Oceanside Nutrition
Matt Priven, MS RDN
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An Ode to Fiber


Fiber has a bit of a stigma.

When we think of fiber, we tend to think of Metamucil powder and the bowel habits of the elderly. I’ll go so far as to say that fiber is the most overlooked aspect of food. Protein has certainly had its time in the sun. Fat is now having a real moment. Carbohydrates are constantly embroiled in media controversy. But fiber is standing in the back, maintaining a stoic disposition, while the world underestimates its potential.   

Dietary fiber is indigestible, but it exerts a powerful influence on our health, even though it never makes it past our digestive tract. Fiber is a carbohydrate, but it is distinguished by its indigestibility. Therefore, it doesn’t effect our blood sugar levels like “normal” carbohydrates do. 

We have historically classified fiber as either soluble or insoluble. For the average person, I don’t think it’s very important to distinguish between the two classifications, Many foods contain both soluble and insoluble fiber anyway. The more interesting story is how fiber influences our health in ways that are not so obvious..


To quickly explain the difference between soluble and insoluble fiber, here is an example: let’s say you had a few tablespoons of ground flaxseed in a bowl and mixed in some water. A few minutes later you’d notice that the mixture has become thick and gel-like. That is soluble fiber in action. The soluble fiber dissolves in the water. But flax also has insoluble fiber, like the harder bits of husk, which do not dissolve and remain visible. This characteristic becomes important when we talk about digestion. Soluble fiber sources absorb water in the digestive tract, which can slow down digestion. Insoluble fiber sources move through the digestive tract without absorbing water, helping things move along. We need both types for good health and this push/pull effect helps us stay nice and regular. 


Fiber grows healthy gut bacteria

Just because we humans can’t digest fiber doesn’t mean the same applies for all species. Many of the billions of bacteria that reside in our digestive tract can digest fibers. When we eat fiber-rich foods, we encourage the growth of a robust and diverse population of healthy gut bacteria. People who host a rich population of gut bacteria have been shown to have better cholesterol levels, lower body fat, less insulin resistance, and lower overall inflammation compared to people with less robust/diverse gut bacteria populations. Science is still unravelling why certain types of bacteria populations are associated with improved health. Much of the research is focused on the byproducts created by this bacterial feeding frenzy, like short-chain fatty acids. The important take away: we can take steps to improve our gut bacteria population by increasing our fiber intake. If we take care of them, they take care of us! 

Fiber helps control our blood sugar levels

Despite being a carbohydrate, fiber can’t increase our blood sugar levels because it never makes it past the digestive tract and into the blood stream. But its indigestibility isn’t the only way fiber helps. Soluble fiber makes our food thicker and more viscous as it moves through the digestive tract, which slows down the rate at which "normal" carbohydrates can enter our blood stream. This means blood sugar levels don’t go quite as high as they would if you skipped that fiber. Sure enough, large scale studies show that high-fiber diets lead to lower risk of type 2 diabetes compared to diets low in fiber. 

Fiber protects our heart

High dietary fiber intake is associated with lower rates of coronary heart disease. The data here is pretty clear, but the reasoning is not. How can eating fiber possibly protect our heart? One theory focuses on the fact that soluble fiber increases bile acid excretion. Since bile acid is comprised of cholesterol, our cholesterol levels should go down in order to support this increased need for bile production. Another theory focuses on the aforementioned byproducts of the bacterial fermentation, short-chain fatty acids. Through a complex series of reactions, these short-chain fatty acids may decrease blood pressure and total cholesterol, which would be protective for the heart. 

Fiber can help us feel better

Fiber fills us up and makes us feel satisfied. By engaging stretch receptors in our stomach, high-fiber meals tell our brain that we are fed. But fiber may also be more directly responsible for good feelings. Research has been pouring in recently showing that there is a strong connection between our guts and our brains. Studies have begun showing that rates of anxiety, depression and other stress-related disorders may be influenced by the healthfulness of our gut bacteria populations, which we know are influenced by our fiber intake. Memory and cognition may be similarly influenced by our gut health. This all adds up to another reason to build a healthy gut bacteria population by getting plenty of fiber. 

So where do we find fiber? 

Fiber is found in plant-based foods, like whole grains, beans and lentils, nuts and seeds, fruit and vegetables. Some notably high-fiber foods are flax and chia seeds, beans and lentils, raspberries and blackberries, avocados, and leafy greens. There are some other interesting high-fiber foods out there and I’m going to be posting soon about techniques for increasing your daily fiber intake. So stay tuned for a much more comprehensive reflection on high-fiber eating. I may even put myself to the test and see just how much fiber I can eat in a week; a dietitian friend of mine does not think I can eat 500 grams of fiber in a week (and she's probably right), but the gauntlet has now been thrown...