02 June 2016 09:07:35

Another nail in the coffin of unsaturated fats?

The results of a study which got media headlines this week certainly seem to fly in the face of accepted medical wisdom on diet - far from cutting death rates, the people in the low-saturated fat diet were more likely to die. But the trouble with research is that there are so many variables to take into account.

Another day, another headline about how doctors and health gurus have had it wrong all these years. This time it's from a second look at data from a study dating back to the late 1960s, in which 2,355 people were randomly assigned to one of two diets for at least a year. Half ate a diet that had 'standard' amounts of saturated fat, with the polyunsaturated fat linoleic acid added; the other half ate a diet in which much of the saturated fat was swapped for linoleic acid in the form of corn oil and corn oil polyunsaturated margarine.

The results certainly seem to fly in the face of accepted medical wisdom on diet - far from cutting death rates, the people in the low-saturated fat diet were more likely to die. But the trouble with research is that there are so many variables to take into account.

We have to be very careful in science to tease out the difference between 'associations' and 'causal links'. There are lots of studies which show that substituting unsaturated fats for saturated lowers cholesterol (1). There have also been multiple studies showing that risk of heart attack, and death from heart disease, is inversely related to cholesterol levels. But does that automatically mean that cutting down on saturated and increasing unsaturated fat automatically protects against heart disease?

Let's take it to extremes to explain my concerns about direct causal links. Women who wear tights are statistically much more likely to get breast cancer. This is an association - but can we say there's a causal link (in other words that wearing tights actually causes breast cancer)? To make this leap we have to exclude so-called 'confounding factors' - a third factor that is related to both the others and which makes it look as if the two are connected. In the case of tights and breast cancer, the 'confounding factor' is where you live and how this affects your lifestyle. In very hot countries like Africa (where it's often too hot to wear tights), women are less likely to be obese, which we know contributes to breast cancer. They tend to have more children, which protects against breast cancer. And on average they die younger, so are less likely to live long enough to get breast cancer. It's these confounding factors, not the wearing of tights, that contribute to the apparent connection.

On the plus side, the people in the two halves of this diet study were 'matched', so there were similar numbers in each group of a given age and gender. The study also matched the participants for other factors, such as blood pressure, cholesterol levels and body mass index, confounding factors which can affect the risk of heart disease. Importantly, though, we have no idea of whether similar numbers of people in the two groups smoked.

But there are lots of unknowns that remain. The link between lower cholesterol level and higher death rate seems only to have affected those aged over 65 - among the under 65 year olds, cholesterol changes didn't have any impact on death rates. Nor was there any link between the degree of atherosclerosis (furring up of the arteries) or heart attack and cholesterol levels in autopsy studies - although these data were sketchy and therefore shouldn't be relied on.

The patients in the study were inpatients either at nursing homes or state-run psychiatric hospitals. They are hardly representative of the population at large, and may have suffered from very different medical conditions from the average man on the street today. The study didn't look at what they ate apart from the study food they were given - if they didn't like the taste of their corn-oil diet, they might have regularly persuaded visitors to bring them other food instead. It's also possible (but unlikely) that the patients in the low-saturated fat diet were fed higher levels of 'trans fats', which could have increased their risks.

So overall, I don't plan to change the recommendations I give my patients on the basis of this study. Other studies in the last couple of years have tried to turn our diet thinking on its head, only for us to discover that we needed to look at the bigger picture. For instance, two years ago a study suggested there was no difference in heart attack rates between a low- or high-saturated fat diet - but failed to tell us if this finding was because people on the low-sat fat diet ate more sugar.

Last year another study showed that replacing 5% of energy intake from saturated fats with the same number of calories from either polyunsaturated fats, monounsaturated fats, or carbohydrates from whole grains did lower your risk of heart disease. Swapping 5% of saturated fat calories for sugar calories didn't (2).

This study is certainly not saying that saturated fats are good for us - and it's only looking at a tiny part of the diet picture. There's ample evidence that a Mediterranean diet which includes lots of fruit and veg, unrefined carbohydrates, less meat, more oily fish, more unsaturated fat and less saturated, does protect against heart disease (3).

There's no doubt that a Mediterranean diet contains less saturated fat than the average British one. It also has a great deal less sugar. Recent guideline updated definitions of 'added sugar' to include fruit juice, but not whole fruit - while juice can be a useful source of vitamins, it's best to stick to just a glass a day, and it only counts as one of your 'five a day' no matter how much you have. Low-fat spreads are a low-sat-fat alternative to butter, and versions containing plant sterols can lower your cholesterol. Low-fat yoghurts provide protein and calcium, but some are high in sugar - and these days it's about the whole picture of what you eat, not just one element.

Of course, national guidelines may change next year when the Scientific Advisory Committee on Nutrition (SACN) publishes its latest review on saturated fats. Until then, I suggest, it should be business as usual as far as your diet is concerned - with a pronounced Mediterranean twist.

References

1) Keys A, Anderson JT, Grande F. Serum cholesterol response to changes in the diet: IV. Particular saturated fatty acids in the diet. Metabolism1965;14:776-87. doi:10.1016/0026-0495(65)90004-1 pmid:25286466.

2) Saturated fat as compared to unsaturated fats and sources of carbohydrates in relation to risk of coronary heart disease: A prospective cohort study," Yanping Li, Adela Hruby, Adam M. Bernstein, Sylvia H. Ley, Dong D. Wang, Stephanie E. Chiuve, Laura Sampson, Kathryn M. Rexrode, Eric B. Rimm, Walter C. Willett, Frank B. Hu, Journal of the American College of Cardiology, online September 28, 2015, doi: 10.1016/j.jacc.2015.07.055

3) Estruch R et al. Primary prevention of cardiovascular disease with a Mediterranean diet. NEJM 2013 http://www.nejm.org/doi/full/10.1056/NEJMoa1200303

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