Printer icon

But Where do you Get your Fiber?

What is fiber, why do you need it, and how can you get it on a Paleo diet?

If you’ve spent any amount of time reading conventional weight-loss blogs and magazines, “filling fiber” is probably right up there with “heart-healthy whole grains” or “artery-clogging saturated fat.” You’re probably familiar with the advice to eat more fiber, or even take a fiber supplement if you’re constipated. Like other conventional wisdom, all of this advice ought to be analyzed with a healthy amount of skepticism, but in this case, it’s not all wrong.

Mainstream nutritionists aren’t as mistaken about fiber as they are about grains and fat – there is a lot of truth to the “filling fiber” catchphrase, and fiber certainly isn’t dangerous if it’s eaten in moderate quantities from non-toxic food sources. But there’s a limit to the benefit; more is not necessarily better, especially more of the wrong kind. As this article explains, fiber is definitely not a one-size-fits-all positive, and there’s a healthy and an unhealthy way to go about including it in your diet.

What is Fiber?

Most basically, fiber is a kind of carbohydrate found in the cell walls of plants, but it’s a carbohydrate that you can’t digest. The diet-magazine claim is that you don’t get any nutrients from it, but you also don’t get any calories (although as seen below, this is only actually true for one kind of fiber). This is why the diet industry loves it – fiber adds “free” volume, so you can eat larger portions without eating more calories.

This analysis of fiber as a miracle food is great for selling products, but for good health it’s useful to have a more comprehensive idea of what fiber actually is. First of all, talking about “fiber” is imprecise, because fiber actually has two varieties: soluble and insoluble. When you eat soluble fiber, it reacts with water and turns into a gel during the digestive process. Insoluble fiber, on the other hand, absorbs the water and swells up to many times its own size. Soluble fiber slows digestion, while insoluble fiber speeds it up. If you’re getting them in reasonable quantities from healthy foods, both types of fiber are healthy for the gut, but they behave in very different ways. Soluble fiber nourishes the gut flora, and insoluble fiber helps add bulk to your stool, which can help prevent or treat constipation.

Since fiber is a major component of plant cells, almost all fruits and vegetables are good source, and most of them contain both soluble and insoluble varieties. Whole grains are also fiber-rich (if you ask a conventional nutritionist how to get more fiber in your diet, you’ll probably get “whole grains” as the first response), but since this benefit doesn’t cancel out all the other ways grains are bad for you, whole grains aren’t recommended on a Paleo diet.

Soluble Fiber and Gut Flora

One of the main benefits of soluble fiber doesn’t actually directly affect you – it’s a bonus for the gut flora that live in your large intestine and support everything from mental health to your immune system. Soluble fiber is these bacteria’s favorite food – eating plenty of fiber gives them all the nourishment they need to live long and prosper. This is one reason why fiber is so effective for constipation: healthy bowel movements are largely made up of dead gut flora, so supporting the growth of these bacteria is the best way to keep your digestive system functioning properly.

Unfortunately, there’s a darker side of this as well. Many people have too few bacteria in their digestive system, but some people also have too many. This condition is called Small Intestinal Bacterial Overgrowth, and it’s a major factor behind IBS symptoms and several food intolerances. Feeding an already overgrown population of bacteria can make the problem even worse. Thus, if you have SIBO or other bacterial overgrowth problems, it might be wise to go easy on the soluble fiber until your gut heals. Many people with IBS or SIBO find relief by restricting their intake of certain vegetables (most notably FODMAPs); this is in part because these vegetables are high in soluble fiber, and restricting them reduces the food available to the overgrown gut flora.

Benefits of Fiber

As well as nourishing your gut flora, fiber is also beneficial in several other ways. As noted above, insoluble fiber can help alleviate constipation by increasing the amount of matter in your colon; this is why getting more “roughage” helps many people have regular bowel movements.

Both soluble and insoluble fiber are also excellent for weight loss because (unlike “heart-healthy whole grains”) fiber actually lives up to its epithet. Fiber is very filling, partly because it requires a lot of chewing, so it slows down the meal. Think of eating six oranges in one meal (which nobody but the most ardent of raw vegans does on a regular basis) vs. drinking one glass of orange juice (which thousands of perfectly normal people do every day at breakfast). The difference is the fiber: orange juice has had all the fiber removed by the juicing process, so it’s not nearly as filling as the whole fruit.

The high fiber content of fruit is actually one reason why the fructose in fruit is not as damaging as the fructose in artificial sweeteners like sugar or corn syrup: the difference is that almost nobody eats enough fruit to feel the detrimental effects of fructose toxicity, because the fiber in the real fruit makes it much more filling than a soft drink or candy bar. Fiber also helps make carbohydrate safer to eat by controlling blood sugar spikes, making very important for anyone with diabetes, metabolic syndrome, or other difficulties in digesting carbohydrates.

Soluble fiber also increases the production of butyrate in the colon. Butyrate is a short-chain fatty acid that helps control appetite and metabolism; it also has anti-inflammatory properties and has been suggested as helpful in inflammatory gut disorders. It even seems to offer some protection against colon and breast cancer, although this is still being studied. Butyrate (also found in butter) is absorbed and used for energy by the colon. This is why soluble fiber doesn’t actually have 0 calories: the fiber itself has none, but it is converted to butyrate, which does. Thus, the soluble fiber from many plant foods actually provides a significant number of calories from fat. In Paleo terms, this can be seen as yet another benefit: fiber adds a significant source of healthy fats to the diet in addition to all its other properties.

On the whole, soluble fiber is slightly more beneficial, because it nourishes the gut flora and provides an excellent source of butyrate. Both kinds of fiber are useful, though, as long as you’re getting them from whole foods rather than from supplements.

Fiber and Cholesterol

Avocado are high in soluble fiber

One of the most commonly touted benefits of fiber according to conventional medical advice is its ability to lower cholesterol. But as anyone within 10 miles of a Paleo dieter knows by now, “lowering cholesterol” is not always a benefit and may actually be negative – it all depends on which kind of cholesterol we’re talking about. There’s no reason to be afraid of fiber because it lowers cholesterol, but it’s also not a ringing endorsement.

Fiber and Paleo

Even though Paleo dieters know better than to spend much time worrying about their cholesterol levels, getting plenty of dietary fiber is clearly beneficial. Luckily, Paleo includes plenty of fiber, even if it doesn’t always seem that way to the outside world. “But where do you get your fiber?” might very well be the Paleo equivalent of every vegan’s least favorite question, “but where do you get your protein?” Since fiber is so strongly equated with whole grains in most people’s minds, a grain-free Paleo diet seems at first glance to be sorely deficient.

But this is ignoring that Paleo dieters eat more vegetables than some vegetarians! All vegetables are rich in fiber, and if you’re eating only real foods it’s both simple and easy to get plenty of fiber in your diet without even worrying about it. For example, look at the menu below:

This is a fairly standard day of Paleo meals – it’s certainly not a diet that goes to any extremes to include huge amounts of fiber. There aren’t any “fiber-rich whole grains,” and all the vegetable portions are modest. But even this diet includes 33 grams of fiber – well within the standard recommendations of 20-40 grams. It’s clearly perfectly possible to get plenty of fiber on Paleo without resorting to any kind of dietary extreme or even making a special effort to do it.

Fiber Supplements vs. Real Food

Even though it’s so easy to get enough fiber from food alone, many people feel that they ought to take fiber supplements, or worry that they might be missing out on something if they don’t. Fiber supplements aren’t necessarily as beneficial as fiber-rich foods, though – not only do they lack the other nutrients found in foods, but they can also be overwhelming to the gut.

It’s important to note that many foods praised for being high in fiber are also high in several other potentially beneficial nutrients. One cup of blueberries, for example, has 3.6 grams of fiber, but also high levels of Vitamins C and K, and several beneficial antioxidants. It’s just not possible to judge whole foods entirely on the basis of their fiber content, since “a diet rich in fiber” is also a diet rich in all the vitamins and minerals packaged with that fiber. The fiber itself is also beneficial, but it’s far from the only factor at play. Unlike blueberries, Metamucil doesn’t have much in the way of nutrition, so it won’t deliver the same benefit as an actual food source of fiber.

The benefits and drawbacks of supplementation also depend on whether you’re supplementing with soluble or insoluble fiber. Supplementing with moderate amounts of soluble fiber (also known as prebiotics) is generally not problematic – it nourishes your gut flora and can help keep your bowel movements regular, especially if you’re suffering from a FODMAPS intolerance or some other food condition that prevents you from eating as many vegetables as you’d like.

Supplementing with insoluble fiber, on the other hand, is not nearly as useful. In such large doses, insoluble fiber (the kind found in fiber supplements) is actually a gut irritant. The additional bulk of the fiber scrapes against the lining of the gut, which can rub away the mucous lining the digestive tract and cause inflammation and irritation in the intestine.

To make things worse, relying on these megadoses of fiber for regular bowel movements can create a self-fulfilling prophecy: rather than moving properly on its own, your gut becomes dependent on high doses of fiber to force everything through your system.

The upshot of all this is that fiber-rich foods are healthy and nutritious, but the benefit doesn’t necessarily extend to fiber as a processed, purified supplement taken on its own. Soluble fiber supplements are fine for most people (although anyone with bacterial overgrowth problems should be cautious with them), but insoluble fiber is unnecessary and even harmful to supplement. This should be unsurprising: humans were designed to eat food, not Metamucil. That kind of concentrated dose of insoluble fiber isn’t available anywhere in nature, and for good reason: it’s just too much for our digestive systems to process regularly.

With that said, there is one case in which a temporary boost of insoluble fiber can be helpful. In the case of transient constipation (namely, constipation brought on by travelling, a sudden change in diet, or some other one-time reason), a fiber supplement can be a helpful band-aid to get everything moving again. For long-term health though, insoluble fiber supplements aren’t advisable.


Overall, fiber is everyone’s favorite kind of important nutrient: one that’s easy to get plenty of without thinking of, comes in so many different forms that everyone can find something they like, and doesn’t require a lot of laborious preparation or processing. Both soluble and insoluble fiber have benefits, although soluble is probably slightly better for you unless you’re struggling with SIBO or another bacterial overgrowth condition. Just eating plenty of fresh fruits and vegetables will give you as much fiber as your body can use, so grab a bowl of fresh strawberries and celebrate having one less diet problem to worry about.