Does Too Much Fiber Cause Loose Stools?

You've probably heard it time and time again: "Eat more fiber!" And while that's great advice, given all the health benefits of incorporating fiber into your diet, it doesn't tell the whole story. There's no doubt that fiber is essential. It keeps your digestive system regular, decreases cholesterol levels and helps balance blood sugar, but if you're new to following a high fiber diet, loose stools can be an unwelcome side effect.

That doesn't mean that you should avoid fiber. It just means you may have to take it easy and work it into your diet a little more slowly and strategically.

In simple terms, fiber is a type of carbohydrate that the body can't digest. Instead of getting broken down into smaller components, like proteins, fats and other carbohydrates, fiber moves through the digestive tract untouched, so it doesn't get absorbed. Because it doesn't get absorbed, it may seem as though fiber doesn't really do much, but that couldn't be further from the truth.

Because fiber is ‌undigested‌, it slows the rate at which ‌other foods‌ that you eat with it are digested. This decreased transit time helps keep you full longer and helps control both blood sugar and insulin levels, because glucose doesn't enter the bloodstream as quickly. Fiber also balances gut bacteria, reduces the risk of heart disease and diabetes and helps eliminate constipation.

However, if you eat too much fiber too quickly, it can actually have the opposite effect and cause diarrhea or loose stools, instead. Before jumping into the physiology of how fiber causes loose stools, it's helpful to know the different types of fiber and what they do.

There are two major types of fiber: soluble and insoluble.

‌Soluble fiber‌ dissolves in the water in your body and forms a gel-like substance. This type of fiber slows down the digestion of food and the transition of food from your stomach to your small intestine. Decreased cholesterol and balanced blood sugar are the most notable health benefits associated with soluble fiber. Oats, beans, nuts, flaxseed, apples, oranges and Brussels sprouts are all sources of soluble fiber.

‌Insoluble fiber‌ does not dissolve in the water in your body. Instead, it pulls water into your digestive tract and holds onto it. Unlike soluble fiber, insoluble fiber ‌speeds up‌ digestion‌.‌ Often described as "roughage," insoluble fiber is the type that helps you go to the bathroom. Examples of insoluble fiber include green vegetables, carrots, beets, fruit skins, whole grains and wheat bran.

Loose stools, or diarrhea, occur when you have too much water in your digestive tract, and your food is digesting too quickly. Insoluble fiber triggers both of those things. While that's a good thing in normal amounts, if you eat too much of it, it can work the other way. That's why some people experience a loose stool after eating vegetables in large amounts.

If you have a lot of insoluble fiber in your digestive tract, it will pull a bunch of water from ‌outside‌ your intestine to the ‌inside‌ of your intestine and speed up transit time to the point at which you may end up with a bathroom emergency.

On the opposite end of the spectrum, if you eat too much fiber, constipation may develop. If you're constipated, it's possible that you're eating ‌too much soluble fiber‌, and your rate of digestion has slowed too much. Other possible symptoms of eating too much fiber include:

• Abdominal pain and cramps
• Bloating
• Gas
• Excessive flatulence
• Dehydration
• Acid reflux/GERD
• Decrease in appetite
• Temporary weight gain
  

Low intake of fiber is a much more common problem than eating too much. According to the 2015-2016 NHANES survey from the United States Department of Agriculture, most adults get between 14 and 19 grams of fiber per day, on average.

The Academy of Nutrition and Dietetics recommends that women under the age of 50 get at least 25 grams per day and men aim for 38 grams daily. After the age of 50, those numbers decrease to 21 grams and 30 grams, respectively.

Going slightly over your fiber needs probably won't be enough to cause problems, but when your intake is excessive (or you eat too much, too quickly), you may experience loose stools and some of the other uncomfortable symptoms of too much fiber.

Duke Student Health and Nutrition Services says that a dietary intake of 70 grams or more per day is when you'll start to see some of the negative effects of consuming too much fiber.

Besides lowering your intake of fiber to normal ranges, there are some other things you can do to avoid or eliminate loose stools from too much fiber. Make sure you're getting a good balance of both insoluble fiber and soluble fiber. While insoluble fiber speeds up digestion, soluble fiber slows it down. Because they work as opposing forces, getting a combination of both helps you hit a digestion sweet spot that's not too fast and not too slow.

If you're not used to eating fiber, it's also a good idea to increase your intake gradually, instead of all at once. Your body may not be ready to go from eating very little fiber to trying to meet your daily recommended intake overnight, so it's best to slowly incorporate more fiber-rich foods over a few weeks, so that your digestive system has time to adjust. Other things that can help digestion get back to normal include:

• Drinking more water
• Avoiding fiber supplements and/or foods fortified with extra fiber
• Engaging in light exercise, like walking
  

Stool hardeners are over-the-counter, anti-diarrhea medications that work by slowing down the speed at which things move through your digestive system. When digestion is slowed down, it allows your body to re-absorb some of the excess water that was pulled into the digestive tract. As a result, your stool becomes harder and more solid.

Typically, loose stools from eating too much fiber will resolve themselves on their own with a few diet and lifestyle changes, so stool hardeners aren't necessary, in most cases. Always talk to your doctor about any new symptoms and whether medication is right for you.