Whole and refined grains and disease
Interest in the health effects of grain consumption, and particularly whole grains, is growing. Whole grains contain the germ, bran, and endosperm, and contrast to refined grains which have the germ and bran removed during the milling process.
Much of the nutrient content in grains is contained in the germ and bran and for this reason the nutrient content of refined grains is between 60-80% lower than that of whole grains.
Previous epidemiological studies have suggested a lower risk of type 2 diabetes, cardiovascular disease, colorectal cancer, and all-cause mortality with a higher whole grain intake, while intake of refined grains has shown no benefit in relation to risk of type 2 diabetes or mortality.
NIH-AARP Diet and Health Study findings
A published article in BMC Medicine by Huang and colleagues investigated the association between the intake of whole grains and cereal fiber and the risk of total and cause-specific mortality based on participants in the NIH-AARP Diet and Health Study.
During 14 years of follow-up, 46,067 deaths were documented among 367,442 participants who were free from cardiovascular disease, cancer, type 2 diabetes or end-stage renal disease at baseline.
The authors found a reduced risk of mortality from all causes, from cardiovascular disease to cancer, for those who had the highest intake of whole grain versus the lowest (1.20 ounces versus 0.13 ounces per 1000 kcal/day, which amounts to 2.2 and 0.25 servings per day on a 1800 kcal/day diet, respectively).
Similar inverse associations were observed for the intake of cereal fiber. The association between whole grain intake and mortality was substantially attenuated by adjustment for cereal fiber intake and therefore the association may at least partly be explained by cereal fiber.
However, caution should be made in attributing effects of food groups to single nutrients as cereal fiber and whole grain intake is highly correlated, and whole grains contain many other nutrients and biologically active components which may be beneficial.
The many null results from randomized trials of vitamin and antioxidant supplementation which contrasts considerably with epidemiological studies of fruit and vegetable intake in relation to chronic disease prevention should serve as a reminder that foods are much more complex than single nutrients and therefore their health effects should not be expected to be equal either.
Implications and recommendations
Few previous studies have been large enough to investigate the association between whole grain intake and less common causes of death with sufficient statistical power, but the current study is an exception.
In the Iowa Women’s Health Study, the Nurses’ Health Study and the Health Professional’s Follow-up Study there were suggestive inverse associations between whole grain intake and respiratory and infectious disease mortality. An interesting finding in the NIH-AARP Study is that there was an inverse association with all other/unknown causes of death as well.
This suggests that additional investigations should investigate whole grain intake in relation to even more refined categorizations of mortality.
Some of the proposed mechanisms that may explain the observed health benefits of whole grain and cereal fiber intake include (but may not be limited to) cholesterol and blood-pressure lowering effects, improved body weight control and insulin sensitivity, antioxidant effects, reduced inflammation, increased stool bulk, dilution of fecal carcinogens, production of short chain fatty acids by bacterial fermentation and effects on the gut microbiota.
The current study adds to a growing body of evidence of a beneficial effect of whole grain and cereal fiber consumption on all-cause mortality as well as specific causes of death.
a good choice to replace the refined grains with their whole grain counterparts
It has important public health implications as choosing whole grains instead of refined grains is highly feasible and could have a large effect if a large part of the population increased the intake.The study also provides further support for recommendations to the general population to increase intake of whole grains.
Most of the current literature on whole grains and health outcomes is from American cohort studies, with only a few European studies published. Whole grain intake is higher in Northern Europe than in the US, so these populations may be promising for further studies investigating the association between whole grains and health outcomes, both in terms of examining more extreme intakes and specific types of whole grains.
Further studies are needed to understand whole grain intake in relation to risk of specific diseases and less common causes of death, for incorporating biomarkers of whole grains and to investigate the underlying mechanisms involved. Meanwhile it is probably a good choice to replace the refined grains with their whole grain counterparts, such as whole grain breakfast cereals, brown rice and brown bread.