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In this article, I tell you everything you need to know about dietary fiber, its benefits, myths, and how much fiber you need per day. I also talk about the risk of ingesting too much fiber; I give you a list of foods high in fiber and much more.
In a nutshell, fiber is not an essential nutrient because the body can’t absorb it and its absence does not cause any detectable deficiency state. However, adequate fiber intake can reduce the risk of certain medical conditions. So continue reading to learn more.
If you are constipated, you want to lose weight, control your blood sugar, or if you suffer from cardiovascular disease, the standard advice is to eat more fiber. So it seems like fiber is essential but is fiber a nutrient or a vitamin? And what are the side-effects of not consuming enough?
Fiber has a similar molecular structure as carbohydrates, but the human body lacks the necessary enzymes to digest it. As a result, fiber is not a nutrient (1).
However, fiber has many health benefits, and scientists regularly find more data to suggest that increasing your fiber intake is a good thing.
The Academy of Nutrition and Dietetics recommends 14 grams of total fiber per 1,000 kcal or 25 grams for adult women and 38 grams for adult men (2). Scientists have based this recommendation on research that demonstrates the ability of the fiber to lower the risks of certain diseases.
However, if you stop consuming fiber today, you won’t suffer from any deficiency or immediate side effect. Instead, you might increase your long-term risk of specific chronic diseases.
Some recent studies have found that excessive fiber intake may reduce the absorption of certain nutrients. But the effect is insignificant, and, as a result, you shouldn’t worry about consuming too much (natural) fiber.
Fiber is a collective name for a large number of molecules that are structurally similar to carbohydrates.
There are two categories of dietary fiber (soluble and insoluble), and there is also a synthetic version we can often find in supplements.
You can find synthetic fiber in various supplements and processed food. Manufacturers create synthetic fiber from non-digestible carbs that are similar to their natural counterparts. However, synthetic fiber might not be as effective and may cause more adverse side-effects. So, I recommend choosing natural sources of fiber for the same reasons as doctors recommend getting vitamins and minerals from food instead of from supplements. There are a ton of micronutrients and chemicals in natural food sources that can influence the bioavailability and how the body absorbs those nutrients that you might not get from synthetic sources.
Insoluble fiber does not dissolve in water, and thus it does not change to a gel-like structure. This type of fiber is typically responsible for increasing stool volume and improving bowel movements. We can find it in most seeds, whole-grain products, and in the skin of most fruits. Insoluble fiber does not have any effect on your blood sugar levels, but it can slow down the rate of which your body absorbs glucose. As a result, it lowers the so-called glycemic index (GI) of food. (3).
Soluble fiber has a similar function as insoluble fiber because it remains in your stool and improves gastrointestinal functions. However, it forms a gel after contact with water. As a result, it binds with bile, a fluid that aids digestion and that holds a lot of cholesterol molecules. Thus, when your body excretes soluble fiber, all of the bile attached to it goes away, preventing cholesterol from returning to the bloodstream. That’s how soluble fiber can reduce cholesterol levels and improve cardiovascular health (3). You can find soluble fiber in nuts, apples, beans, oatmeal, and other foods.
Note that all fiber-rich foods contain both types of fiber at varying degrees. While the distinction between those two types is a bit out of date, manufacturers need to declare what kind of fiber they are offering in their products, especially if there are health claims on the package (4).
After making the distinction between the two natural types of dietary fiber, let’s have a look at what fiber does to your body.
Both soluble and insoluble fiber can help if you’re trying to lose weight. One of the best ways to accomplish that is to reduce the carbohydrate intake and to create a negative caloric balance. In other words, you eat fewer calories than what your body burns.
Fiber is not absorbed or degraded, so it makes your stool bulkier and slows down your gastric emptying, thus making you feel satiated for a longer time. That helps to avoid those sugar cravings that would likely cause a rebound effect (2).
For many years, fiber has been an ally if you suffer from cardiovascular disease or have an increased risk. Scientific studies and clinical trials determined that insoluble fiber coming from cereals and vegetables protect us from cardiovascular disease and coronary heart disease. On the other hand, fiber coming from fruits predominantly prevents cardiovascular disease (4).
Fiber reduces the glycemic index of foods, which keeps sugar levels and triglycerides in control. Plus, fiber also improves cholesterol levels, as we have discussed previously, reducing specific types of it, such as LDL, and preventing atherosclerosis (2). There’s also evidence that dietary fiber lowers your blood pressure if you suffer from hypertension, especially when your systolic blood pressure is elevated (5).
Increasing your fiber intake can help you as a preventative measure to reduce your risk of type 2 diabetes. But it also helps diabetic patients to control their disease. Fiber improves our insulin sensitivity, and increasing dietary fiber intake also slows down gastric emptying, improves digestion, and slows down the absorption of glucose. Thus, if accompanied by fiber, food has a lower glycemic index, which means that instead of experiencing violent spikes in your sugar levels, carbs will be absorbed at a steadier pace, allowing your body to keep sugar levels under control. Scientists still don’t fully understand the exact role fiber plays in reducing the risk of developing type 2 diabetes. However, there is strong evidence that fiber can benefit those who suffer from insulin resistance or pre-diabetes.
Both soluble and insoluble fiber can improve your digestive health. They create extra bulk in your stool, thus making it easier for your body to regulate bowel movements. Fiber also reduces the internal pressure in your gut. As a result, there’s a lower chance for you to develop hemorrhoids, polyps, and other intestinal problems. Both soluble and insoluble fiber are beneficial if you’re constipated, but if you’re experiencing diarrhea, doctors recommend soluble fiber (2).
Research suggests that fiber is an effective means to prevent different types of cancer, especially colon cancer and breast cancer. According to the World Cancer Research Fund, there is convincing data that eating more fiber will reduce your risk of developing colorectal cancer (6). For every 10 grams of fiber, we consume per day, the risk of cancer is reduced by an impressive 9% (7).
Researchers explain that fiber increases fecal bulk, which reduces the time of intestinal transit. That in turn, decreases the time your intestines are exposed to particular carcinogens (2). On the other hand, fiber reduces the risk of breast cancer because it removes excess estrogens and modulates the activity of specific enzymes in the gut (8).
As we will further discuss, our gut microbiota is capable of degrading some types of fiber, especially β-glucans, oligosaccharides, and others. After fermenting fiber, your healthy gut bacteria start to produce short-chain fatty acids, a unique variety of fatty acids with a range of benefits, as we will discuss further (2).
Scientists have recently discovered that modulating the type of bacteria that colonize our intestines can have a positive effect on the rest of the body. That’s why there are so many studies about probiotics and prebiotics, and a slew of manufacturers offering unique probiotic strains directed towards a given ailment.
A prebiotic is a substance that selectively stimulates the growth of certain bacteria in the gut, and that’s precisely what inulin and fructo-oligosaccharides do. Their structure is similar to that of carbohydrates, but only certain (healthy) bacteria have the enzymes to degrade and use them. We can find these prebiotic fibers in garlic, onions, oats, leeks, peas, and other foods (10).
Given its properties, we can also consider fiber as a naturally-occurring prebiotic because even if our body can’t synthesize and absorb it, it is a useful energy source for our healthy bacteria, and it promotes their growth (2).
But that’s not the end of the story. When bacteria break down fiber, they release short-chain fatty acids, as I have mentioned earlier. Propionate, one of those fermentation metabolites has implications for your health, reaching far beyond the gut. (9) Specifically, those fatty acids can improve the normal function of the immune system, reduce cellular adhesion, and protect from inflammation (11).
As I mentioned previously, too much fiber does not cause significant health problems. However, in some cases, excessive fiber intake can cause specific symptoms and change the way our gastrointestinal system works.
It’s also worth noting that fiber is not the silver bullet and solution to all of our gastrointestinal problems. There are inflammatory bowel diseases such as Crohn’s disease that will not benefit from fiber intake because the added bulk in the stool can cause intestinal blockages. Also, patients suffering from gastritis can aggravate their symptoms after eating fiber-rich foods because it slows down the gastric emptying.
Too much fiber can also cause intestinal gas and may increase the frequency of your stools. If you have a food intolerance, it may cause excessive bloating and flatulence. For instance, inulin can cause discomfort, abdominal pain, and bloating in sensitized patients, and when that happens, I recommend to stop consuming inulin-rich foods, such as asparagus, leeks, and sugar beets (12). I realized a few months ago that I get incredibly bloated after eating asparagus and artichokes and I think inulin is the reason.
Below are high fiber food charts, based on information from the Mayo Clinic. All amounts are in grams.
|Fruits||Serving Size||Total Fiber|
|Vegetables||Serving Size||Total Fiber|
|Green peas, boiled||1 Cup||9.0|
|Broccoli, boiled||1 Cup chopped||5.0|
|Turnip greens, boiled||1 Cup||5.0|
|Brussels sprouts, boiled||1 Cup||4.0|
|Potato, with skin, baked||1 Medium||4.0|
|Sweet corn, boiled||1 Cup||3.5|
|Cauliflower, raw||1 Cup chopped||2.0|
|Carrot, raw||1 Medium||1.5|
|Legumes, Nuts, and Seeds||Serving Size||Total Fiber|
|Split peas, boiled||1 Cup||16.0|
|Lentils, boiled||1 Cup||15.5|
|Black beans, boiled||1 Cup||15.0|
|Baked beans, canned||1 Cup||10.0|
|Chia seeds||1 ounce||10.0|
|Almonds||1 Ounce (23 nuts)||3.5|
|Pistachios||1 Ounce (49 nuts)||3.0|
|Sunflower kernels||1 Ounce||3.0|
|Grains||Serving Size||Total Fiber|
|Spaghetti, whole-wheat, cooked||1 Cup||6.0|
|Barley, pearled, cooked||1 Cup||6.0|
|Bran flakes||3/4 Cup||5.5|
|Quinoa, cooked||1 Cup||5.0|
|Oat bran muffin||1 Medium||5.0|
|Oatmeal, instant, cooked||1 Cup||5.0|
|Popcorn, air-popped||3 Cups||3.5|
|Brown rice, cooked||1 Cup||3.5|
|Bread, whole-wheat||1 Slice||2.0|
|Bread, rye||1 Slice||2.0|
According to studies, most Americans do not consume enough fiber in their diet. While increasing your fiber intake is relatively easy, there are some pitfalls you should try to avoid.
For example, common advice to increase your fiber intake is to eat whole grains and legumes, such as beans and lentils. While those foods are an abundant source of fiber, they come at a cost. For example, most types of grains contain gluten, especially wheat. Raw legumes are toxic, and only cooking makes them eatable. But even after soaking and cooking them, they include plenty of anti-nutrients, which is why I recommend staying away from them. So here are some better ways to increase your fiber intake.
Nuts and seeds are easy to carry around, and seasonal fruits are delicious snacks that can help to increase your fiber intake.
Raspberries, for example, are high in fiber (8 grams per 1 cup) and they don’t have a lot of sugar. So you can even have them if you are on the Keto Diet!
Unpeeled pears and apples are also a good source of fiber, but they contain more sugar than raspberries.
Avocados, another fruit, also contain a decent amount of fiber (10 grams per cup) and tons of healthy fats.
If you are consuming fruits, remember most of the fiber is located in the skin. In most cases, peeling fruits would only waste that valuable feature of those foods.
While it is true that natural juices are an excellent source of nutrients, they lack fiber. Without it, your body gets hit with a ton of (fruit) sugar and your insulin levels spike. As a result, I recommend blending over juicing.
Broccoli is one of my favorite vegetables because it can modulate the gene expression in humans. Plus, one cup of it has about 5 grams of fiber.
If you are struggling with maintaining a decent fiber intake, you can also supplement with natural psyllium husk powder. I usually have two teaspoons of it mixed with water or as part of a protein shake after a workout. Psyllium husk is a natural product with three grams of water-soluble fiber per teaspoon.
Look at your stool! According to the Bristol Stool Chart, your stool should look like type 3 or 4 in the chart below.
If it doesn’t, try eating a few spoons of psyllium husk powder and notice the difference the next day. Of course, there are numerous reasons why your stool looks like it does and it’s not always a lack of fiber. But unless you have a medical condition, chances are, that fiber (or lack thereof) may play an important part in that.
Fiber is not an energy source for us, and we can’t consider it an essential nutrient to sustain life. However, fiber feeds our healthy gut bacteria and scientists have demonstrated the positive impact of fiber on reducing certain chronic diseases. As a result, I consider fiber an excellent addition to an already healthy diet.
I was born and raised in Austria. I speak German, English, and Spanish. Since moving to the U.S., I have lived and worked in Alpharetta, GA. In my twenties, I was a professional 100m sprinter. These days I do mostly CrossFit. I'm a technologist and Apple fan. I love science and don't believe anything unless there is proof. I follow the Paleo diet and intermittently fast every day. I'm married and have two trilingual kids. My goal with this blog is to share what I learn so that you can spend time on something else.