evidence does not support the hypothesis that dairy fat or high-fat dairy foods contribute to obesity or cardiometabolic risk” (i.e. heart disease or type II diabetes). In fact, the analysis suggested that dairy fat was beneficial.

I suspect the problem here is oversimplification – we have a bewilderingly complex biochemistry, and to demonise a selected food is not likely to work. And a selected food isn’t usually homogeneous – for example, dairy fat contains a multitude of fatty acids and nutrients, many of which will be good for us. It is telling that, although the low-fat dairy message is now accepted (whole-milk consumption fell a whopping 71% in the USA during the last century), obesity continues to rise, as does heart disease and type II diabetes.

The beneficial effects of dietary fibre, and the dangers of dietary salt and cholesterol, are also on increasingly shaky ground. Sugar is probably another matter though.

My view is that foods are created more or less equal, and balance matters (as does exercise).

By now it’s too late to correct popular misconception. The definitive science gets published too late, and is easily misunderstood as ‘just another study’. Lucrative industries have evolved around established dietary rules. Powerful lobbies aren’t to be deterred. Social and cultural pressures apply guilt to keep us in line. Simplistic slogans (‘eat less fat’, ‘eat more fibre’) are in demand, and will remain so.

Therefore, I have a simple food-slogan of my own for you to ponder:

Relax, be sensible, enjoy.


References:
1) PW Siri-Tarino, Q Sun, FB Hu, RM Krauss (2010) “Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease” Am J Clin Nutr, dx.doi.org/10.3945/ajcn.2009.27725
2) M Kratz, T Baars, S Guyenet (2013) “The relationship between high-fat dairy consumption and obesity, cardiovascular, and metabolic disease”, Eur J Nutr 52:1–24.