Dietary fiber is a type of carbohydrate found in many foods. Human beings cannot digest dietary fiber because we don’t have a digestive enzyme that can digest it (Soliman, 2019). But, as we will discuss on this page, dietary fiber has many, many health benefits. There is a long list of very important health benefits that can follow from a diet high in soluble fiber. So much so that I feel strongly that we should be recommending a diet high in soluble fiber to all our patients.
Note: There are a few medical situations (not discussed here) in which a patient may have been told to avoid a high-fiber diet.
Firstly, it is important for us to realize that there are two very different types of dietary fiber and they are very different in their effects on the body and their potential health benefits. These are:
– Soluble (or water-soluble) fiber, and
– Insoluble (or water-insoluble) fiber.
It is very important to confuse these two types of fibers. Insoluble fiber helps with constipation. Soluble fiber has an impressive list of other health benefits (see below).
What is soluble fiber?
In the intestinal tract, soluble fiber dissolves in water to form a gel. This gel delays gastric emptying, slows the movement of food through the intestines, slows the digestion of the food, and decreases nutrient absorption (Soliman, 2019). Also, the gel formed from soluble fiber feeds healthy gut bacteria
Benefits of soluble fiber
Soluble fiber is helpful for so many important problems that it is very important that all clinicians encourage a high intake of soluble fiber. Each of the following conditions for which soluble fiber is helpful has a high prevalence in the population: irritable bowel syndrome, hypercholesterolemia, type 2 diabetes, high blood pressure, breast cancer, and non-alcoholic fatty liver disease.
– For irritable bowel syndrome with constipation, soluble, poorly fermentable fiber is recommended as a first-line treatment, but insoluble and fermentable fiber is not (Lacy et al., 2021).
– Foods high in soluble fiber are associated with at least a moderate reduction in LDL cholesterol (Schoeneck and Iggman, 2021). They reduce cholesterol by many different mechanisms (Soliman, 2019).
A high proportion of my patients have high LDL cholesterol and many of them are taking a statin medication. Clinical trial data suggest that adding psyllium to a statin may lead to a greater lowering of LDL cholesterol compared to a statin alone (Brum et al., 2018). Adding psyllium fiber to the statin was associated with reductions in LDL cholesterol that were equivalent to doubling the statin dose (Brum et al., 2018).
– In adults with type 2 diabetes, soluble fiber supplementation reduces hyperglycemia (Xie et al., 2021), including postprandial hyperglycemia (Vlachos et al., 2020).
– Many clinical trials have found that soluble fiber reduces both systolic and diastolic blood pressure (Khan et al., 2018).
– Soluble fiber may also reduce body weight and body fat in overweight and obese persons (Thompson et al., 2017).
– Higher intake of soluble fiber is associated with a reduction in the risk of breast cancer (Farvid et al., 2020).
– High soluble fiber foods are also part of dietary strategies to reduce intrahepatic fat in non-alcoholic fatty liver disease (Worm, 2020).
– High intake of dietary fiber appears to reduce the risk of colorectal cancer (Arayici et al., 2021).
Total dietary fiber
Here’s an amazing statistic: In the United States, > 97% of men and > 90% of women don’t meet the recommended daily intake for total dietary fiber (source). We have to do something about this!
We should tell our patients what their target daily intake of dietary fiber should be.
With regard to total dietary fiber—soluble plus insoluble—a simplified rule of thumb is that for adolescents and adults, males should take about 35 g/day and females should take about 25 g/day of dietary fiber.
After the age of 50 years, the intake can go down to about 30 g/day for males, and 20 g/day for females.
1. We should tell our patients which foods contain a lot of soluble fiber and how they can increase their intake of soluble fiber.
Soluble fiber is found in many foods including (Soliman, 2019):
– Cereals like oats, oat bran, barley. So,oatmeal is a good source of soluble fiber.
– Beans, lentils, peas.
– Nuts and seeds
– Most fruits (source), including apples, bananas, berries, citrus fruits, pears ,strawberries.
– Some vegetables like artichokes, broccoli, carrots, onions,, potatoes.
Patients should be encouraged to eat foods from all or most of the above categories every day.
2. We can consider recommending a soluble fiber supplement
Taking soluble fiber in the form of food is better on principle and should be emphasized for the long run. But, if needed, a fiber supplement can be taken.
Note: Various types of fiber supplements are very different from each other in what they contain and what effects they will have. For example, some of them are fermentable, feed the gut bacteria, and are considered to be “prebiotics”. Others are not fermentable and even though they have many benefits, modulating the gut flora is not one of them.
For a practical guide to fiber supplements, please see the following article on this website:
Insoluble (or water-insoluble) fiber
What is insoluble fiber?
Insoluble fiber increases the rate of emptying of the stomach, makes the stool bulkier, and makes the contents of the intestines move on more quickly. So, it helps to reduce constipation.
Examples of insoluble fibers include lignin, cellulose, and hemicellulose (Soliman, 2019).
Where is insoluble fiber found?
Insoluble fiber is found in whole grains, wheat bran, oat bran, nuts, seeds, and vegetables like cauliflower, and potatoes (Soliman, 2019). The insoluble fiber in wheat bran and oat bran may be more effective than that from fruits and vegetables (source).
Two precautions are recommended when a person’s intake of insoluble fiber is increased (source):
– The increase should be made gradually to allow the body to adjust.
– Fluid intake should be increased at the same time since insoluble fiber absorbs water.
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