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Whole grains and health

03 September 2014


Carrie Ruxton PhD RD

Freelance Dietitian

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.

Figure 1

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.

Health impacts

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.

Discussion

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.

Figure 2
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.

Acknowledgement

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.

References

1 AACCI (1999) Whole grain task force, 1999. Definition of whole grain. Available at: http://www.aaccnet.org/initiatives/definitions/Pages/WholeGrain.aspx

2 Healthgrain Consortium (2010) Whole grain definition. Available at:http://www.healthgrain.org/webfm_send/44

3 Grains & Legumes Nutrition Council (2013) Grains & Legumes Nutrition Council: Positioning Statements. Available at: http://www.glnc.org.au/4th/positioning-statements

4 National Food Institute (2008) Summary report. Available at, http://www.wholegrainscouncil.org/files/WholeGrainsinDenmark.pdf

5 Swedish Wholegrain Guidelines (2012) Available at, http://www.slv.se/sv/grupp3/Nyheter-och-press/Nyheter1/Nytt-rad-om-fullkorn-ersatter-brodrad/

6 Whole Grains Council (2012) Existing dtandards for whole grains. Available at: http://www.wholegrainscouncil.org/whole-grains-101/existing-standards-for-whole-grains

7 UDSA (2013) Food Guide Pyramid. Available at: http://www.cnpp.usda.gov/FGP.htm

8 Flint AJ et al. (2009) Whole grains and incident hypertension in men. Am J Clin Nutr 90: 493-8.

9 Lang R et al. (2003) Consumption of whole-grain foods by British adults: findings from further analysis of two national dietary surveys. Pub Health Nutr 6: 479-84.

10 Mellen PB et al. (2007) Whole-grain intake and carotid artery atherosclerosis in a multiethnic cohort: the Insulin Resistance Atherosclerosis Study. Am J Clin Nutr 85: 1495-502.

11 Gil A et al. (2011) Wholegrain cereals and bread: a duet of the Mediterranean diet for the prevention of chronic diseases. Pub Health Nutr 14: 2316-22.

12 Kelly SA et al. (2007) Wholegrain cereals for coronary heart disease. Cochrane Database Syst Rev 18: CD005051.

13 Tighe P et al. (2010) Effect of increased consumption of whole-grain foods on blood pressure and other cardiovascular risk markers in healthy middle-aged persons: a randomized controlled trial. Am J Clin Nutr 92(4): 733-40.

14 Parker ED et al. (2013) The association of whole grain consumption with incident type 2 diabetes: the Women's Health Initiative Observational Study. Ann Epidemiol 23: 321-7.

15 Aune D et al. (2013) Whole grain and refined grain consumption and the risk of type 2 diabetes: a systematic review and dose-response meta-analysis of cohort studies. Eur J Epidemiol 28: 845-58.

16 Giacco R et al. (2014) A whole-grain cereal-based diet lowers postprandial plasma insulin and triglyceride levels in individuals with metabolic syndrome. Nutr Metab Cardiovasc Dis. (E-pub).

17 Ekström LM et al. (2013) On the possibility to affect the course of glycaemia, insulinaemia, and perceived hunger/satiety to bread meals in healthy volunteers. Food Funct 4: 522-9.

18 Venn BJ et al. (2010) The effect of increasing consumption of pulses and wholegrains in obese people: a randomized controlled trial. J Am Coll Nutr 29: 365-72.

19 Maki KC et al. (2007) Effects of high-fiber oat and wheat cereals on postprandial glucose and lipid responses in healthy men. Int J Vit Nutr Res 5: 347-56.

20 McKeown NM et al. (2009) Whole-grain intake and cereal fiber are associated with lower abdominal adiposity in older adults. J Nutr 139: 1950-5.

21 Johansson EV et al. (2013) Effects of indigestible carbohydrate in barley on glucose metabolism, appetite and voluntary food intake over 16 h in healthy adults. Nutr J 12: 46.

22 Aune D et al. (2011) Dietary fibre, whole grains, and risk of colorectal cancer: systematic review and dose-response meta-analysis of prospective studies. BMJ 343: d6617.

23 Kyrø C et al. (2013) Intake of whole grains from different cereal and food sources and incidence of colorectal cancer in the Scandinavian HELGA cohort. Cancer Causes Control 24: 1363-74.

24 Fardet A (2010) New hypotheses for the health-protective mechanisms of whole-grain cereals: what is beyond fibre? Nutr Res Rev 23: 65-134.

25 Gunness P & Gidley MJ (2010) Mechanisms underlying the cholesterol-lowering properties of soluble dietary fibre polysaccharides. Food & Function 1: 149-55.