Overlooked “X” factor in grains may add to their health benefits
Whole grains have always been a foundation of the “health food” movement.
That’s been true since the movement’s emergence in the 1870s through its strong resurgence in the late 1960s, and mainstream research has confirmed the health benefits of whole grains.
The core principles of the venerable health food movement remain simple and clear: minimize refined/processed/artificial foods in favor of whole, unprocessed, natural foods.
Once considered fringe or even flaky, those principles now enjoy overwhelming evidence, and guide dietary advice from establishment institutions like the USDA and the National Institutes of Health.
However, grains — including whole grains — are opposed by prominent advocates of the Paleo diet.
To be sure, the gluten protein in wheat, barley, rye, and some other grains is dangerous to people with celiac disease.
And there’s been a rise in the (still-tiny) proportion of people who are genuinely sensitive to gluten — as opposed to other potentially gut-upsetting food factors such as FODMAPs.
But those problems affecting a very small minority of people have increasingly become the irrational rationale for demonization of all grains, including whole grains.
In fact, the available evidence strongly suggests that diets rich in whole grains are actively healthful for the great majority of people, reducing the risks for cardiovascular disease — and, somewhat counterintuitively — the risk for diabetes.
Background to the new findings
Earlier this year, a U.S.-China research team examined the available epidemiological evidence concerning possible links between various diets and types of food and the risk for breast cancer (Xiao Y et al. 2019).
As they wrote, “High intakes of red meat, animal fats, and refined carbohydrates [e.g., white bread and pastries] have been shown to be associated with an increased risk, whereas intake of fruits, vegetables, whole grains, and dietary fiber has been linked with a reduced risk of breast cancer.”
Now, the results of an evidence review published last fall by many of the same researchers add more evidence that diets relatively rich in whole grains may help reduce the risk of breast cancer.
Here’s how the American-Chinese team expressed the motivation for their study: “Epidemiological studies have found that high whole grain intake may be associated with a reduced risk of breast cancer. However, the evidence has not been consistent.”
Ironically, their January 2019 evidence review did not include the results of their own September 2018 evidence review — probably because the 2018 paper hadn’t been peer-reviewed and accepted for publication before they completed their 2019 review.
As we’ll see, their 2018 paper linked diets relatively rich in whole grains to modestly but significantly reduced risks for breast cancer.
After examining their review of the evidence, will look at the plausible reasons why whole grains might reduce the risk for cancer, including breast cancer.
U.S.-China review links whole grains to reduced breast cancer risks
This evidence review was conducted by researchers at Harvard University, Missouri’s Washington University School of Medicine, and the Center for Disease Control and Prevention in Shenzhen, China (Xiao Y et al. 2018).
Theirs was the first review of evidence from observational studies that had specifically looked for links between whole grain intakes and the risk of breast cancer.
They conducted a relatively sophisticated type of evidence review — called a meta-analysis — to look for possible links between the risk for breast cancer and diets with different intakes of whole grains.
Out of more than 400 potential candidates for inclusion, they selected 11 studies that met their quality criteria, including four cohort and seven case-control studies that involved 131,151 participants in total. (See “About cohort and case-control studies” below to learn about the differences between them.)
After analyzing the data from all 11 studies, the American-Chinese team calculated that the risk of developing breast cancer was 16% lower among the participants with high versus low self-reported intakes of whole grains.
As they wrote, “The results suggest that intermediate and high intake levels of whole grain were associated with a modest reduction of breast cancer risk.”
Their analysis of the seven case-control studies showed that the risk for breast cancer was 31% lower among the participants whose self-reported diets contain relatively high proportions of whole grains.
In contrast, their analysis of data from the smaller (less than 2,300 participants) studies — tended to be less well-designed — detected a much smaller 4% drop in breast cancer risk among participants whose self-reported diets were relatively high in whole grains.
That finding of only a small reduction in risk was especially true of the case-control studies, possibly because such studies are more susceptible to errors in people’s memories of their diets.
Given the limitations of many of the studies included in their review — which were the best available — they acknowledged the need for more large-scale cohort studies.
Why would whole grains reduce breast cancer risk?
The authors of the 2018 evidence review proposed several reasons why whole grains might reduce the risk of breast cancer.
For starters, whole grains contain various micronutrients, antioxidants, fibers, and “bioactive” fibers that are lost in the refining process but may help prevent cancer.
They cited five specific characteristics of whole grains that could explain any anti-cancer effects:
- Higher blood insulin levels have been linked with increased breast cancer risk, and whole grains reduce the glucose and insulin responses to meals, leading to better blood-sugar and insulin control.
- Whole grains have been linked to lower levels of markers for systemic inflammation and liver enzymes, higher levels of which have been linked to higher cancer risks.
- Whole grains are rich in fiber, which has been linked to reduced breast cancer risk, and is known to produce of a variety of effects associated with reduced cancer risk.
- Whole grains are rich in antioxidants, including vitamins C and E, beta-carotene, polyphenols and trace minerals (selenium, zinc, copper, and manganese) that are essential components of the body’s own antioxidant enzymes
- Whole grains provide significant amounts of phytoestrogens and lignans, which play important protective roles against cancer through their antioxidant properties and ability to inhibit cell proliferation and angiogenesis (the creation of blood vessels to feed tumors), to trigger apoptosis (cellular “suicide”) in cancer cells, and to modulate hormonal pathways in ways known to discourage cancer.
Another possible cancer-preventive factor in whole grains
In recent years, Danish researchers discovered that whole grains contain a once-obscure family of antioxidants called benzoxazinoids or BX.
Importantly, they also found BX in baked whole-grain bread and other whole-grain products, that BX passes through the gut wall and circulates in the body, and that the metabolic byproducts of dietary BX end up in various organs.
Why would the presence of BX in whole grains matter to cancer prevention?
It turns out that diets rich in BX make certain immune system cells to react more strongly to some types of bacteria and that BX compounds appear to suppress the growth of prostate cancer. (Its effects on other types of cancer have not been studied.)
For example, men with prostate cancer who ate six slices of whole rye bread daily showed lower levels of PSA (prostate-specific antigen), which indicates that the cancer is being suppressed.
In addition, lab studies found that diets rich in whole rye suppressed prostate cancer in mice, while test tube studies found that BX compounds suppress the growth of prostate cancer cells.
About case-control and cohort studies
Case-control and cohort studies are two different “observational” types of epidemiological studies.
While their findings aren’t as reliable as the results of randomized, controlled clinical trials, the evidence produced by well-designed cohort and case-control studies is taken seriously, especially if findings are consistent across several such studies.
Case-control studies examine the health outcomes among two groups — one with a given disease and one without that disease — and then look for statistically significant differences in their exposure to a potential risk or prevention factor within each group, such as intakes of whole grains.
Cohort studies can be retrospective (backward-looking) or prospective (forward-looking). In retrospective cohort studies, researchers analyze the data collected during the study to see whether exposure to a risk factor resulted in a statistically significant difference in the risk for a given disease.
Prospective cohort studies are more common and generally produce more reliable results. Researchers will follow the study participants over time to see how many develop the disease in question and then compare the disease rates between people with different characteristics — such as substantially different intakes of whole grains.
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