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Not all carbs are bad

Getting enough fiber might do more than keep you regular.

This brief post was written as a popular science article for a class on science communication. My own research is currently focused on exactly this topic: describing microbial community dynamics associated with acarbose treatment and the production of butyrate.

A quick internet search search for “low-carb diets” comes back filled with promises to make you sleek, spry, and slim just by cutting out this entire category of foods. The popularity of these diets shouldn't surprise you. Recent research has implicated overconsumption of sugars, the simplest form of carbohydrates, and starchy foods, which can quickly be broken down into sugars, in the increased risk of heart disease, obesity, even some forms of dementia. Americans have responded quickly, with 50% trying to limit their intake of sugars and carbohydrates according to a 2014 survey. That same survey found only 74% of respondents believe that a healthy diet can include moderate amounts of sugar, down from 84% in 2013. If these trends lead to decreasing calorie consumption as a whole, we could see a decline in obesity, diabetes, and heart disease.

Unfortunately, there may be an unintended consequence of these quickly shifting attitudes. The fraction of Americans that are actively trying to consume fiber and whole grains in their diet dropped to 53% in 2014 from 62% the year before. Dietary fiber is also a carbohydrate, made-up of individual sugars combined like daisies into long chains and branching structures. What makes these carbohydrates special is that, due to the links holding them together, the human body is not capable of breaking fiber down into individual sugars to be absorbed into the blood stream.

Without any way to digest it, fiber is often thought of as an inert component of our food, increasing feelings of fullness but not contributing to the calorie content, all while improving our digestive health by absorbing water and pushing everything smoothly along. While some of the benefits of fiber consumption may be due to its inertness, the problem with that description is that it entirely ignores the multitudes of microbes living in our gut, collectively referred to as the microbiome. Many of these bacteria can break down and feed on dietary fibers, in the process, producing a huge variety of byproducts. The discomfort and flatulence associated with the consumption of beans, for instance—or in lactose intolerant individuals, milk—is due to the carbon dioxide and hydrogen gas belched out by trillions of microbes in the large intestine while they feast on carbohydrates inaccessible to our bodies. Despite the discomfort, the breakdown of these compounds by fermentation produces a number of small molecules called short-chain fatty acids, each of which you can already recognize by smell: acetate (the acid in vinegar), lactate (which gives cheese its sharpness), propionate (similar to body odor), and butyrate (the distinctive stink of vomit). Despite its odious stench, butyrate has a number of health benefits, notably its ability to reduce inflammation, a fact that should come as welcome news to the almost 2 million Americans who suffer from inflammatory bowel diseases. The ability of these fibers to stimulate the growth of certain beneficial gut bacteria puts them into the category of dietary supplements called "prebiotics" (rather than probiotics).

For years, diabetics have been prescribed a drug called acarbose to temper the glucose spikes that occur after starch-heavy meals. Acarbose operates by blocking the breakdown and absorption of starch, effectively (and unintentionally) transforming it into dietary fiber to be broken down by the bacteria in our gut. Unsurprisingly, flatulence and diarrhea are commonly reported side effects. A little discomfort might be worth it, though, if the finding that treatment with acarbose extends lifespan in mice—by as much as 20%—applies to humans as well. Why it has this effect is unclear, but if it is due to the increased production of butyrate by gut bacteria, increased fiber consumption could have similar effects. Studies are currently underway testing the longevity effects of inulin, a dietary fiber from chicory root. If this and other fibers also increase lifespan in mice, butyrate, and by extension the microbiome, will be new targets for the growing field of life-extending therapies.