1. Constipation
There are two main types of fiber: soluble and insoluble. Soluble fiber is broken down and fermented in the colon, while insoluble fiber travels through the digestive tract unabsorbed, providing bulking and causing stool to move more quickly through the gut. Primary sources of soluble fiber include oats, barley, nuts, seeds, peas, plus some fruits and vegetables.
While an appropriate amount of soluble fiber helps aid in weight loss as it attracts water and forms into a gel, slowing the digestive process, too much soluble fiber can cause constipation, particularly for those who already struggle with the problem.
A 2012 study of 63 subjects found that patients with constipation while on a high-fiber diet found relief when decreasing their daily fiber intake significantly.
If you regularly struggle with constipation on a high-fiber diet, it’s possible that decreasing your overall fiber intake and/or reducing the amount of soluble fiber you eat might help relieve your symptoms.
2. Diarrhea
On the inverse, another symptom of too much fiber is diarrhea. You may have guessed it, but while constipation is often a symptom of having too much soluble fiber, diarrhea occurs many times when people have too much insoluble fiber.
Insoluble fiber travels undigested through your system to the bulk stool and speed digestion. It is found in whole grains, most fruits and vegetables most frequently. Too much insoluble fiber without proper soluble fiber can potentially provide a speed that leads to diarrhea.
3. Bloating and Gas
Another common problem with extra fiber intake, especially right after a rapid increase, is digestive discomfort, including bloating and flatulence. This is especially true for people with irritable bowel syndrome, or IBS.
4. Abdominal Pain/Cramps
Usually related to constipation issues, cramping is another possible effect of eating too much fiber. Reducing overall fiber intake can help lessen abdominal cramping.
Patients with IBS who are trying to increase their fiber intake may also want to consider increasing soluble fiber more than insoluble, because there is limited research that shows a high level of insoluble fiber may actually worsen the stomach pain associated with IBS.
5. Mineral Deficits
Fiber binds to minerals. Through the body’s normal digestive process, this is not generally a problem. However, there is some evidence that too much fiber can bind to minerals at such rates that it’s possible you might not absorb them efficiently.
For example, one study found an increase in calcium absorption for diabetes patients increasing their fiber intake too quickly.
This is generally not going to be a problem if you balance your fiber intake by eating a huge variety of colors in your food (meaning you maintain variety), because too much insoluble fiber is often the culprit in these situations.
Another complication with mineral absorption is also related to the source of your fiber — non-sprouted grains and legumes often contain high levels of phytate (also known as phytic acid), a group of antinutrients that are known to block the body from absorbing iron, calcium, and zinc. To avoid consuming high levels of phytate, stick to sprouted grains wherever possible, and soak beans and nuts for at least 30 minutes before consuming.
6. Dehydration
I’ll touch more on water intake below, but it’s important when increasing fiber intake to also up the amount of water you drink each day. When you stay hydrated, the fiber you eat can absorb the water it needs without leeching it from other necessary systems.
However, if you drastically increase your fiber without increasing your water intake, or consistently eat more than 50 grams of fiber a day, you are more likely to become dehydrated. This symptom is also connected to diarrhea and constipation, so they may appear together.
7. GERD/Acid Reflux
While traditional medicine tells us that acid reflux is caused by too much stomach acid, there is actually a growing body of evidence that it may be caused (or exacerbated) by low stomach acid and carbohydrate malabsorption.
A low amount of stomach acid results in poorly digested food, including carbohydrates that contain the fiber your body needs. When those only-partially-digested carbohydrates get into the intestines, they can result in SIBO (small intestinal bacterial overgrowth), which, in turn, causes intra-abdominal pressure. That pressure is likely the underlying cause of acid reflux.
On the one hand, reducing overall carbohydrate intake will decrease the amount of fiber in the diet and can help relieve some of these symptoms. However, in these cases, it’s probably best to figure out why your stomach wasn’t producing enough acid in the first place in order to deal with that issue.
8. Intestinal Blockage
The worst possible scenario (an extremely rare one, at that) for too much fiber is a risk for intestinal blockage, a medical emergency requiring immediate attention. It’s unlikely that most people will ever experience this, but there are people at risk for shrinkage of the intestines, known as colon stricture, which increases the chance of intestinal blockage. That is mainly a risk for people with Chron’s disease and can be managed with a low-fiber diet.
People with poor intestinal function are also potentially at risk for a specific kind of blockage known as a phytobezoar, a collection of indigestible plant fiber, skins, and seeds. One treatment method for phytobezoars is to drastically reduce fiber intake for a period of time.