When nutrition experts review the same data and come to different conclusions we have to ask why?

Nutrition experts review scientific evidence for different reasons. On the one hand a review might be conducted during the development of healthy eating advice for the general public – this has occurred recently in Australia during the preparation of the new Dietary Guidelines. In the United States and Canada, another reason for reviewing scientific evidence is to apply for a health claim – a government-sanctioned claim for use on food packaging. Such claims are a boon to marketing and are highly sought after. Whatever the motive for a scientific review on a particular topic, the results should be pretty much the same. Evidence is evidence. But it doesn’t always turn out that way.

Canadian wholegrain review for a health claim

In July, Health Canada published the results of a systematic literature review of the evidence linking the consumption of wholegrains with protection against coronary heart disease. The results make interesting reading.

When all the good intervention trials were assessed together it was clear that wholegrains lowered total and LDL-cholesterol, which would be expected to lower risk for heart disease. However, on closer inspection it became clear that blood cholesterol was only lowered in those trials of grains that were rich in beta-glucan – a soluble dietary fibre abundant in oats and barley. Health Canada had already accepted a health claim for oat products and cholesterol-lowering but it was obviously not in any mood to broaden its health claim to include all wholegrains.

Cholesterol-lowering caused by grains high in beta-glucan fibre cannot be generalized to other grains, including wheat, the predominant grain consumed in Canada. A whole grain and coronary heart disease health claim would, therefore, be misleading if applied to grains that are not high in beta-glucan fibre.

Ouch! I think that was Health Canada telling the food companies to stop making unproven health claims in their marketing of wholegrain products.

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Dietary Guidelines review

Although the systematic review of wholegrains and heart disease conducted for the new Dietary Guidelines (page 244) covered the same territory it came to a different conclusion. There was agreement on the cholesterol question:

Almost all the [randomised controlled trials] have been conducted with oats, and there is evidence of beneficial lowering of levels of LDL and total cholesterol, but longer term studies and with other grains are needed.

So far, so good – the two reviews are in line. But then:

Nonetheless there is a strong body of evidence from cohort studies of the protective effect of wholegrain foods in general, which in the studies from the US are primarily wheat-based, and therefore relevant to the Australian diet. Therefore the evidence statement is not restricted to cereal foods high in soluble fibre.

The Canadian view on the same series of studies was:

Only six prospective cohort studies were eligible for inclusion and these were limited by potential bias due to confounding factors and poor applicability to the general population of Canada.

Health Canada’s position is conservative. For them, there is a missing piece of the puzzle – a clear mechanism of action by which wholegrain wheat provides protection against coronary heart disease. It is just not clear whether there is a true effect. If there is, we need to know whether the benefit is driven by fibre or some other component of whole wheat. With oats, the mechanism is clear.

Is our advice on wholegrains ‘misleading’?

Where does this leave our Dietary Guidelines’ advice to eat ‘mostly wholegrain’ cereal foods, which is at least partly based on the premise that these foods are protective against heart disease? If Health Canada’s argument is carried through it would suggest that the Dietary Guidelines’ embrace of all wholegrains is to some extent ‘misleading’. Perhaps this language is too strong but the local nutrition authorities have certainly made a leap of faith that the Canadians were not prepared to take.

What leads nutritionists to take that leap of faith when the scientific evidence is not quite good enough?

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The whole foods philosophy

In recent years a whole foods philosophy has crept into the practice of nutrition. This argues that a whole food, such as whole wheat, is a more nutritious option than a foods derived from that whole food, such as white flour. In some cases this is undoubtedly true. However, it’s also an argument that appeals to something deep in the souls of nutritionists – minimally processed, whole foods must be good for us. It’s what nature intended. But as a consequence both subjectivity and objectivity are now shaping our nutrition advice. Let’s subject the whole foods philosophy to a little scrutiny.

Take the oats example. If the cholesterol-lowering component is the outer fibre-rich part of oats, wouldn’t oat bran be a useful part of a diet for someone at high risk for heart disease? It may not be a whole food but so what? Oat bran may actually be better than whole oats in this instance.

More examples come to mind. Nuts and seeds are good for heart health but so are the vegetable oils derived from them. Oils may not be defined as whole foods by those who make the definitions but the evidence for their benefit is strong. And they are pretty useful around the kitchen too, increasing the likelihood that people will actually consume them.

If recommending whole foods is to be our guiding philosophy, shouldn’t nutritionists recommend whole dairy products rather than low fat-dairy foods? As discussed in my last post, this would be hard to justify. Should nutritionists recommend whole meat products, fat and all? I don’t think so.

Philosophy or science?

Superficially, the whole foods philosophy is very appealing and it also ties in with a nostalgic desire among sections of the general public for simpler foods and a simpler life in a hectic modern world. The food companies are responding to the same need by applying the words ‘natural’ and ‘organic’ wherever they can. In this mist of mellowness, marketing and heartfelt beliefs it is worth asking the question of whether nutritionists’ professional advice should be guided by a philosophy at all, or guided by their science.

Isn’t the role of the scientifically-trained nutritionist to remain clear-headed and provide accurate, objective nutrition advice to those who depend on us? Shouldn’t we be saying that ‘natural’ has no meaning in nutrition science – it’s a marketing term – and that ‘organic’ denotes a means of production and is not an indicator of nutritional quality. And on the issue of wholegrains and heart health, the message is that the evidence is encouraging but not convincing. It’s okay to say that. You won’t be excommunicated.

Nutrition is a science, not a faith.