The latter effect is the result of fiber soaking up water and slowing digestion to the point that it causes what Glassman calls a “traffic jam of waste” in your small intestine.
However, like all good things, it's possible to get TOO much of it. And now that it’s being added to foods where you wouldn’t normally find it—like yogurt and diet sodas—under un-recognizable names like polydextrose and inulin, you may not be aware of how high your daily tally actually is.
“When you have too much fiber, it can work against you," says Keri Glassman, R.D. Overdosing on this indigestible plant material can actually strip your body of other essential nutrients as food gets pushed through your system too quickly for your body to absorb vitamins and minerals, and, ironically enough, even constipate you. The latter effect is the result of fiber soaking up water and slowing digestion to the point that it causes what Glassman calls a “traffic jam of waste” in your small intestine. Other side effects of too much fiber at once include bloating and diarrhea. Fun!
If you want to avoid that fate, aim for between 21 and 25 grams of the stuff per day, and try to get it mostly from whole foods. It can be tough—for instance, it’d take 80 baby carrots, or five and a half oranges, or two cups of peanuts to meet that quota—but most whole foods contain a natural mix of soluble and insoluble fiber, and your body needs both. (Soluble fiber soaks up water, slows digestion, and helps manage blood sugar, while insoluble fiber acts like a broom that sweeps through your digestive system.) Beans, legumes, and whole grains like oatmeal are all great sources of both kinds.
Functional fibers added to foods may have similar effects, but the jury’s still out. They resist digestion like fiber but don’t have the other nutrients you find in fiber-rich foods, says Glassman. And often, the products they’re being added to aren’t all that healthy to begin with. To make sure you’re not inadvertently gumming up your GI tracts with extra fiber, read labels (here's a guide from the FDA of common culprits), especially when a product makes “added fiber” claims.