Carrie Ruxton PhD RD
Public health recommendations are now more likely to include mention of whole grain (WG) foods while the term is a more common feature of food labels, helping to draw the public's attention to dietary sources. Yet confusion remains about what constitutes a portion of WG and how foods rich in these may impact on health. This article will consider the scientific evidence for WG, particularly with reference to medical conditions for which dietitians are often asked to give advice.
What does 'whole grain' mean?
The term WG refers to the edible grain of cereals which include the entire bran, endosperm and germ (the hull and glume which are inedible are removed)1. After processing, and when different varieties of the same grain are combined, similar proportions of the bran, endosperm and germ must be present in the end product (compared with the original grain) in order to qualify for a 'whole grain' classification. Similar definitions are used by the Healthgrain Consortium2 which suggests that WG "shall consist of the intact, ground, cracked or flaked kernel after the removal of inedible parts such as the hull and husk".
Recommendations and intakes
The UK currently has no specific recommendation for WG, instead promoting a daily fibre dietary reference value of 18g non-starch polysaccharide (range 12-24g). However, official websites, such as NHS Choices, do advice that more WG foods are consumed as part of a heart health diet. Other countries, such as the US, Australia and the Nordic nations recommend three to four portions of WG daily (see Table 1) while Germany, France and Switzerland give general advice about including WGs in a healthy, balanced diet.
Intakes of WG foods remain lower than recommendations. In the US, a national survey noted that adults ate 6.7 servings of grain products daily, but only one of these was WG. Breakfast cereals and bread constituted around one-third of WG servings, grain-based snacks represented one-fifth, while smaller amounts came from pasta, rice and baked foods8. In the UK, WG intakes were calculated from a secondary analysis of two national dietary surveys9. The results showed that 90% of adults were not meeting the US recommendation of three WG portions per day. This may be due to confusion about what foods contain WG, or the lack of official guidelines.
Evidence supporting increased WG consumption has tended to come from observational studies, which are not suitable for determining cause and effect. However, more recently, the evidence-base has been boosted by several randomised controlled trials (RCT) and mechanistic studies.
Heart health is one of the main targets of WG advice as observational studies have found clear links between WG consumption and reduced risk of hypertension8 as well as slowed atherosclerotic progression10. A systematic review11 found that adults eating three or more portions of WG cereals daily had a 20-30% reduced risk of cardiovascular (CV) disease compared with those eating the lowest intakes, while a meta-analysis of 10 RCT concluded that WG foods, particularly oats, consistently reduced total and low-density lipoprotein cholesterol12. More recently, a 12 week RCT13 in 206 adults with CV risk factors found that three portions of WGs daily significantly reduced blood pressure, to an extent which was estimated to lower stroke risk by 25%.
Prospective observational studies report consistent associations between WG intake and a lower risk of type 2 diabetes. In a US study of 72,215 women14, a dose-dependent relationship was seen between daily WG consumption and risk of developing type 2 diabetes. Eating at least two servings daily was associated with a 43% reduced risk. This was confirmed by a meta-analysis15 which found a 32% lower risk of developing type 2 diabetes when three portions of WGs were eaten daily. A few intervention studies have looked at the impact on glycaemic control when WG consumption is increased. A 12-week RCT16 in 61 middle-aged adults found that switching to WG cereals significantly reduced postprandial insulin and triglyceride responses, but not glucose. In one acute study17, boosting the WG and fibre content of white bread resulted in significantly lower postprandial glucose and insulin levels.
Interest in weight management and satiety has led to several intervention trials using WG foods. An 18-month RCT in 113 overweight women noted a significantly reduced waist circumference when refined carbohydrates were swapped for four daily servings of WG foods18. In a 12-week study, two portions of breakfast cereals containing 3g oat beta-glucan daily led to significant reductions in waist circumference but no change in body weight19. A 3-year double-blind RCT in older adults found that eating three servings of WGs daily led to reductions in body mass index, percentage body fat and trunk mass20. The mechanisms involved may relate to improved satiety as a RCT found that consumption of WG barley significantly increased levels of satiety hormones and reduced feelings of hunger21.
Finally, WG intakes have been linked with better gut health. A meta-analysis22 of 25 prospective observational studies found that the risk of colo-rectal cancer reduced by 20% for every three daily servings of WG foods (considered to be 90g a day). This was confirmed by a longitudinal survey of 108,000 Scandinavian adults23.
The evidence suggests that WG consumption offers benefits for disease risk reduction, as well as contributing to weight management. The mechanisms may relate to food structure (e.g. faecal bulking, intestinal transit time, glycaemic response), antioxidant effects, or the presence of bioactive compounds, such as oligosaccharides which promote healthy colonic bacteria species. WGs are also a rich source of methyl donors and lipotropes, including methionine, betaine, choline, inositol and folate, which may protect against heart disease24. There is also the possibility that WG foods replace less healthy options, such as higher glycaemic carbohydrates. In relation to cholesterol-lowering effects, it is likely that soluble fibres in WG foods, such as beta-glucan, inhibit bile salt re-absorption from the small intestine leading to enhanced cholesterol excretion in faeces25.
While the benefits may be relatively clear from scientific evidence, WG foods in the UK are all but invisible due to the lack of specific guidelines on intakes and an inconsistent approach to WG portion labelling. Consumers may wish to eat more WG foods but cannot easily identify good sources or obtain reliable information on portions. This is where dietitians could play an important role in advising which foods are rich in WGs and how many portions are appropriate for different age groups. Table 2 presents a selection of foods that provide a portion of WGs.
Source: Whole Grains Council http://wholegrainscouncil.org/whole-grains-101/what-counts-as-a-serving
Several European health claims have been approved for specific WG ingredients which could increase consumers' exposure to WG claims on labels. These include claims that oat beta glucan lowers blood cholesterol, wheat bran fibre accelerates intestinal transit and increases faecal bulk, and rye fibre contributes to normal bowel function.
In conclusion, WG foods are a valuable part of a healthy, balanced diet and could help to lower chronic disease risk. Excellent sources of WG include breakfast cereals, bread and grain-based snacks. Improved labelling and an official WG recommendation would help to boost WG intakes in the UK.
This work was funded by the Breakfast Cereal Information Service, an independent information body set up to provide balanced information on breakfast cereals. It is supported by a restricted educational grant from the Association of Cereal Food Manufacturers. The content reflects the opinion of the author.
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