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Digesting the evidence

In prizeing scepticism over credulity, one must acknowledge that scepticism is but a hop, skip and jump from cynicism, a warm bath in which it is easy to wallow. At some point, however, meaningful engagement in a civilised society entails getting out of the bath and deciding whether there is a role for trust, and if so, where it might be safely placed. 

So it is reassuring to know that where medical science is concerned, if there’s one secure repository of the truth that can be trusted, it is in a gigantic virtual filing cabinet labelled ‘Published, peer-reviewed evidence’ and available at the click of a mouse. This means that those of us seeking, say, nutritional certainty and guidance need only consult published evidence that has been sifted, reviewed and rigorously assessed before publication and there — we are told — we’ll find the truth.

So evidence-based certainty hung heavy in the air when Malhotra et al in the British Journal of Sports Medicine (online 25 April 2017) published a piece whose title proclaims: ‘Saturated fat does not clog the arteries… ’ and whose text observes: “Despite popular belief among doctors and the public, the conceptual model of dietary fat clogging a pipe is just plain wrong.”

Having digested this encouraging news, it was with a conscience as light as a dinner in a high-priced eaterie that I helped myself to a generous slice of full-fat cheese. Another slice followed when I learned that Malhotra et al’s paper was published some months after Noakes and Windt adduced in the same journal: ‘Evidence that supports the prescription of low-carbohydrate, high-fat (LCHF) diets: A narrative review.’

To this mere scribbler, the above invited the inference that a high-fat diet is good for one’s health. My near-conversion to a LCHF diet, plus being a keen runner, ticked the recommended nutritional and exercise boxes, and all was well with the world. I even mounted another raid on the Camembert, having read Demmer et al’s report in the Journal of Nutritional Science (2016, 5:9) titled ‘Consumption of a high-fat meal containing cheese compared with a vegan alternative lowers postprandial C-reactive protein in overweight and obese individuals with metabolic abnormalities: A randomised, controlled, cross-over study’. They concluded that although current dietary recommendations promote reduced saturated fat consumption to lower the risk of cardiovascular disease, “saturated fat in the form of cheese lowers postprandial inflammation compared with plant sources of saturated fat”.

So it was with a heavy — and possibly atherosclerotic — heart that I remembered reading Dr Muiris Houston, writing in The Irish Times (11 April 2017), describe a Lancet study (online 17 March 2017) as adding “to a growing belief that a diet high in carbohydrates [my italics] is actually good for our health”.

Heaving a sigh, I abandoned my plans to consume a side of beef and whipped up a bowl of pasta, but not before I found the Lancet study cited by Dr Houston. Titled ‘Coronary atherosclerosis in indigenous South American Tsimané: A cross-sectional cohort study’, it concluded that this “ forager-horticulturalist population of the Bolivian Amazon with few coronary artery disease risk factors have the lowest reported levels of coronary artery disease of any population recorded to date”.

Dr Houston highlights “a growing belief that a diet high in carbohydrates is actually good for our health”. Yet it is a belief whose growth could risk being arrested by the downward pressure of evidence which appears to challenge this view. The latest addition I can find (at the time of writing) is from Dehghan et al, whose report in The Lancet (online 29 August 2017) investigated ‘Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): A prospective cohort study’. They found that “high carbohydrate intake was associated with higher risk of total mortality, whereas total fat and individual types of fat were related to lower total mortality”. The authors also suggest that “global dietary guidelines should be reconsidered in light of these findings”.

But reconsidering the guidelines — and given the heated debates that the current controversy over saturated fat versus carbohydrates is generating — it seems that once an evidential tipping-point has been reached, one way or the other, there may be large helpings of (low-fat or low-carb?) humble pie on offer. Someone brave enough to sample it is Prof Tim Noakes, whose book Lore of Running (2001) promoted high levels of carbohydrate consumption to boost athletic performance — advice which he now says is wrong.

Meanwhile, many nutritional authorities appear to view the presence of saturated fat in food as a dietary bête noire. For example, the www.diabetes.ie website states: “The Department of Health keeps dietary recommendations under review as part of its role in promoting evidence based public health.” In relation to the recently-published new Irish food pyramid for healthy eating, the Department’s evidence base in support of, for example, its recommendation that where milk, yogurt and cheese are concerned, “choose reduced-fat or low-fat varieties” appears, let’s say… selective.

Is medical science’s nutritional evidence base as trustworthy as many would have us believe?

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