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Does Eating Too Much Fiber Cause Mineral Deficiencies?

The Institute of Medicine hasn’t set an upper limit on fiber, meaning that there’s no amount at which it’s considered toxic. But could excessive fiber block absorption of minerals from foods?

By
Monica Reinagel, MS, LD/N, CNS,
Episode #532
fiber foods

Today’s topic was proposed by Lisa, who called in on the Nutrition Diva Listener line with this great question:

"Hey Monica, I have a question about whether too much fiber causes nutrient deficiencies. I read that an excessive amount of fiber binds with minerals including calcium, magnesium, zinc, and iron preventing them from being absorbed by the body. Is there validity to these claims? What can we consider to be excessive fiber?"

How Much Fiber Do You Need?

The recommended daily allowance for fiber is around 30 grams per day. The average intake in the U.S. is somewhere in the neighborhood of 15 grams per day. So the typical American would need to double their fiber intake to meet the recommended amount. But is the recommended amount of fiber too low?

The Recommended Daily Allowance for fiber was set decades ago and was based on research showing that fiber was beneficial for digestive health, weight management, and can also play a role in moderating cholesterol levels. 

How Fiber Supports Health

Fiber from foods absorbs water and takes up a lot of space. This helps with weight management by filling up the stomach and making us feel more full on fewer calories. Fiber also binds to certain food elements, such as cholesterol. That means that as the food moves through the small intestine, less of the cholesterol is absorbed into the bloodstream. As it continues through the digestive tract, fiber also helps move waste matter through the large intestine, which prevents constipation and reduces our risk of colon cancer.

In recent years we’ve become aware of another important way that fiber supports our health: by supporting the health of our gut bacteria.

However, in recent years we’ve become aware of another important way that fiber supports our health: by supporting the health of our gut bacteria. The greater the amount and variety of plant fiber we include in our diet, the more varied and robust that population of beneficial bacteria becomes. Studies have shown that increasing fiber to 50 grams a day positively changed the composition of the gut microbiome in a surprisingly short period of time. 

But Lisa raises an interesting question: Is there any danger of consuming too much fiber? 

Can You Eat Too Much Fiber?

The Institute of Medicine hasn’t set an upper limit on fiber, meaning that there’s no amount at which it’s considered toxic. That doesn’t mean that an excess of fiber couldn’t make you uncomfortable, though! People who quickly and drastically increase their fiber intake sometimes experience gas, cramping, and bloating. Taking very large amounts of fiber supplements could also cause intestinal blockages, especially if you don’t take them with sufficient water. 

The Institute of Medicine hasn’t set an upper limit on fiber, meaning that there’s no amount at which it’s considered toxic.

If you suffer from constipation, you may have tried increasing your fiber intake. If it doesn’t help, it may be that your constipation is not due to inadequate fiber, in which case, too much fiber may actually be making the problem worse. One study of patients with chronic constipation found that many of them were taking quite high amounts of fiber—to no avail. In this situation, reducing the amount of fiber in the diet often led to an improvement in symptoms. 

Does Fiber Block Nutrient Absorption?

But could excessive fiber also negatively affect nutrient absorption? The idea is that fiber could bind to minerals in way that prevents their absorption in the small intestine. And some studies have confirmed that increasing dietary fiber resulted in more calcium being excreted in the stool. But this might not be the best way to assess the impact. Another study found that, while subjects eating more fiber lost more calcium in their stool, they still showed net gains in calcium, magnesium, and zinc levels. This is probably because the higher fiber diet was also a lot higher in these nutrients. 

Even though fiber may trap some of the minerals in foods, because these foods are more nutritious to begin with, you are likely to come out ahead nutritionally.

So, even though fiber may trap some of the minerals in foods, because these foods are more nutritious to begin with, you are likely to come out ahead nutritionally. And that’s before we take into account all of the other positive effects of a higher fiber diet. But keep in mind that this assumes you are getting your fiber from whole foods, and not supplements, which wouldn’t offer the same nutritional advantages.

How Much Fiber Should You Eat?

Taking all this into consideration, it would appear to be safe and beneficial for most people to eat as much as 50-60 grams of fiber per day. If you’re currently only taking in the typical 15 grams a day, that may seem like a pretty big lift. But you could still get a lot of benefit from working your way up to the recommended 30 grams a day. 

Good Sources of Fiber

The best sources would be whole, nutrient rich foods such as legumes, whole grains, nuts, seeds, fruits, and vegetables—the greater the variety the better. Increase your fiber gradually to give your gut (and its inhabitants) time to adjust. Also, be sure to drink plenty of fluids. And if gas and bloating are still an issue, you might need to do a little sleuthing to discover which types of fiber your belly can most comfortably handle. For that, there’s no better guide than the Bloated Belly Whisperer, by Tamara Duker Freuman.

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