Whole Grains, Dietary Fibre and Health

REGISTERED DIETITIANS

Dr. Slavin is a Professor in the Department of Food Science and Nutrition, University of Minnesota, St. Paul, MN. She has conducted many human feeding studies on dietary fibre, whole grains, fruits, vegetables, soy and flax. Besides her pursuit of the "scoop on poop", she teaches Life Cycle Nutrition and Human Nutrition. She has published more than ioo referreed scientific articles and speaks widely on choosing your carbohydrate sources wisely.

FIBRE: AN INTEGRAL PART OF WHOLE GRAINS

Whole-grain foods can be valuable sources of nutrients that are in short supply in the North American diet, including dietary fibre, vitamins, minerals and other phytochemicals.1 The most commonly consumed grains are listed in Table 1, and include wheat, oats, rice, corn, and rye, with wheat constituting 66-75% of the total.2

Buckwheat, wild rice, and amaranth are not botanically true grains, but are typically associated with the grain family due to their similar composition. The typical architecture of a whole grain is shown in Figure 1.

All grains have a bark-like, protective hull beneath which are the endosperm, germ, and bran. The endosperm represents 80% of the grain and is mostly composed of starch and protein, and contains comparatively few vitamins. The germ is a relatively minor contributor to the dry weight of most grains (typically 4-5% in wheat and barley) and is rich in unsaturated fats, protein, micronutrients and phytochemicals. The bran content varies depending on the grain but it provides all of the fibre and a large concentration of phytochemicals. The fibre content of whole grains can vary greatly, with whole wheat being concentrated in fibre and brown rice containing very little (Table 2).

Generally, grains are ground into flour and processed into grain-based foods. Most of the dietary fibre is lost in the milling process, with significant losses in calcium, magnesium, and potassium also particularly evident. White flour is enriched in thiamin, riboflavin, niacin, iron, and folic acid; some whole grain products are also enriched with folic acid. Processing of whole grains does not remove biologically important compounds.3 Analysis of processed whole grain breads and cereals indicate that they are a rich source of antioxidants.4 Processing may open up the food matrix, thereby allowing the release of tightly bound phytochemicals from the grain structure.5

While the most common source of dietary fibre in the North American diet is whole grain, consumption is still not enough to meet recommended fibre intakes. However, many processed foods can be enriched in dietary fibre through the addition of the bran portion of whole grain. This practice allows foods to achieve dietary fibre levels that are otherwise unattainable through the use of whole grains alone.

THE HEALTH BENEFITS OF WHOLE GRAINS & FIBRE

In reporting the health benefits of whole grain, epidemiologists have traditionally defined whole grain as cereals containing 25% or more whole grain or bran content by weight.6,7 As such, evaluation of the health benefits of whole grains has inherently included the benefits of fibre-rich bran. It is therefore important that dietary guidance reflect the need for whole grain foods to contain dietary fibre, to ensure that consumers derive the full range of benefits attributed to whole grains. It is important to keep this in mind as one reviews the health effects that follow.

Whole Grains, Fibre and Cardiovascular Disease

There is strong epidemiological and clinical evidence linking consumption of whole grains to a reduced risk for coronary heart disease (CHD).8 It is likely that the combination of compounds in grains, rather than any one component, explains its protective effects. Whole grain feeding studies have looked at biomarkers relevant to cardiovascular disease. Katz et al.9 measured the effect of oat and wheat cereals on endothelial responses in human subjects. Month-long, daily supplementation with either whole-grain oat or wheat cereal prevented postprandial impairment of vascular reactivity in response to a high-fat meal. Food consumption patterns that include whole grains also are protective for cardiovascular disease. Intake of refined diets without whole grains was associated with higher serum cholesterol levels and lower intakes of micronutrients.10 A prudent dietary pattern, including intake of whole grains, was associated with lower C-reactive protein levels and endothelial dysfunction, an early step in the development of atherosclerosis." Whole grain food intake was also associated with lower levels of C-reactive protein in the Nurses Health Study.12

Since whole grains are the predominant dietary fibre source in most countries, it is difficult to separate out the protection of dietary fibre from whole grains. Jensen et al.13 examined intakes of whole grains, bran, and germ and risk of coronary heart disease from food frequency data in the Health Professionals Follow-up study. Added germ was not associated with CHD risk and the authors conclude that the study supports the reported beneficial association of whole-grain intake with CHD and suggests that the fibre-rich bran component of whole grain could be a key factor in this relation.

Whole Grains, Fibre and Blood Glucose

Epidemiologic studies consistently show that risk for type 2 diabetes mellitus is decreased with consumption of whole grains.14 Intake of fibre from whole-grain cereals has also been found to be inversely related to type 2 diabetes. In a long-term study of almost 90,000 women, researchers found that those with higher intakes of cereal fibre had about a 30% lower risk for developing type 2 diabetes, compared to those with the lowest intakes15. There are several reports linking whole grain and dietary fibre intakes to a decreased risk of type 2 diabetes.16,17

Whole Grains, Fibre and Cancer

There is substantial scientific evidence that whole grains reduce risk of cancer. In a meta-analysis of whole grain intake and cancer, whole grains were protective in 46 of 51 mentions of whole-grain intake.18 Potential mechanisms linking whole grains to reduced cancer risk include large bowel effects, antioxidant content, alterations in blood glucose levels via mediation in the blood glucose response, weight loss via satiety mechanisms, and hormonal effects. Hormonally active compounds in grains called lignans, which are found in the fibrerich bran layer, may protect against diseases. Lignans are converted to enterolactone, a mammalian lignan, in the gut. Serum enterolactone concentrations increased with consumption of whole-grain products.19 Variability in serum enterolactone concentration was great, suggesting the role of gut microflora in the metabolism of lignans may be important.

Few studies have looked at the direct effects of feeding defined whole grain diets to humans. Mclntosh et al.20 fed rye and wheat foods to overweight middle-aged men and measured markers of bowel health. The men were fed low-fibre cereal grain foods providing 5 grams of dietary fibre for the refined grain diet and 18 grams of dietary fibre for the whole grain diet, either high in rye or wheat. This was in addition to a baseline diet that contained 14 grams of dietary fibre. Both the highfibre rye and wheat foods increased fecal output by 33-36% and reduced fecal β-glucuronidase activity by 29%. Postprandial plasma insulin was decreased by 46-49% and postprandial plasma glucose by 16-19%. RYe foods were associated with significantly increased plasma enterolactone and fecal butyrate, relative to wheat and low-fibre diets. The authors conclude that rye appears more effective than wheat in overall improvement of biomarkers of bowel health.

Although few studies have specifically sought to determine which components of whole grain play a role in bowel health, two large epidemiologic studies21,22 suggest that fibre plays a particularly important role in reducing the risk of colon cancer. Bingham et al.2 showed that colorectal cancer incidence in a sample of 519,978 European subjects could be reduced by up to 40% by doubling fibre intakes from the average baseline intake of ~12 g/day. Similarly, Peters et al.22 showed a 27% reduction in the incidence of colorectal adenoma with a doubling of dietary fibre intakes from a baseline of ~12 g/day. Both studies suggest the importance of dietary fibre from grain, cereal and fruit and vegetable sources.

Whole Grains, Fibre and Body Weight Regulation

Whole grain intake is positively associated with the regulation of body weight. Several factors may explain the influence of whole grains on body weight regulation. The high volume and low energy density of whole grain foods may promote satiation (regulation of energy intake per eating occasion through effects of hormones influenced by chewing and swallowing mechanics). Additionally, whole grains may enhance satiety (delayed return of hunger following a meal) for up to several hours following a meal. Grains rich in viscous soluble fibres (e.g., oats, barley and psyllium) tend to increase intraluminal viscosity, prolong gastric emptying time, and slow nutrient absorption in the small intestine.

Weight gain among men in the Health Professionals Follow-up Study was followed over 8 years and compared to changes in whole-grain, bran, and cereal fibre intake.23 The increased consumption of whole grains was inversely related to weight gain, and the associations persisted after changes in added bran or fibre intakes were accounted for. This suggests that there are components in whole grains beyond dietary fibre which may contribute to favorable metabolic changes that reduce long-term weight gain. Nonetheless, the important role of dietary fibre cannot be underscored.

COMPONENTS IN WHOLE GRAINS THAT DELIVER HEALTH BENEFITS

The "fibre hypothesis", published in the early 19705, suggested that whole foods, such as whole grains, fruits, and vegetables, provide fibre along with other constituents that have health benefits. The bran and germ fractions derived from conventional milling provide the majority of the biologically active compounds found in a grain. Specific nutrients include high concentrations of B vitamins (thiamin, niacin, riboflavin, and pantothenic acid) and minerals (calcium, magnesium, potassium, phosphorus, sodium, and iron), elevated levels of basic amino acids (e.g., arginine and lysine), and elevated tocol levels in the lipids. Numerous phytochemicals, some common in many plant foods (phytates and phenolic compounds) and some unique to grain products (avenanthramides, avenalumic acid), are responsible for the high antioxidant activity of whole grain foods.4

Dietary fibre is essentially the polysaccharide leftovers of digestion. The physiological effect of fibre in intact foods is often greater than that found with isolated fibre fractions. In epidemiologic studies, whole grains, vegetables, and fruits are often more protective against diseases than fibre supplements. Thus, fibre intake may be a marker of a healthy diet, rather than just a nutrient that can be isolated and added back to the diet.

In 2002, the Dietary Reference Intakes (DRIs) for the first time included fibre as a nutrient.24 Dietary fibre was defined as nondigestible carbohydrates and lignin from plants. Foods high in dietary fibre may include whole grains, legumes, vegetables, and fruits. Another class of fibre, functional fibre, was also defined in 2002.24

Functional fibre is found in bulk laxatives, fortified foods, beverages, and dietary supplements. However, Health Canada does not currently recognize this catgory of fibre; therefore; functional fibres do not contribute to the total dietary fibre declared in the Canadian Nutrition Facts panel.

Previously, dietary fibre was divided into soluble and insoluble fibre in an attempt to assign physiological effects to chemical types of fibre. Oat bran and psyllium, two mostly soluble fibres, have health claims for their ability to lower serum lipids. Wheat bran and other more insoluble fibres are linked to laxation. Yet, scientific support that soluble fibres lower blood cholesterol, while insoluble fibres increase stool size, is inconsistent at best. Resistant starch and inulin, both considered soluble fibres do not lower blood cholesterol. Thus, not all soluble fibres lower blood cholesterol, and other traits such as viscosity of fibre play roles. Constipation is more likely on low fibre intakes and risk of colon cancer is inversely related to stool weight. The association of insoluble fibre with laxation also is variable. (Table 3)25

Many fibre sources are mostly soluble but still enlarge stool weight, such as oat bran and psyllium. Furthermore, besides food intake, other factors can also affect stool size.

RECOMMENDATIONS FOR INTAKE OF WHOLE GRAINS AND FIBRE

Increased whole grain consumption is widely promoted. Grain products are featured in mypyramid.com from the U.S. Department of Agriculture (USDA) and are a significant part of "Canada's Food Guide to Healthy Eating". Whereas the USDA Food Guide Pyramid and the 2005 Dietary Guidelines for Americans place particular emphasis on eating more whole grain foods (Table 1)2, Health Canada's draft nutrition recommendations on carbohydrates26 emphasize the importance of fibre-containing whole grain products, legumes, fruit, and vegetables. The 2005 Dietary Guidelines for Americans recommends that at least 3 servings, or one-half of grain foods consumed daily, be whole grain with the remainder either enriched or whole grain. Health Canada's draft nutrition recommendations on carbohydrates emphasizes the need to increase dietary fibre intake at a level of 14 grams per 1000 kcal per day, a level also recommended in the 2005 Dietary Guidelines.

Despite these recommendations, both whole grain and fibre consumption remain low. According to a survey of Americans 20 years and older, total grain intake was 6.7 servings per day with only i.o of these servings being whole grain.27 Only eight percent of the study participants consumed 3 servings of whole grains on a daily basis. Furthermore, average fibre intakes in the United States are only about 14 g/day, falling woefully short of the 14 grams per 1000 kcal recommended in the 2005 Dietary Guidelines for Americans. In Canada, average fibre intakes are in the range of 10-21 grams per day28, also short of the 14 grams per 1000 kcal recommendation.

THE BOTTOM LINE

It is clear that dietary fibre is a key component in whole grain that delivers health benefits. As such, dietary guidance should encourage consumption of whole grain foods. Additionally, dietary guidance must stress the importance of dietary fibre and acknowledge that whole grains are important fibre sources.

Eating both whole grain and high fibre foods can help promote good health. Whole grain foods are made with all three parts of the grain kernel - the fibre-rich outer bran layer, the middle endosperm and the inner germ. While some health benefits come from the parts of the whole grain working together, studies show that many benefits are attributed to the fibre.

DO WHOLE GRAIN AND FIBRE MEAN THE SAME THING?

Some people think this is the case. The truth is not all whole grain foods are high in fibre. The fibre content of whole grains can vary greatly. The easiest way to confirm that a food provides the benefits of whole grain and fibre is to check the Nutrition Facts table. When looking for whole grain products with a high source of fibre, choose foods that list a whole grain as the first ingredient and provide at least 4 grams of fibre per serving.

HEALTH CANADA RECOMMENDS WHOLE GRAINS AS A SOURCE OF FIBRE...

Health Canada recommends Canadians eat 5 to 12 servings of grain products each day, and identifies whole grains as a u " source of dietary fibre.1 Similarly, Health Canada's proposed recommendations on carbohydrates indicate whole grain products, among others, should be the primary sources of carbohydrates and fibre in the diet.2

EATING A HIGH FIBRE DIET CAN HELP...

* Promote regularity and maintain a healthy digestive system

* Lower blood cholesterol levels and reduce the risk of heart disease

* Reduce the risk of colon cancer and other types of cancer

* Control blood sugars in people with diabetes

* Assist with weight control

Fibre Claims

Fibre is considered a nutrient because it has been assigned a Daily Value. The Nutrition Facts table lists the grams fibre and % DV per serving.

Source of fibre - product with at least 2 g of fibre per serving and per reference amount High source of fibre - product with at least 4 g of fibre per serving and per reference amount Very high source of fibre - product with at least 6 g of fibre per serving and per reference amount

Whole Grain Claims

The Ingredient List will include the words "whole" or "whole grain" before the name of the grain, such as "whole wheat" or "whole grain oats." You can't tell the amount of whole grain that is present in a food; however, the best sources will list a whole grain as the first ingredient.

THE BOTTOM LINE

It is important that Canadians check the Nutrition Facts table for the grams of fibre per serving when choosing whole grain foods that will provide the greatest health benefits.

[Reference]

REFERENCES

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25. Cummings JH. The effect of dietary fibre on fecal weight and composition. In: CRC Handbook of Dietary Fibre in Human Nutrition, Ed. Spiller GA. CRC Press, Boca Raton, FL, pp 263-333, 1993.

26. Office of Nutrition Policy and Promotion. Nutrition Recommendations for Canadians: Draft Recommendations on Carbohydrates. Health Canada, 2004.

27. Harnack, L, Waltersm S, and Jacobs, J.R. Dietary intake and food sources of whole grains among US children and adolescents: Data from the 1994-1996 continuing survey of food intakes by individuals. J Am Diet Assoc 2003;10:1015-1019.

28. Mendelson R, Tarasuk V, Chappell J, Brown H, Anderson GH. Report of the Ontario Food Survey, June 2003.

[Author Affiliation]

JOANNE SLAVIN, PHD, RD