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Do Whole Grains Hurt or Help Gut Health?
Contrary to common claims, new evidence sees whole grains as potential gut-health allies

02/16/2017 By Craig Weatherby

Grains have really taken it on the chin over the past decade.

Chief among the charges is that gluten is problematic for more people than once thought.

And the sugar-like carbs in refined grain foods such as white breads and pastries deserve a good deal of blame for America’s epidemic of obesity and diabetes.

Finally, advocates of Paleo-style diets assert that humans haven’t been eating grains long enough for our genes to have fully adapted, which allegedly causes various health problems.

Such critiques often downplay the huge differences between refined "white" grains and whole grains — while some anti-grain jeremiads either lack strong evidence or paint with too broad a brush.

Unlike refined grains, whole grains are rich in fibers and the same kinds of antioxidants that help make colorful fruits and vegetables so healthful.

We explored these concerns in Do Grains Get a Bum Rap?, Is Gluten Really so Guilty?, Do Grains Help or Harm Health?, and Whole Grain Foods Found High in Antioxidants.

Dozens of large population studies link diets rich in whole grains to reduced risks for heart disease, diabetes, and some cancers.

Whole grains also appear to dampen inflammation — and to normalize blood sugar levels, especially in people with prediabetes.

Most evidence on whole grains is positive, which explains why the 2015-2020 Dietary Guidelines for Americans recommends that Americans replace refined grains with whole grains.

Specifically, Americans are advised to eat at least three ounces (women) to four ounces (men) of whole grains daily, which you'd get from 1½ to 2 cups of brown rice or oatmeal.


Quick primer on whole grains
Whole grains feature an outer, highly nutritious layer rich in fiber and antioxidants, which is mostly — though not fully — retained in whole-grain flours and oatmeal.

Milling whole grains into whole-grain flour — which is clearly healthier than white flour — blunts some of their health benefits.

But the milling of wheat, rice, oats, rye, corn, or barley into white flour strips them of most fiber and nutrients, making them nutritionally "empty" starches that exert sugar-like effects on health.

Specifically, refined grain products — such as white flour, white bread, and white rice — lack the ample fiber, fats, iron, B vitamins, minerals, and antioxidants found in whole grains.


Tufts trial probes gut and metabolic effects of whole vs. refined grains
Earlier this month, Tufts University researchers reported the results of a small but well-designed clinical trial.

Let’s review the design of the trial and then summarize the encouraging results of its two parts:

  1. The effects of whole grains versus refined grains on gut health.
  2. The effects of whole grains versus refined grains on metabolic health.

As you read, please keep in mind the well-proven phenomenon called "bio-individuality".

In short, the health effects of foods and nutrients can vary pretty widely, due to differences in people's genetic profiles and lifestyles.

Something that's healthful or at least harmless for most people may sicken some, even seriously.

That's obviously true of celiac disease patients, to whom gluten is severely damaging — and for the apparently growing number of gluten-sensitive people who suffer some adverse effects.

Still, the vast majority of people have little or no problem eating whole grains in moderation.

If you suspect whole grains — or any food — of causing trouble, avoid it for a few weeks to see what happens, and consult a professional if the problem hasn't disappeared.

Trial design ensured a fair comparison
Tufts researchers recruited 81 volunteers for a clinical study lasting eight weeks.

The participants in this randomized, controlled trial were 49 men and 32 postmenopausal women aged 40–65 years.

For the first two weeks, all of the participants ate the same type of food, and their individual calorie needs were determined.

After two weeks, the participants were randomly assigned to eat a diet that included either whole grains (41 people) or refined grains (40 people).

It’s critical to note that the whole grain foods given to the 41 participants in the whole-grain diet group were supermarket goods featuring mostly whole grain flour.

The study authors believe that if the volunteers in the whole-grain group had received whole grain kernels — such as wheat berries — instead of whole-grain flour products, they would likely have experienced benefits greater than the ones actually observed, which we’ve detailed below.

The two diets were otherwise very similar in terms of total calories, macronutrient (protein, fat, carbs) composition, and the types and proportions of foods.

The meals complied with the Dietary Guidelines for Americans, and were designed so that the participants would maintain their weight.

(People who participated in prior studies like this one lost weight and enjoyed reduced levels of inflammation when they ate more whole grains. It wasn’t clear whether the anti-inflammatory benefits were simply a side effect of weight loss, which is known to reduce inflammation.)

The study volunteers picked up specially prepared meals three times each week, and were given instructions on how to reheat them properly to avoid nutrient loss.

During each meal, participants completed a food checklist, which the researchers used to determine the actual amount of food consumed by each participant.

The participants also recorded the presence (if any) and severity of six gastrointestinal symptoms, maintained their usual level of physical activity, and abstained from anti-inflammatory medicines (including aspirin and antihistamines) 72 hours before their blood was collected for testing.

Before proceeding, we should note that a somewhat smaller clinical trial published two years ago did not find that eating more whole grains changed the subjects' gut microbiology, blood markers, or body composition (Ampatzoglou A et al. 2015).

However, the new two-part clinical study from Tufts produced significantly different — and more encouraging — outcomes.

Part 1: Whole grains improved participants’ gut bacteria profiles
This part of the Tufts study was designed to measure the effects of diet rich in whole grains — versus a diet high in refined grains — on immune and inflammatory responses, gut microbiota, and regularity (Vanegas SM et al. 2017).

Your gut microbiota is the diverse community of bacteria and other microbes that live in the digestive tract. The specific types and proportions of gut microbes exerts strong effects on mental and physical health.

Effects on gut microbiota: modestly positive
To see how a whole-grain diet might influence gut microbiota, the Tufts team analyzed the participants stool to measure its bacterial content and levels of fatty acids.

Both gut microbiota and short-chain fatty acids are considered vital contributors to healthy immune and inflammatory functions.

(Whole grains have previously been shown to boost the variety of gut microbes and raise production of short-chain fatty acids, which serve as a crucial source of energy for bacteria in the colon.)

Encouragingly, the participants who ate the whole-grain diet showed increases in the numbers of a bacteria that produces short-chain fatty acids, known as Lachnospira.

The Tufts researchers speculated that this increase in Lachnospira bacteria resulted from a more favorable stool pH (acid-alkaline balance) due to consuming a diet rich in whole grains.

Better yet, the volunteers in the whole grains group enjoyed a drop in the numbers of a pro-inflammatory bacteria called Enterbacteriaceae.

The authors suggested that this decrease in pro-inflammatory bacteria might be linked to the higher concentration of acetate in the stool samples of those who ate the whole grains diet.

Effects on immune responses: modestly positive
Blood samples taken from both groups revealed modest differences in two key measures of healthy immune response: levels of memory T cells and levels of TNF-alpha production.

The whole-grain group showed higher (healthier) levels of memory T cells, while the refined grains group suffered a (negative) drop in TNF-alpha production.

The researchers stressed that the differences were very modest.

And they suggested further studies using whole grains (such as barley) with higher levels of soluble fibers to help identify how different whole grains affect gut microbiota and immune responses.

Although the differences in microbiota and immune responses were modest, the results confirm that whole grains do not exert any negative effects on either of those important health factors.

Part 2: Whole grains helped metabolic health
Compared with the refined-grains group, the participants in the whole grains group displayed two benefits:
• Met the recommended dietary allowance (RDA) for fiber.
• Lost almost 100 more calories daily, due to faster metabolisms and greater fecal elimination.

According to senior author Susan B. Roberts, Ph.D., “The extra calories lost by those who ate whole grains was equivalent to a brisk 30-minute walk, or [avoiding] an extra small cookie every day.”

The extra energy losses related to elimination of more feces were caused not by the extra fiber in the whole grain products (which was accounted for in the scientists’ calculations) but because the extra fiber affected the digestibility of other foods.

Sensations of hunger, fullness, and satisfaction were essentially the same for people in both groups.

Based on prior research, it’s likely that the whole-grain group would've experienced greater satisfaction had they eaten whole grain kernels instead of whole grain flours.

The Tufts trial was supported by the USDA Agricultural Research Service and the General Mills Bell Institute of Health and Nutrition.


Sources

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