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.

When it comes to recommendations about grain foods the draft Australian Dietary Guidelines keep it simple: eat mostly wholegrain. The rationale is simple too: wholegrains are nutrient-rich and protective against a range of modern diseases such as cardiovascular disease, type 2 diabetes and obesity.

But is it true? How evidence-based is this advice?

Wholegrains and the prevention of disease

Superficially the wholegrain story looks very good. Prospective cohort studies consistently show that higher intake of wholegrains is associated with lower risk for cardiovascular disease, finding a reduction in risk of about 20 percent when high wholegrain-eaters are compared to those who eat little wholegrain. The risk for type 2 diabetes also appears to be lowered by eating wholegrains, the risk reduction being 27-30 percent. And higher wholegrain intake is associated with lower body mass index. In some studies higher intakes of wholegrains have also been associated with lower levels of inflammatory markers in the blood, suggesting that wholegrain consumption is damping down underlying disease processes. Based on studies such as these the draft Australian Dietary Guidelines report states: The evidence for the association of grain (cereal) foods (mostly wholegrain) with reduced risk of cardiovascular disease, type 2 diabetes and excess weight gain has strengthened …

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Reality check

It is important to remember that all of the above evidence is drawn from observational studies. They show associations between wholegrain consumption and health benefits but this type of study can’t prove that the wholegrains are actually responsible for the benefits. In fact, the results can be interpreted in two ways: (1) eating wholegrains makes people healthy, or (2) healthy people eat wholegrains.

As it turns out wholegrain-eaters are indeed healthy people – they tend to smoke less, exercise more and have better diets. Although researchers do their best to allow for these confounding factors in their analyses there is always the risk of a false signal about wholegrains that is actually due to the cluster of healthy behaviours typical of wholegrain-eaters.

Other lines of evidence need to be considered, so let’s look at the intervention studies. If wholegrains provide protection against cardiovascular disease and diabetes they would be expected to operate through the well-established risk factors for these diseases. So the task for the researcher is relatively straightforward – feed people wholegrains and measure changes in risk factors. The first large scale intervention study of this kind was the WHOLEheart Study conducted in the United Kingdom. The subjects substantially increased their wholegrain intake for four months and the researchers looked for effects on body weight, percentage body fat, waist circumference, blood lipids, glucose and insulin, and indicators of inflammation, coagulation and blood vessel function. The results? Nothing. Eating wholegrains had no significant effect on any of these markers of risk. There were similar findings in a more recent intervention study, although this study did find a beneficial effect of wholegrains on blood pressure.

Phytochemicals

Health benefits from wholegrains are sometimes attributed to phytochemicals – minor components found in the bran and germ of grains such as betaine, phenolic acids, carotenoids, tocotrienols and plant sterols. They are not essential nutrients but are biologically active and could conceivably affect health. However, the limited effects of wholegrains on risk factors for cardiovascular disease and diabetes seen in the intervention studies suggest any effects of these phytochemicals are relatively weak.

So rather than the evidence strengthening, it appears that the different lines of evidence linking wholegrains with prevention of disease are inconsistent. Importantly, a plausible mechanism for protection is yet to be identified.

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Dietary fibre and health

Lest this review be seen as a bit too sceptical we should consider the known health effects of one of the best-researched components of wholegrains – dietary fibre. The insoluble fibre found in the bran of wheat has a beneficial effect on bowel function, promoting the regular passing of soft stools. Bearing in mind the relatively high prevalence of constipation there is only benefit to be gained from dietary recommendations encouraging increased intakes of all sources of wheat fibre.

Another well-researched subject is the blood cholesterol-lowering effect of the soluble fibre in oats, achieved via changes in bile acid metabolism. This would be expected to lower the risk for coronary heart disease. In the draft Australian Dietary Guidelines the discussion on the mechanism by which wholegrains lower heart disease states Almost all the high level trials were conducted with oats. Therefore the benefit can be ascribed to oats or to soluble fibre, but cannot be generalised to wholegrains. Many wholegrains are not good sources of soluble fibre. All wholegrains are not alike.

The known health benefits of dietary fibre suggest that any effects of wholegrains may be due, at least in part, to their fibre content. In fact, some of the prospective studies showing that wholegrains are protective also show  that cereal fibre is protective, to a similar degree to wholegrains. Digging deeper, the definitions of wholegrain in these studies vary widely and in some instances include bran-based foods (which by definition are not made from whole grains). To muddy the waters further, wholegrain foods vary widely in their dietary fibre content.

So should dietary recommendations for cereal foods be based around the concept of wholegrains or dietary fibre? Or maybe both?

The shortcomings of wholegrains

There are a couple of shortcomings of wholegrains that nutritionists seldom mention but I will address them here. The draft Australian Dietary Guidelines refers to wholegrains as highly nutrient-dense and they are contrasted with low nutrient-dense foods like white bread. This statement is simply not true. All unfortified cereal foods are relatively nutrient-poor, with wholegrain foods being marginally better than refined cereals. White bread is actually more nutrient-rich than many wholegrains because it is enriched with nutrients. When dietary surveys find that cereal foods are major sources of B group vitamins and iron it is important to remember that this is largely due to the enrichment of breads and breakfast cereals with these nutrients.

Many wholegrain foods also have high glycaemic indices and when eaten in quantity contribute to a high glycaemic load. Evidence is accruing that this issue may be relevant to the risk for coronary heart disease, diabetes and weight management. I’ll address glycaemic index in depth next week.

When all the variables are considered it is apparent that the term wholegrain is not necessarily synonymous with good. For example, brown rice is a wholegrain and is a good source of fibre, but it is also a nutrient-poor food and has a high glycaemic index. Is it good?

Recommending cereal foods

The recommendation to eat mostly wholegrains sits comfortably with the dietetic mind-set that minimally processed, whole foods must be nutritious. But one has to question the wisdom of embracing the concept of wholegrains to the exclusion of other important aspects of the nutritional quality of grain-based foods, such as dietary fibre, nutrient density and glycaemic index. Several submissions on the recent draft Australian Dietary Guidelines encouraged the authorities to recommend both wholegrain and high fibre cereal foods. The latest Dietary Guidelines for Americans recommend wholegrains and enriched grains, which at least addresses the nutrient density issue.