Fiber, not just for your guts!

Soybeans are a great source of dietary fiber. Image courtesy of Wikipedia.
Soybeans are a great source of dietary fiber. Image courtesy of Wikipedia.

Allergies have been on the rise in the last few decades – not just the highly publicized increase in peanut and gluten allergies, but also allergic asthma caused by allergens in the air (like dust mites, mold, or pollen). Poorly controlled asthma and asthma emergencies result in many ER visits. What is causing this trend of increasing allergies?

The hygiene hypothesis – lack of exposure to potential allergens due to homes that are too clean leads to kids being hypersensitive is popular with many folks.

A new paper, however, tests an alternative  hypothesis about the increased rate of allergies. Could low fiber consumption in Western diets be to blame?

To test this hypothesis, the research group fed mice three different diets: low fiber, high soluble fiber, and high insoluble fiber. Why three diets? Soluble fiber goes through a processing step in the gut that may have an effect the mouse’s allergic response. Insoluble fiber does not go through this processing step. The high insoluble fiber diet allows the researchers to distinguish between the effects due to amount of fiber consumed and the processing of soluble fiber when comparing to the low fiber diet.

The researchers found that mice eating the high soluble fiber diet showed a reduced allergic response in their lungs to house dust mite extract relative to mice eating the low fiber diet. The low fiber mice also had increased airway hyper-reactivity, which results in wheezing and a struggle to breathe, or what most people know as an asthma attack. The mice on the high insoluble fiber diet had the same allergic responses as the low fiber mice, suggesting that it’s something about soluble fiber that is affecting the allergic response, not just fiber in general.

So how do get from eating a diet rich in soluble fiber to decreased allergy responses in the lungs? There are a lot of data and experiments relevant to this question. So, I’ll try to highlight the main points. Eating soluble fiber results in a change in the gut microbiota (the bacteria living in our guts). What we eat forms their environment. When that environment (ie, our diet) changes, the gut microbiota change too. In this case, the balance shifts towards Bacteriodetes, which are adept at fermenting soluble fiber into short chain fatty acids (SCFA).

Are SCFAs the reason why animals with a high soluble fiber diet are less allergic? When you give mice SCFAs without changing the fiber content of their diet, they show a similar decrease in allergic response just like those on a high soluble fiber diet. It appears that SCFAs influence the development of immune cells in such a way that they can’t sustain a highly active response in the lungs.

Now, I know that the gut microbiome is all the rage right now. According to the media and many grant applications, problems with gut microbiota are the cause of almost all health issues. Normally, I’m skeptical, but I’m intrigued by this preliminary study in mice.

The increase in human allergies does coincide with this dietary trend. It’s likely that a study like this could be done soon in humans, as the risks associated with altering a diet’s fiber content should be minimal for healthy individuals. It’s possible that this particular mechanism is only acting mice. If not, this research could be the first step toward helping kids manage their asthma symptoms through their diets and not through expensive medications alone.

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