Fiber's Trusty Sidekicks

by Kristina C. - January 25, 2016

Kristina Campbell is a freelance science writer who create quality content on gut microbiota for

Photo Credit: by Kristina C
Photo Credit: by Kristina C

Fiber's benefits for the human body have long been a conundrum in the world of nutrition science. It's clear enough that fiber boosts health: people with a diet high in fiber tend to weigh less, have better blood sugar balance, and have a decreased risk of inflammatory bowel disease, cardiovascular disease, and colon cancer, among other conditions. What hasn't been clear is the line that links fiber to these benefits.

SEE ALSO: Gastroparesis: Dietary Tips to Reverse Your Life

Our bodies can't digest dietary fiber. We lack the necessary enzymes. But somehow as they pass through us, these ‘non-digestible carbohydrates' – found in plant foods from oranges to leeks – end up helping our bodies.

Science has made some advances, though, by studying the gut microbiota -- the collection of friendly microorganisms that live in each person's digestive tract. Research, especially in the last decade, has shown that we normally live in a happy symbiotic relationship with our gut microbiota. And these microbes are responsible for turning fiber into something that keeps us ticking.

Here’s how it works: the fiber you eat travels through the stomach and small intestine intact. When it enters the large intestine, the bacteria living there produce enzymes that chop up the fiber into smaller components – in other words, they ferment it. And here's where the magic happens: in this act of fermentation, some of the 'waste' that bacteria produce is a key factor in the health of your body. The most important group of waste products are called short-chain fatty acids, or SCFAs.

Never mind that the name 'short-chain fatty acid' sounds like it came from a random word generator. SCFAs do a lot of good in the body. Different microbes in the gut produce different versions of them after their fiber meals: bacteria from the Bacteroidetes group (or phylum) mainly produce SCFAs called acetate and propionate, while those from the Firmicutes group produce butyrate.

When SCFAs are produced in your colon, some are excreted out the bottom end, but the vast majority (95%) are slurped up right away by cells of the large intestine. Butyrate, in particular, provides energy for these cells and helps keep the inner layer of the colon in good repair. After this happens, the leftover parts of SCFAs end up in the blood, travelling to various organs where they are used as materials or are employed as messengers.

In many experiments, SCFAs have the effect of improving metabolic health. Giving acetate to healthy mice reduced their food intake, for example, and lowered their chance of becoming obese. Butyrate and propionate prevented weight gain in mice on a high-fat diet. And some SCFAs balanced blood sugar in diabetic mice and rats. From this evidence, it’s plausible that SCFAs are one of the mechanisms by which fiber protects us against obesity.

From this comes the realization that fiber is probably just a means to an end. It's important to eat fiber – lots of it – but it's the trusty SCFA sidekicks that are responsible for its health benefits (with the help of microbes).

But it’s also important to know that SCFAs are a source of energy for the body, providing about 10% of our daily calorie requirements. This should mean the more SCFAs your gut bacteria produce, the more calories you extract from your diet and the more weight you gain. Indeed, people who are obese seem to produce more SCFAs, possibly harvesting more energy from a given diet than those who are lean.

Do SCFAs protect against obesity, or do they contribute to obesity? At this point, data supports both. This apparent contradiction shows we haven’t yet cracked the code of how SCFAs work. Likely, their advantages and disadvantages depend on which species of bacteria produce them and interact with them in the digestive tract.

We still have a lot to learn about SCFAs to really harness their power. According to some researchers, what we really need is to observe these metabolites going about their business in real time; only then will we understand how fiber accomplishes its good work.


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