Let's Talk About Fiber

What is fiber?

The definition of dietary fiber is contentious and evolving. Let’s start with the broadest definition from the American Association of Cereal Chemists. Fiber is the:

“edible parts of plants or analogous carbohydrates that resist digestion and absorption in the small intestine, with complete or partial fermentation in the large intestine”

A mouthful, but all of that is important. Let’s dig in.

1) Yes, fiber is in the ‘carb’ family, but it’s more complex than that, pun intended. This is where the resistance to digestion comes in. Sucrose is a carb, but it is digested quickly into glucose and fructose, directly entering your bloodstream through the small intestine and either used for energy, or stored as fat.

Fiber refers to carbs that are not digested. In fact, they go all the way to your large intestine, your gut, where they get broken down, not your own body, but by bacteria that live there! This also means that dietary fiber, while sometimes tasting quite sweet, does not contribute to increasing your blood sugar, and most of the time, does not contribute to calories in your diet.

Dietary fiber goes by another name, Prebiotics. This name points to the fact that fiber is food for your gut bacteria. So, by eating fiber, you are directly feeding and growing a population of beneficial microbes that live inside of you. Conversely, if you’re not eating fiber, you’re not feeding your bug buddies, which can lead to “good” bacteria dying off, and “bad” bacteria growing. This condition, called ‘dysbiosis’, will be the topic of a future blog post.

The final important piece of the definition is ‘fermentation’. Many things are fermented, think beer, wine, kombucha, or kimchi. This is the term for breaking down carbohydrates without the presence of oxygen, also is called anaerobic digestion. The products of fermentation can reduce inflammation throughout the body, a process covered in our post on diet and the immune system.

Remember this:

  • Fiber is a complex carb from plants that doesn’t get broken down into sugar
  • It generally contributes 0 calories to your diet
  • It’s food for the beneficial bacteria in your gut


Why is fiber good?

  • Low (or no) Calories
  • Microbes eat fiber and make mucin, strengthening your gut barrier and preventing foreign molecules from leaking out of your gut into your bloodstream.
  • Fiber is bulky and it absorbs water, leaving you feeling full for longer. There is also evidence that high fiber foods will alter hunger hormones, making you crave food less.
  • Fermentation leads to Short Chain Fatty Acid production (SCFAs)
  • Many SCFAs are anti-inflammatory