People who eat more whole grain foods are less likely to suffer from coronary heart disease, type 2 diabetes and some cancers. The data supporting this statement are mostly from detailed analyses of large-scale population (epidemiological) studies carried out in the US and Europe, which show very strong inverse relationships between whole grain consumption and risk of disease - in other words the more whole grains consumed, the lower the risk of disease.
The evidence is perhaps strongest for heart disease with estimates suggesting about a 25% reduction in risk for the highest whole grain consumers compared with non consumers. The reduction in risk of type 2 diabetes is of a similar magnitude, and risk of a range of different cancers is also lower, but is more variable between different cancer types. Recent evidence has also emerged suggesting that whole grains may help in weight maintenance.
Although these health benefits are generally supported by the scientific community, governments across Europe have been reluctant to incorporate dietary targets for increased whole grain consumption as part of national dietary guidelines. This may, in part, be due to the lack of data from dietary interventions to support the epidemiological data.
In contrast, in the US, the message to increase the consumption of whole grain foods is now enshrined in the Dietary Guidelines for Americans, which recommend consuming three ounce equivalents of whole grains per day (three servings per day equivalent to 48g of whole grain dry matter per day). This level of intake has been derived from the epidemiological data although, once again, it has not been tested under experimental conditions in the general population.
Consumption of whole grains is low
The impact of this dietary advice has yet to be evaluated, but it is likely to take some time for the population target to be reached since whole grain consumption is known to be well below this level. In the UK, the latest information (from the 2000-1 National Diet and Nutrition Survey) shows that intake was just above one serving per day and more than 30% of the population ate no whole grains at all. Less than 5% of men and women surveyed achieved the three servings per day target.
Whole grains are the entire, edible seeds (kernels) collected from cereal plants of the Gramineae family of grasses. The most commonly consumed grains include wheat, maize (corn), rice, oats, barley, sorghum, spelt, and rye. Some other seeds used less frequently in foods but which have similar nutritional composition to true grains, include wild rice, buckwheat, quinoa, and amaranth, and are sometimes also included in the whole grain category.
The majority of grains are milled to flours for use in food manufacturing. To be included (and identified specifically) as whole grains in foods, the flours must contain all three anatomical parts of the grain: the bran, germ and endosperm in the same proportions as in the original grains.
Clear labelling and identification of whole grains in foods is essential in helping consumers correctly identify whether or not a food contains whole grains, as is establishing a standard definition that can be used by manufacturers.
The definition currently used in the US was produced by the American Association of Cereal Chemists in 2005 and emphasises the need to re-constitute the components of the grain described above. No similar definition exists in the UK and Europe, but this is urgently needed.
In addition to a standard definition of whole grain there is also a need for a definition and labelling system which can be used to identify whole grain foods. Health claims in the US and UK currently can only be used on foods that contain more than 51% whole grain; in Sweden a food must contain more than 50% whole grain to carry the claim.
To date, apart from some breakfast cereals, the claim is not extensively used, and the number of foods that qualify for the claim is, in any case, relatively small. In our analyses of the recent NDNS surveys, we identified 347 foods that contained more than 10% whole grain. However, the food categories covered were dominated by breakfast cereals, breads and baked products, which together accounted for over 90% of all whole grain foods eaten.
The Whole Grains Council in the US (www.wholegrainscouncil.org) is a consortium of industry, scientists and chefs dedicated to promoting increased whole-grain consumption.
The Council has petitioned for the use of stamps for use on food packaging on foods to help the consumer make informed choices on which foods to purchase. The stamps range from those for use of foods made with less than 100% whole grain but delivering a minimum of 8g of whole grain per serving up to those for use on foods made with 100% whole grain and delivering 16g of whole grain per serving.
These stamps provide a simple and quick way for consumers to identify healthy foods. A similar system endorsed by European governments would be a major step in helping consumer awareness. The EU is currently receiving petitions for inclusion of health claims under article 13 of the Nutrition and Health Claims Regulation and the current whole grain health claims used in the UK and Sweden will be submitted by those countries as well as petitions from other Member States.
Agreement on a unified whole grain health claim, together with standard definitions and labeling rules will be of great benefit to industry and the consumer.
A number of mechanisms have been proposed for how whole grains exert their health-promoting effects. The epidemiological evidence demonstrates the relationship between whole grain intake and improved health, however, such relationships do not demonstrate causality.
So far, mechanisms are speculative because evidence from large controlled dietary intervention studies does not exist although plausible explanations have been proposed. These include increased intake of antioxidant substances, bioactive components, such as plant lignans, phyto-oestrogens, soluble and insoluble fibre, phytates and phenolics.
Higher intakes of many of these components have been linked to improved immune function, antioxidant status, endothelial function and blood pressure, tumour suppression, and inflammation - all of which are known to affect chronic diseases. Some data from small studies with 'at risk' subjects have shown beneficial effects, such as reduced blood pressure in mildly hypercholesterolemic men and women and lower concentrations of systemic inflammatory markers in diabetic women.
Whole grain food consumption has also been linked to weight management and obesity control, but most of the evidence is from cohort studies which use self-reported weight and has not been confirmed in either weight loss studies or long term intervention studies. One major nutrient present in higher concentrations in whole grains compared with refined grains is dietary fibre. For some grains, such as oats, barley and rye, the fibre is predominantly soluble whereas for other grains, such as wheat, the fibre is predominantly of an insoluble form. The effect of consuming these grains, therefore, may be quite different.
The cholesterol-lowering effects of oats and barley have been well documented and health claims based on the presence of the soluble fibre beta-glucan is allowed for oats (in the UK and US) and for barley (in the US only).
Eating a fibre-rich diet has been promoted as a way to reduce cancer risk, especially for cancers of the lower bowel, although the effect of specific cereal products in reducing the risk of cancer is inconclusive.
Fermentation properties of fibre from whole grain may be a factor in many cases, lowering colonic pH and producing the short chain fatty acid, butyrate. The fibre in whole grains may also reduce intestinal transit time and increase faecal bulk, both of which dilute and reduce exposure to carcinogens.
Antioxidants present in whole grains may reduce oxidative stress in the gut lumen and intestinal cells. For some hormone-related cancers the beneficial effect of whole grains may be due to the presence of bio-active phytochemicals such as lignans, but more research is needed.
There is a common misconception that all whole grain foods are low glycaemic index (GI) compared with foods made with refined grain. Although some whole grain foods do have a lower GI than refined grain alternatives, this is not true for all of them. For example, wholemeal bread has a very similar GI to white bread, but has a slightly lower glycaemic load (GL). In contrast, whole grain (brown) rice has a lower GI than polished rice and whole grain pasta has a lower GI than refined pasta.
Thus, some whole grain foods may have an impact on GL and consequently on postprandial glucose and insulin responses. Long-term consumption may result in improved insulin sensitivity, and this may be the mechanism for the observed reduction in risk of type 2 diabetes but this has not been confirmed in a healthy population.
Studies in healthy subjects have shown that plasma insulin responses are significantly lower after eating wholegrain rye bread compared with a white wheat bread, and we have recently shown that eating whole grains for 16 weeks resulted in lower fasting insulin concentrations. We are currently testing this again in a much larger population as part of the WHOLEheart study, a wholegrain intervention funded by the UK Food Standards Agency.
There is strong evidence from epidemiological studies that consumption of whole grains is beneficial to health, reducing the risk of developing heart disease, type 2 diabetes and some cancers.
How whole grains exert these effects is unclear, but a number of processes may be involved which act synergistically to improve health. Whole grains and foods prepared from them are a complex mixture of nutrients and bioactive components.
Although individual components, such as dietary fibre, are known to have effects in isolation, it is the cumulative (and perhaps additive) effects of the different components that are key to providing the health benefit. This is a key area for future research.
Health claims are currently available for use of foods containing more than 51% whole grain, but identifying whole grains and whole grain foods is currently not easy for manufacturers and consumers alike. Standardisation and acceptance of these health claims across Europe and improving labelling on foods will be essential in the development and implementation of public health messages aimed at increasing consumption.
Dr Chris Seal is a senior lecturer in food studies at Newcastle University, UK.