Epidemiological studies reveal a clear association between increased wholegrain consumption and a reduced risk of colorectal cancer, cardiovascular disease, elevated blood pressure, type 2 diabetes and weight gain.
However, short-term randomized controlled intervention trials looking at biomarkers for these conditions are not quite as conclusive - at least those looking at weight control, blood pressure and fasting insulin, delegates at the Whole Grains on Every Plate conference in San Antonio were told last week.
But this may have more to do with study design than anything else, said Paul Jacques, professor at Tufts University in Boston, who gave a presentation summarizing the science on whole grain consumption and health benefits.
“Once you start looking more closely at these intervention studies, they are often on very small groups of people that already have metabolic disorders, and they are too short for any meaningful effects to be observed”, he said.
“Whereas the observational studies are looking at thousands of healthy people studied over several years to see if they develop diabetes, obesity and so on.”
They are not comparing like with like
He added: “Take blood pressure. The drug companies will often do a two year study, whereas some of the ones for whole grains and blood pressure are only eight week studies. It’s just not enough time.”
Similarly, with weight control, a 12-week study on a handful of obese people is not really long enough to demonstrate a meaningful effect, and is also unlikely to square with results from an epidemiological trial looking at weight gain over many years in a large cohort of people that start at a healthy weight, he suggested.
“Also, they are not all comparing like with like. Some are feeding people oats, others barley, others whole wheat, others dark rye bread and so on, and the bioactive constituents in these grains are different.”
Is the data in large observational studies reliable?
But should we view the results of some of the large observational studies looking at whole grains with caution?
After all, aren’t people that have high intakes of whole grains also likely to have healthier diets and lifestyles overall, which could be skewing the results?
Perhaps, he said. “Obviously this is a potential problem with these kinds of studies, but when you look at the data and try and control for those [potentially confounding] factors, you still see a strong correlation between whole grain intakes and reduced risk of heart disease, type 2 diabetes and colorectal cancer.”
Ideally of course, you would put thousands of people in lab conditions, randomize them, assign them into groups (high whole grains, low or zero whole grains etc), rigidly control their diet and lifestyle for 10 years and then see how many go on to develop heart disease, diabetes or obesity, he said.
But in reality, this would be prohibitively expensive and impractical, so we have to rely on a combination of epidemiological studies and shorter term intervention trials that use surrogate endpoints for identifying risk reduction (or biomarkers) such as fasting blood glucose concentrations, cholesterol or insulin resistance.
So what kind of intervention studies are practical?
It depends on the biomarker, he said, but ideally they should look at hundreds of healthy people over a period of months rather than 10s of unhealthy people over a period of weeks.
Whole grains and health claims
Currently there are two approved health claims about whole grains:
‘Diets rich in whole grain foods and other plant foods and low in fat, saturated fat, and cholesterol, may help reduce the risk of heart disease and certain cancers.’ (1999)
‘Diets rich in whole grain foods and other plant foods, and low in saturated fat and cholesterol, may help reduce the risk of heart disease.’ (2003)
While the 2010 Dietary Guidelines for Americans state that: "Limited evidence also shows that consuming whole grains is associated with a reduced incidence of type II diabetes”, there is no government-approved health claim to this effect.
However, ConAgra Foods has recently petitioned the FDA to approve a qualified health claim about the relationship between whole grain consumption and a reduced risk of type II diabetes - and is waiting to hear back.
Click here for more on ConAgra’s petition.