Fiber is an important part of a healthy diet.
In fact, eating enough fiber is said to keep your gut healthy and protect against type 2 diabetes and weight gain ().
It's recommended that men aim for 38 grams of fiber per day, while women should aim for 25 grams ().
However, not all dietary fiber is created equal, and different types have different health effects ().
This article explores how dietary fiber works to protect your health and how much of it you should eat.
Different Types of Fiber
Dietary fiber is a group of carbs that humans can't digest. It's found in all plant foods, including fruits, vegetables, nuts, seeds and grains.
Because dietary fiber refers to a diverse group of different types of carbs, it can be categorized in various ways.
However, it's usually grouped into one of the following categories, according to its solubility:
- Insoluble fibers: These fibers don't dissolve in water. They generally pass through your gut unchanged and add bulk to your stool.
- Soluble fibers: These fibers absorb water in your gut to form a gel-like paste. This slows down the digestion of nutrients in your food.
Most foods contain both soluble and insoluble fibers, but usually have more of one type than the other.
In general, foods that contain mostly insoluble fibers include whole grains, wheat bran and some fruits (like avocados) and vegetables (such as celery and cauliflower).
Good sources of soluble fibers include oats, flaxseeds, beans and lentils, as well as some fruits (such as berries and bananas) and vegetables (like broccoli and carrots).
Bottom Line: Dietary fiber is usually classified as soluble or insoluble. It's found in all plant foods, including fruits, vegetables, nuts and seeds.
Fiber Can Help Keep Your Gut Healthy
Eating fiber is said to help maintain regular bowel movements and relieve constipation.
Additionally, people with constipation who don't eat much fiber can usually benefit from eating more (, ).
In fact, one study found that as many as 77% of people with chronic constipation experienced relief by simply eating more fiber ().
Furthermore, it's thought that sufficient amounts of some types of fiber help promote the growth of "good" bacteria in your bowel ().
For example, soluble fibers known as prebiotics feed your gut's beneficial bacteria. By helping your good gut bacteria thrive, they can benefit your health (, ).
They also increase the production of some important nutrients, including short-chain fatty acids like butyrate, which are thought to promote a healthy immune system and good gut barrier function (, , ).
Having a strong gut barrier is important. It helps keep you healthy by preventing things like viruses and harmful bacteria from entering your body.
Some prebiotic foods include oats, bananas and berries.
However, it's currently not fully known which types and amounts of fiber best promote the growth of good bacteria in your gut ().
Bottom Line: Eating adequate amounts of dietary fiber can prevent constipation. Soluble, prebiotic fibers help maintain the balance of good bacteria in your gut.
Fiber Can Make You Feel Full and Help You Lose Weight
Including fiber-rich foods in your diet can help you lose weight.
In fact, observational studies show that people who eat lots of fiber tend to weigh less and have less body fat than those who don't (, ).
This may be because high-fiber foods are both lower in calories and more filling than low-fiber foods. This means that high-fiber foods could help you eat less, without you even noticing ().
This was reflected in one review of over 50 studies, which estimated that people who ate 14 grams more fiber per day automatically reduced their calorie intake by around 10% ().
Interestingly, this effect was larger in people who were overweight or obese.
However, a recent review found that only around 39% of fibers helped reduce hunger. Of these, just 22% resulted in a reduction in the amount of food eaten at a meal ().
Viscous, soluble fibers — which form a thicker, sticky gel in your gut when they absorb water — are the most effective at keeping you full ().
Food sources of viscous, soluble fibers include flaxseeds, legumes and oats.
Emerging research is also investigating whether supplementing with specific types of fiber may help weight loss ().
However, in general, fiber supplements haven't always been found to be particularly useful ().
One exception to this is a fiber supplement called glucomannan, which has been shown to help people lose a small amount of weight in the short term ().
Nevertheless, it can't be presumed that fiber supplements have the same health benefits as whole-food fibers. This is because whole-food fibers come with many other beneficial nutrients ().
Bottom Line: Viscous, soluble fibers are thought to be the most helpful fibers for weight loss. If you don't eat much fiber, increasing your intake by around 14 grams per day could help you lose weight.
Fiber Can Lower Blood Sugar Levels and Protect Against Type 2 Diabetes
Regularly eating the recommended amount of fiber is thought to help prevent and treat type 2 diabetes.
Observational studies have linked eating more fiber with a lower risk of developing type 2 diabetes (, , , ).
One study followed over 75,000 people for 14 years and found that those who ate more than 15 grams of fiber per day had a significantly lower risk of developing diabetes ().
Additionally, this risk was lowest in the group that ate the most insoluble fiber.
Another study found that people eating 3–5 servings of whole grains per day had a 26% lower risk of type 2 diabetes ().
If you already have diabetes, it's also thought that eating more fiber could help you control your blood sugar levels.
This is because soluble fibers slow down the digestion and absorption of sugars, resulting in a more gradual rise in blood sugar levels and fewer blood sugar spikes.
Studies show that increasing fiber intake, especially soluble fiber, can lower blood sugar levels and improve metabolic health in people with type 2 diabetes (, ).
Bottom Line: Regularly eating dietary fiber may help prevent type 2 diabetes. Eating fiber may also improve blood sugar control in people with type 2 diabetes.
Does Fiber Have Any Negative Effects?
While increasing the amount of fiber in your diet should benefit your health, doing so can sometimes cause problems.
If you aren't used to eating a lot of fiber, suddenly increasing your intake by a large amount could result in digestive symptoms like bloating, pain and gas.
Moreover, if you are chronically constipated, you may find that increasing the amount of fiber you eat doesn't help. It may be that reducing your fiber intake is the best way to improve your symptoms ().
However, this is usually only the case if you have chronic constipation that isn't caused by an inadequate fiber intake ().
Also, those with irritable bowel syndrome (IBS) may find fiber-rich foods problematic.
This is because many high-fiber foods are also high in fermentable carbs known as FODMAPs. These are known to make IBS symptoms worse (, ).
Bottom Line: Eating too much fiber can be a problem, especially if you have a functional bowel problem like IBS.
So How Much Fiber Should You Eat?
Unfortunately, most people don't eat much fiber. In the US, most people eat less than half of the recommended daily amount ().
That said, the current evidence does not indicate which type or amount of fiber is optimal for your health.
Fiber from whole foods comes with many other healthy nutrients. So it may be that the type of fiber and where it comes from is more important than the total number of grams.
Therefore, for most people, eating enough fiber doesn't require obsessing over each and every gram.
Simply aiming to include healthy high-fiber foods with most of your meals should be sufficient.