The idea that saturated fat is good and that sugar is bad is very fashionable at the moment. I want to have a bit of a wander through the evidence surrounding this, specifically the effects of saturated fat and sugar on the incidence of heart disease.

When you’re looking at health interventions in human beings, the place to start is a meta-analysis, where the results of lots of studies are pooled together to try to find the big picture of what’s happening. When it comes to saturated fat and heart disease, there are a plethora of these analyses. One that has been reported upon a great deal is this “meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease”, which reports no correlation between saturated fat and heart disease. Significantly, this study makes no differentiation between patients who replaced saturated fat in their diets with poly- and mono-unsaturated fats, and those who replaced it with carbohydrates, meaning that it is difficult to separate the role of saturated fat in these cases from the role of sugars: if decreasing saturated fat intake also increased sugar intake, then the effect on cardiovascular disease could be due to either of these factors (or lots of other factors), so this doesn’t help us answer the question.

A meta-analysis which does look at the difference is this one from the Cochrane collaboration, which found that replacing saturated fats with unsaturated fats decreased risk of heart disease, while replacing saturated fats with carbohydrates did not decrease the risk.

I couldn’t find a meta-analysis of studies on the link between sugar and heart disease – perhaps because interest in this idea is quite new. I’ve found a recent study that showed a link between consumption of added sugar (i.e. sugar not from fruit, milk, or other intrinsic sources, but from sugary drinks and processed foods) and heart disease. It seems like far more studies on this are needed. It would have been surprising to find out that sugary drinks are good for you, but it’s not necessarily obvious that they would correlate with heart disease. The current theory seems to be that sugar, like saturated fat, increases the level of “bad” cholesterol in the bloodstream.

There’s quite a good meta-metaanalysis, where scientists pick over the evidence for dietary advice regarding heart disease risk. The conclusion basically encapsulates my favourite systems biology maxim: “it’s a bit more complicated than that”. In general, replacing saturated fats with poly-unsaturated fats reduces heart disease risk, but some saturated fats may have different effects than others, and other things present in foods high in saturated fats might change its effects. In general, replacing saturated fats with carbohydrates doesn’t reduce heart disease risk, but if the carbohydrates are unrefined (e.g. wholegrains) then perhaps it does reduce heart disease risk.

The multifactorial nature of heart disease probably goes some way to explaining the “French paradox” – the fact that the French diet has more saturated fat than the UK diet, and yet they have lower levels of heart disease (incidentally, there are yearly stats published on heart disease in Europe that make for fascinating reading). There are multiple differences between the diets of our countries, and between other lifestyle factors such as activity levels, smoking, drinking, and weight.

The change in advice regarding cholesterol is brought up a great deal to discredit the link between saturated fat and heart disease (even though the levels of cholesterol in your blood are linked to your risk of heart disease, the levels of cholesterol that you eat aren’t something you need to worry about, so you can be like Gaston and eat five dozen eggs in the morning to help you get large).

Advice on this, and on other things, has changed over the years – trans fats, previously thought to be a healthy alternative to saturated fats, are actually way worse, and high fructose corn syrup is probably far less healthy than sugar (although please note that in the UK we don’t really eat much of either, and that fructose is no reason to avoid eating fruit). There is a lot of controversy over the link between saturated fat and heart disease, and I think it’s fairly clear that it’s not the sole culprit – in human biology, there is rarely ever a single reason for a phenomenon. Broadly, the answer to the question “Which should I be avoiding?” is “both” (to a certain extent – obviously, you need a little of everything in your diet).

From searching through the studies, I can’t find a lot of compelling evidence for increasing saturated fat in one’s diet, so arguing that we’ve been misled about saturated fat entirely feels like rather a leap. I’m looking specifically at heart disease here, not necessarily weight loss (there’s a whole other blog post on that, I think). It’s very possible to reduce saturated fat without increasing your intake of refined carbohydrates, and the evidence supports the idea that replacing saturated with poly-unsaturated fats either decreases one’s risk of heart disease or, at worst, doesn’t have any ill effects on one’s health. Reducing sugar intake is a good idea for multiple other reasons anyway (your teeth and your risk of diabetes, for example), so if it also reduces your risk of heart disease then that’s a bonus.

I think arguments like this come about from attempts to create a healthy diet that looks completely identical to an unhealthy diet, with certain nutrients replaced with others, when the easiest path is to encourage what we know to be healthy – a diet much, much higher in fruits and vegetables than is currently the norm in the UK. Things seem to be slowly moving in that direction, especially with the rise of more convenient ways of getting one’s “five a day”. We have to change the world to either make people’s lives easier, giving them more time to cook and more money for buying lovely fresh ingredients, or make it easier to get healthy food cheaply, by heavily subsidising healthy options or opening heavily-subsidised healthy cafeterias where people can get good food quickly and cheaply.

tl;dr: Eat some salad. Or don’t, it’s your body, not mine.