Nutritionists have embraced the concept of wholegrains with a passion claiming they are protective foods. But the evidence is inconsistent, which may be due to the methodology of the studies that have been undertaken so far. As the dietary fibres found in some grain foods are beneficial for bowel function and lowering blood cholesterol there is a case for basing cereal recommendations on fibre. Other aspects of the nutritional quality of cereal goods, such as nutrient density and glycaemic index are also relevant. Just recommending wholegrains is not enough.
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 …
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.
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.
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.