Fiber For Fat Burning and a Stronger Gut

Fiber For Fat Burning and a Stronger Gut

You likely grew up hearing about how good fiber is for you. It’s the poster child for whole grains, and the sole reason cereals that look and taste like cardboard stay on grocery shelves. Marketers say fiber will make you more regular, lower your cholesterol, stabilize blood sugar, burn fat, and help you live longer.

You’ve probably heard of soluble and insoluble fiber, though you may not know exactly what they are. Conventional wisdom (and marketing) says that soluble fiber is the one that boosts your performance and that you should get plenty of it every day.

The reality is a bit more complicated. Both soluble and insoluble fiber offer benefits, and you really want both in your diet. There are also some more unusual types of fiber that offer cool upgrades to your biology. This article will cover all of the above.

Discover the best ways to fix your gut with this handy gut guide

What is fiber?

A closer look at gut

Fiber is a type of carbohydrate, but unlike sugars and starches, your body can’t break fiber down. Human digestive enzymes don’t work on it. Where most carbs degrade into simple sugars in your stomach or small intestine, fiber makes it through to your large intestine undigested. What it does for you at that point depends on the type of fiber you eat. We do know that fiber won’t take you out of ketosis (the fat-burning mode you get into on the Bulletproof Diet). 

Different types of fiber

Soluble fiber

Soluble fiber gets its name because it dissolves in water, and it’s a powerful biohacking tool. Soluble fiber turns into a gel-like substance in your gut, slowing stomach emptying so that vitamins and minerals spend longer in your system and absorb better[1]. That’s one reason you want soluble fiber with nutrient-dense meals – it helps you get more out of your food.

Soluble fiber also keeps your blood sugar and insulin from spiking after you eat[2], making it especially useful on carb refeed days. It also keeps you full for longer, possibly by modulating your hunger hormone, ghrelin[3].

One of the main reasons doctors push soluble fiber is its effect on cholesterol. Soluble fiber binds to cholesterol and keeps it from circulating through your blood, lowering your levels.

Sounds good at first, but the latest research shows cholesterol isn’t the problem when it comes to heart health. Even the American Heart Association has reversed its stance on cholesterol. Cholesterol is essential to your absorption of fat-soluble vitamins. It’s the building block for every single sex hormone you produce (think testosterone and estrogen). It’s not a problem in and of itself; the bigger issue is inflammation oxidizing your cholesterol. So unless your numbers are off the charts, you’re probably better off eating an anti-inflammatory diet than you are trying to lower your cholesterol.

Speaking of, fiber intake (soluble and insoluble) correlates with decreased inflammation, in the form of lower C-reactive protein[4]. It also links to lower risk of death from all causes[5].

Best sources of soluble fiber:

  • Brussels sprouts
  • Broccoli
  • Kale
  • Cabbage
  • Zucchini
  • Squashes
  • Carrots
  • Sweet potatoes
  • Almonds

Insoluble fiber

Insoluble fiber is another powerful biohacking tool. Insoluble fiber is even more anti-inflammatory than soluble fiber, and it curbs ghrelin, the hormone that makes you hungry, for several hours post-meal[6]. As a result, you tend to eat less when you get plenty of insoluble fiber. Counting calories isn’t part of eating Bulletproof, but if you can effortlessly decrease your food intake and still feel satisfied, it’ll give you an edge in burning fat, so you might as well do it.

Insoluble fiber may also increase your fat burning directly. A long-term study (on mice, but it’s still interesting) compared two diets over the course of a full year. The diets mimicked a standard Western diet – high refined sugar paired with lots of low-quality fat — except one group’s diet had soluble fiber in it, and the other’s had insoluble fiber. The mice who ate soluble fiber had improved gut bacteria, but they were also obese and pre-diabetic by the end of the year. The mice who ate insoluble fiber barely gained weight and showed far better fat metabolism.

Insoluble fiber may help you stay lean and decrease inflammatory stress on your body. 

Best sources of insoluble fiber:

  • Kale
  • Collard greens
  • Green beans
  • Celery
  • Leeks and spring onion
  • Cabbage
  • Broccoli
  • Cauliflower
  • Brussels sprouts
  • Almonds
  • Walnuts

Prebiotic fiber & resistant starch

Prebiotic fibers and resistant starches are a little more unusual. They can be soluble or insoluble, but what sets them apart is that they ferment in your gut and become food for your gut bacteria instead of just passing through. As a general rule, prebiotic fiber and resistant starch feed beneficial gut bacteria[7], giving them the fuel to colonize your GI tract. Your good gut bacteria ferment the fiber, breaking it down into short-chain fatty acids that fight inflammation and maintain the integrity of your gut lining[8].

Resistant starch works well for some people and causes massive bloating and gas for others. Experiment at your own risk. Learn more about resistant starch with this beginner’s guide

The best ways to get resistant starch:

  • Raw green bananas
  • Raw plantain flour
  • Raw potato starch

Prebiotic fiber promotes weight loss by improving gut bacteria[9][10], although they can also cause GI distress, particularly if you have IBS or Crohn’s Disease. Prebiotic fiber tends to be in less common foods, so you may have to get a little creative with your cooking to incorporate it.

You can find prebiotic fiber in:

  • Raw dandelion greens
  • Raw garlic
  • Leeks
  • Raw jicama
  • Chicory root (which is why there’s chicory root fiber in Bulletproof Collagen Bars)
  • Jerusalem artichoke


Glucomannan also deserves an honorable mention. It’s a type of soluble fiber derived from the root of the konjac plant. What sets glucomannan apart is that it’s far more viscous than any other fiber. Glucomannan gels in your gut, slowing GI emptying long enough to dramatically increase nutrient absorption. It curbs hunger hormones[11], ferments as a prebiotic to promote good gut bacteria[12], and triggers fat loss more than most other fibers[13][14].

Plus, glucomannan makes a tasty carb-free substitute for noodles. It’s been a staple of Japanese cuisine for years, where it’s called [15] They absorb the flavor of whatever you cook them with, which makes them perfect for a high-fat sauce, or just with butter and salt. Don’t mind the weird smell when you open them; boiling takes it away. Learn more about shirataki noodles here, or get cooking inspiration with these shirataki recipes

Getting enough fiber comes down to taking your parents’ advice: eat your veggies. Whole grains also have fiber, but grains come with so many other problems that you’re better off avoiding them. Stock up on dark leafy greens and raw nuts, and maybe experiment with some raw plantains or shirataki noodles. Your gut bacteria will thank you for it, and you might lose a couple pounds of fat, too.

For more ways to improve your biology with food, check out the free complete roadmap to the Bulletproof Diet, and get more in-depth hacks to build stronger gut bacteria. 

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