By David Kiefer, MD
Research Fellow,
Department of Family Medicine,
University of Wisconsin;
Clinical Assistant Professor of Medicine,
Arizona Center for Integrative Medicine,
University of Arizona
Dr. Kiefer reports no financial relationships relevant to this field of study.
Synopsis: When adjusted for possible confounding variables, this analysis of U.S. men and women found an association between higher whole grain intake and lower mortality from all causes, including cardiovascular disease, but no association for cancer mortality.
Source: Wu H, et al. Association between dietary whole grain intake and risk of mortality: Two large prospective studies in U.S. men and women. JAMA Intern Med 2015;175:373-384.
Summary Points
- For adult men and women eating the highest quintile of whole grains, their hazard ratio for all-cause morality was 0.91 and for cardiovascular mortality was 0.85.
- In terms applicable to diet, one serving of whole grains (or 28 grams) translated into a 5% lower total mortality or a 9% lower cardiovascular mortality.
- When analyses were done as if whole grains were substituted for refined grains or red meat, significant improvements in total mortality and cardiovascular mortality were seen.
The researchers of this analysis aimed to expand the literature showing a possible connection between dietary whole grains and chronic diseases, such as diabetes and coronary artery disease (CAD), as well as hypothesized benefits on mortality. The researchers analyzed two databases: the Nurses’ Health Study (NHS) data on 121,700 female registered nurses and the Health Professionals Follow-up Study (HPFS), involving 51,529 male health professionals. Participants were excluded if they had baseline cancer, CAD, or stroke; < 500 kilocalories (kcal) or > 3500 kcal daily intake; or incomplete dietary data. After these exclusions, 74,731 participants from the NHS and 43,744 from the HPFS remained.
Food-frequency questionnaires every 2-4 years were used to estimate whole grain intake from all whole grain-containing foods, such as rice, bread, pasta, and breakfast cereals. Added wheat germ and bran were also factored in. Also, from the questionnaires an Alternate Healthy Eating Index (AHEI) score was calculated based on 10 foods and beverages with connections to chronic disease risk; a higher score means healthier eating and better diet quality. A variety of co-variates (i.e., body weight, lifestyle, and medical history) also were included in this analysis.
Study participants who consumed whole grains in the highest quintile were more likely to have a variety of characteristics (see Table 1). In both the NHS and the HPFS, a higher intake of whole grains was associated with a lower total and cardiovascular mortality; the hazard ratios (HR) were 0.91 (95% confidence interval [CI], 0.88-0.95; P < 0.001) and 0.85 (CI, 0.78-0.92; P < 0.001), respectively. The same analysis failed to find an association between whole grain intake and cancer mortality (HR, 0.97; CI, 0.91-1.04; P = 0.43). Quantifying these results, the researchers calculated that every daily serving of whole grains (28 grams = one serving) was associated with a 5% lower total mortality and 9% lower cardiovascular mortality.
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Table 1: Characteristics of Higher Whole Grain Consumers |
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The researchers found some other interesting results. Even refined grain intake was associated with a small reduction in total mortality (HR, 0.98; CI, 0.97-0.99), although there was no association with cardiovascular nor cancer mortality. More striking was the finding that replacing one serving of refined grains or red meat with one serving of whole grains led to 8% (HR, 0.92; CI, 0.88-0.97) and 20% (HR, 0.80; CI, 0.75-0.86) reductions in cardiovascular mortality, respectively. Total mortality reductions were 4% and 10%, respectively, whereas there were no effects on cancer mortality with such substitutions.
By collecting detailed information about specific whole grains, as well as added bran and wheat germ, the researchers were able to separate out the effects of dietary bran, added bran, and added germ. Total bran consumption across the quintiles was associated with lower total (HR, 0.94; CI, 0.90-0.99; P < 0.001 for the trend) and cardiovascular (HR, 0.80; CI, 0.73-0.87; P < 0.001 for the trend) mortalities. The effect of added bran was similar to that of naturally occurring bran, and there was no association between wheat germ intake and mortality.
Commentary
In some respects, this methodologically sound analysis using datasets with information about large numbers of U.S. men and women corroborates what clinicians have been recommending for many years, namely that whole grains are good for our health. The authors of this analysis list some of the shortcomings and conflicting results from other studies. In some cases, dark bread was thought to be the healthiest; in other cases, specific demographics (elderly, diabetic, etc.) had cardiovascular benefits but not improvements in overall mortality. As is often the case, methodological nuances or flaws compromised painting a complete picture of the whole grain effect. That was true until this trial, which now seems to show that eating whole grains as a component of the diet, or when calculated as a substitute for less healthy dietary components, clearly benefits all-cause and cardiovascular mortality in this demographic.
The mechanism fits the results and conclusions. The researchers tie their results into the fact that the bran component of some whole grains contains important phytonutrients (fiber, B vitamins, vitamin E, and magnesium) that have been shown in vitro and in vivo to have protective effects on cardiovascular disease risk and diabetes. The results of this study showed less positive effects with wheat germ, devoid of the above-mentioned bran components, supporting the proposed mechanism. In addition, there is little argument that fiber can benefit people with cardiovascular disease, but this study brings to light the positive effects on all-cause mortality, even if it is less of an effect than cardiovascular effects.
What remains as an important question not necessarily addressed by this study is teasing out how these results are affected by the relative quantity of fiber consumed by study participants. With concerns that grain intake is potentially too high for many people in the United States, is it possible to attain the benefits seen in this study by the phytochemicals and fiber contained in fruits and vegetables? Bran is full of healthy compounds, but for many people it may not be an option, such as in cases of gluten intolerance or celiac disease. The results from the substitution analysis are particularly compelling; any clinician can use these results to convince patients to swap out refined carbohydrates or red meat for whole grains, but a substitution with fruits and/or vegetables would likely convey similar positive effects; it’s just not addressed here.
The researchers were not surprised by the lack of an effect on cancer mortality; many other studies have also failed to find an effect, probably attributable to the varied pathophysiology for each specific cancer as well as what the authors call “population characteristics,” which can vary tremendously and affect results, even in large studies such as this one.
This is a useful study that arms clinicians with practical knowledge about the life-saving effects of one component of a healthy diet. Provided whole grains are an option for someone, the benefits can be pitched in percentages of risk reduction (the hazard ratio), the effect of one serving on mortality over time, or what might happen if refined grains or red meat are replaced with whole grains. This study provides convincing data for why conversations about healthy eating can weave in longevity as part of what we know when it comes to whole grains.
When adjusted for possible confounding variables, this analysis of U.S. men and women found an association between higher whole grain intake and lower mortality from all causes, including cardiovascular disease, but no association for cancer mortality.
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