Saturated fats – friend or foe?

pack-butterA reduction in dietary fat consumption, especially saturated fat, has been at the cornerstone of official dietary advice for as long as most of us can remember. Saturated fats are primarily animal fats, including dairy fat – think cheese, butter, lard, eggs – and from a few vegetable sources such as coconut oil. For example, the NHS Live Well website lists reduction in saturated fats as a key objective, suggesting that: ‘Eating a diet that is high in saturated fat can raise the level of cholesterol in the blood. Having high cholesterol increases the risk of heart disease.’ As a guideline amount the recommendation is set at no more than 30g of saturated fat a day for men, and for women it’s no more than 20g a day. On the other side of the pond the official advice in the Dietary Guidelines for Americans suggests reducing saturated fats so that they supply less than 10% of daily calories. The rationale is similar – saturated fats are associated with obesity and risk factors for cardiovascular disease. This is effectively the consensus opinion from government health agencies across much of the world, not just the UK and US However, the question we have to ask is does the evidence support this advice?

The obvious point is to ask what evidence there is for the core idea that reducing dietary saturated fat intake makes a difference to health. The answer is that there’s not much. A good place to start is to look at the evidence from clinical trials where dietary fat intake has been modified to see what it does in terms of health and those cardiovascular risk factors. What we’re interested in is human data here, not animal models. More than that, we’re interested not in the molecular effects or the variation in differing serum lipid fractions, we’re interested in actual health outcomes. Luckily some of the hard work in looking at the data has been done for us by the good people at the Cochrane Collaboration (a global not-for-profit project involving thousands of scientists working to analyse data and perform systematic reviews of evidence in medicine).

The Cochrane Heart Group have carried out a meta-analysis of clinical trials that looked at both dietary fat reduction and modification to see what effect these had in cardiovascular disease. Where the meaning of fat reduction is clear, fat modification means swapping the relative share of types of fat (saturated, polyunsaturated and monounsaturated) in the diet. The review looked at the data from large interventional trials that tracked health outcomes over a reasonably span of time (> 6 months). In all the data covered 48 randomised clinical trials, and the headline result is plain – there is no clear effect of dietary fat changes on total mortality or cardiovascular mortality.

Does this mean that there are no health outcomes associated with dietary fat change? Not quite. When looking at cardiovascular events (heart attacks, strokes etc) the results are a bit more positive. Overall there is a 14% reduced risk of cardiovascular events due to dietary fat change. Unpacking this result to get at the detail shows that the effect is down to a reduction in risk of men who modify fat intake for more than two years, though it’s not clear what the specific modification should be. While this is certainly a reduction in risk – albeit modest – for a specific population, it is hardly indicative of a major problem in the population at large. And looking at secondary measures, such as cancer incidence or mortality, it appears that dietary fat reduction or modification has minimal effect.

This Cochrane review, last updated in 2012, is not the only one. A more recent, and highly controversial, meta-analysis published in 2014 came to similar conclusions, though not without concerns raised about some of the methodology.

Be that as it may, say you want to lose a few pounds and at the same time reduce your cardiovascular risk factors along the lines of the dietary advice, is there anything we can learn from the data? A couple of recent clinical studies shed more light on the matter.

A team led by Lydia Bazzano and Tian Hu, at Tulane University in New Orleans, conducted a clinical trial in 148 adults without diabetes or cardiovascular disease but at the high end of the BMI scale (30 – 45 kg/m2). One group followed a low carbohydrate diet (less than 40 g of carbs a day) and the other followed a low fat diet (<35% calories from fat, and <7% from saturated fat). Note that both diets are pretty aggressive, and that the low fat diet reduces the saturated fat quite considerably. The trial followed the two groups for a year, and as you’d expect there was a certain degree of dropping out – around 20% in each case. The metrics of interest in the trial are weight loss and cardiovascular risk factors – not disease end points such as heart attacks or strokes.

At the end of the year both groups had lost weight, though the low carb group experienced a greater reduction (-5.3 kg versus -1.8 kg, the difference of 3.5 kg being statistically significant). And it’s not just the weight, the waist sizes shrink more with the low carb diet than the low fat diet – though not to a statistically significant extent.

In the next post we’ll see what else comes from looking at the clinical data.

[A version of this article has previously been published in the Register:]

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