Growing up, fiber was something only my grandparents talked about. Now that I’m getting older, I’m the one talking about fiber. I guess you either die a hero, or live long enough to talk about prunes all day.

In a meta-analysis, fiber reduced overall mortality by 23%. Not only did fiber improve cardiovascular mortality by 26%, surprisingly enough, fiber also reduced risk of cancer by 22%. This meta-analysis covered 64 studies with 3,512,828 total subjects, and the results have been reproduced in other studies. For example, a different study every 10g per day increase in fiber intake, there was a 10% reduction in overall mortality risk. While nutrition science isn’t perfect, the unreasonable effectiveness of fiber has a pretty strong body of evidence behind it.

Dietary fiber drives both cardiovascular and cancer mortality, through different mechanisms

Why does fiber work well for both cardiovascular disease and cancer? In this post, we’ll dive into some of the biochemistry of how fiber works in your body, specific benefits for cardiovascular disease and cancer, soluble vs insoluble fiber, and some ways to get more fiber beyond just eating prunes and broccoli.

Fiber can be both soluble and insoluable. Soluble fiber dissolves in water, and therefore is active in your gut. This will be important to its effect on cholesterol, which we’ll cover in the next section.

What about insoluble fiber? According to the study cited above:

Insoluble fibers have fecal-bulking characteristics that may promote regular bowl movements and avoid constipation.

“Fecal-bulking characteristics.” Interesting. The study itself shows some benefits from insoluble fiber on cancer (not cardiovascular disease); we’ll get into some of the potential mechanims of how stool bulking prevents colon cancer later.

Soluble fiber lowers cardiovascular mortality by driving down LDL cholesterol (or ApoB), partly balancing the effect of saturated fat. Specifically, soluble fiber binds to bile acids in the intestine, which causes your liver to pull LDL out of your blood stream to synthesize new bile acids. Fiber also stabilizes blood sugar by slowing digestion (so that food takes longer to absorb), which has a positive benefit on HbA1c. HBA1c is a biomarker for diabetes which is also an input into cardiovascular risk calculators.

Mechanisms for how soluble fiber lowers LDL cholesterol (or ApoB) by binding to bile in the liver (source)

Finding foods high in soluble fiber is a bit counterintuitive since some high-fiber foods, such as okra or peas, are mostly insoluble. Good sources of soluble fiber are avocados, apples, beans, lentils, and many whole grains.

Saturated fat tends to raise LDL cholesterol. Most heart disease risk comes from three saturated fatty acids in particular: lauric, myristic, and palmitic acid. Each of these decrase LDL receptor activity in the liver, slowing the clearance of LDL cholesterol from the blood. While the balancing effects of fiber and saturated fat occur through distinct biochemical pathways, practically speaking, high fiber foods are low in saturated fat, so shifting your diet toward high fiber foods will often have the “bonus” of naturally reducing saturated fat intake.

If the effects of fiber on cardiovascular disease are well understood, it’s less intuitive why fiber would reduce cancer mortality.

Part of the answer is from the types of cancer that high fiber is associated with reducing.

High intake of dietary fiber is associated with a reduced risk of esophageal, gastric, colon, rectal, breast, endometrial, ovarian, renal cell, prostate, and pancreatic cancers. Colon cancer is probably the area with the strongest evidence, with studies showing that every 10g / day of fiber reduces colon cancer by 10%. Some mechanisms that explain this process are increased stool bulk and dilution of carcinogens in the colonic lumen, reduced transit time, and bacterial fermentation of fiber to short chain fatty acids. Since these studies are all based on correlation, not randomized control trials, it’s also possible there’s residual confounding. One potential variable is folate intake (B12), since folate is involved in DNA synthesis, repair, and methylation and many high-fiber foods are also high in folate.

You don’t need to go to extreme lengths to get enough fiber. Both food and supplements can be sources of dietary fiber, many people use both, and there are now some new options that are more palatable than traditional supplements like metamucil.

The most common dietary fiber supplements are psylium fiber husk, fiber gummies, and olipop (a high fiber, pro-biotic soda). Metamucil is a common brand name of psyilum fiber husk, which you typically mix with water to make a gel. While psylium fiber husk has the highest amount of fiber (14g per oz), gummies (2.5g per gummie) and olipop (9g per can) are more palatable to a lot of people.

Fiber supplements: psyilum fiber husk, gummies, and a high-fiber soda.

Many fiber supplements are inexpensive. For example, you can get a 24oz-container of psyilum fiber husk for $13.

Foods high in soluble fiber including avocados, whole grains, chickpeas, apples, lentils, broccoli, brussels sprouts, certain seeds, and artichokes.

While cooking meals at home is a good option, even fast food chains you may think of as “unhealthy” often have high-fiber options. For example, even Dunkin’ Donuts has some high-fiber options: a multigrain bagel with 8g of fiber and avocado toast with 6g. Eating healthier can be done anywhere, by anyone—don’t let the perfect be the enemy of the good.

You can’t improve what you don’t measure. If you’re trying to get more fiber—or to optimize other heart health nutrients like saturated fat, sodium, and potassium—Empirical Health is designed to make tracking very easy.

Calculating your daily fiber in Empirical Health for iOS or Android. You can track via photo, text, or by logging a supplement.

Fiber reduces all-cause mortality by 23%, including reducing both rates of cardiovascular disease and cancer. You can easily get fiber from supplements and food,