Dietary Fiber 101
Welcome to Dietary Fiber 101, where we’ll learn the basics of what this buzzword is all about! This is intended for a non-medical general audience with little to no biochemistry background. For my medical friends, I’d recommend going to primary sources via PubMed or ask your friendly neighborhood GI doc who is passionate about fiber and the gut microbiome!
My disclaimer. This blog post is a tertiary source of information and science is always changing. This is for educational purposes only and does not substitute for professional medical advice. Talk to your physician if you’re seeking medical advice, diagnoses, or treatment.
What
Dietary fiber is a type of carbohydrate that human cells don’t have enzymes to break down. It’s found in fruits and vegetables in two forms: soluble and insoluble fiber. Soluble means that it’s a fiber that will be able to dissolve in water, insoluble means it cannot. Most fruits and vegetables will have a mix of both types, but some will have more of one over the other. We need both in our diet. Since we’ll be focusing on soluble fiber for its health benefits, we’ll break down the category further into fermentable/fermented and non-fermentable/nonfermented soluble fiber.
Why
So if humans can’t digest dietary fiber, why do we need it? It’s for 2 big reasons: poop and bacteria.
Poop: Fiber helps you have regular, unstrained bowel movements. Constipation and low-fiber diets are common causes of hemorrhoids, and over 50% of adults over the age of 50 in the US have had hemorrhoids. Not fun. Here’s more on the condition by Mayo Clinic. There’s evidence that both insoluble and non-fermentable soluble fiber (“gelling”) help with this.
Bacteria: I’m sure you’ve heard of our friendly colony of gut bacteria, sometimes referred to as our “gut microbiome” or “gut microbiota”. We have more bacteria in our bodies than human cells, so they’re a pretty important ally to keep. I mentioned that human cells can’t break down dietary fiber, well it turns out these gut bacteria LOVE eating fiber. In sum, we feed them fiber, and they give us gifts in return in the form of lowering the risk of colon cancer, lowering bad cholesterol levels, reducing the risk of type 2 diabetes, and keeping a healthy weight.
How
How does dietary fiber work?
Poop: Since fiber isn’t digested, it stays within the GI tract from the moment we eat until it exits, adding bulk to our stool. More bulk and weight of stool increase the natural muscular movement of our GI (gastrointestinal, aka “gut”) tract, literally keeping things moving and preventing constipation. It helps keep our poop in the ideal range of being not too dry and not too wet. On the other side of the spectrum, fiber will absorb extra water in your stool to help with diarrhea. The bristol stool chart is a 1-7 scale that you can use to compare if you’re in the ideal 3-4 range. Here’s a quick link to the chart. The sweet spot is between “corn on the cob” to “sausage”!
Bacteria: Our gut microbiota breaks down fiber into shorter molecules that interact with the human cells in our gut lining, which trigger hormones that signal to our brain and the rest of the body that we’re full. You might have heard them as “satiety” hormones. This is one of the ways high-fiber foods make you feel fuller! There’s also evidence to support that fiber will slow down the absorption of nutrients throughout the entire GI tract, helping further regulate satiety, blood sugar, and insulin levels.
One of these short molecules, in particular, called butyrate, is highlighted to act on many different parts of our body. Notably, butyrate interacts with our fat cells (called adipocytes) in the body to reduce fat accumulation, lower insulin levels, and increase fat breakdown. This effect is compounded by the happy gut bacteria also releasing chemicals that aid in the increase in fat breakdown.
In colon cells, butyrate promotes the self-destruction of pre-cancerous cells, reduces inflammatory chemical markers, and provides energy for the cells. In the liver, it reduces inflammation and fat accumulation, both protective measures for a healthy liver. Butyrate even interacts with skeletal muscle cells to increase lean body mass and decrease fat accumulation.
Here’s what Mayo Clinic has to say about dietary fiber.
Next Steps
The daily recommendation for fiber intake is 25g for women and 38g for men per day.
A really important point to make is that the type of fiber matters. More evidence has been shown against enriching a highly-processed diet with processed fiber supplements, and studies suggest that it may actually cause harm. A common supplement you’ll find is inulin from chicory root. Inulin is a fermentable soluble fiber and evidence suggests it does not have the health benefits that a non-fermentable soluble fiber like psyllium has. There was even a study where they demonstrated 30g of inulin supplementation led to an increase in pro-inflammatory markers in their subjects. Check out the table below for other types of fiber.
The first step in my fiber journey was to understand how much I was eating on a daily basis. Look at every label to track fiber for a week and see if you’re among the 90% of Americans who are eating 15g or less per day. 10g or less is considered a very low-fiber diet. One of my medical school GI doctors told us that we should be getting close to 50g per day as our ultimate goal. That’s a lot of fiber!
My personal goal is 25g and I’ll supplement as needed. Because I’m not a plant-based eater and love my protein, I meet my goal with:
10g from my American diet at baseline
7g from 1T of psyllium husk (bonus of it being a net zero-calorie source of fiber!)
9g from my choice of:
Chia seeds in oat milk, 5g of fiber per serving (1Tbsp)
Granola (current favorite is Bliss cereals), 6g of fiber per serving (1.25cups)
Raspberries (I like them frozen or freeze-dried), 8g of fiber per serving (1cup)
Metamucil fiber thins (I like cinnamon spice), 5g of fiber per package (2 cookies)
Black beans (My favorite are the ones with the green kernels), 15g of fiber per serving (1cup)
It can be a challenge to know where to start, so here’s a link to UCSF’s guide and Mayo Clinic’s guide on how to increase fiber intake and list of fiber-rich whole food.
Great job for making it all the way here! I know it’s a lot of information, so feel free to come back to this page again and again until you’re ready to take your first step. I started calling my journey “chasing fiber”, it’s a whole lifestyle! If you know me, I’m a quality-over-quantity type of person, and this applies to my health. I want to eat and drink the things I want with minimal time in the hospital as I get older, and I think meeting fiber goals is a huge step in the right direction. Let me know if you have suggested edits, comments, or questions!
edited Dec 23, 2022