Packaged diet foods have come a long way since Metamucil drinks and Fiber One bars, but one line of thinking has remained true even as our understanding of diet culture evolves: If you want to lose weight, you should eat fiber, and lots of it.
These days, you can find tasty protein bars packed with more than half of your daily recommended allowance of fiber, as well as easy-to-eat fibrous flatbread crackers for avocado toasts. There are even celebrity-endorsed fad diets — NutriSystem and F-Factor — that encourage people to load up on fiber because it keeps you full for very little calories. But of course, nutrition and weight loss are way more nuanced than just calories. So, what's so special about fiber, and are these fiber-full snacks the answer to all your nutritional concerns?
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Fiber is a unique carbohydrate because our bodies can't digest it, says Wendy Dahl, PhD, associate professor in the department of food science and human nutrition at the University of Florida, who studies fiber. When fiber eventually reaches the large intestines, it's broken down by bacteria in the gut. That fermentation process has a lot of beneficial effects, namely decreasing your risk for a number of chronic diseases, like cardiovascular disease, cancers, type 2 diabetes, and chronic kidney disease, she says.
Compared to other carbohydrates, like sugar, fiber keeps you full for a long time. But the benefits of fiber "go far beyond just having a full stomach," Dr. Dahl says. Studies have shown that fermentation in your gut also leads to hormonal changes that can suppress or regulate your appetite, she says. So, for many people looking to lose weight by eating less food, filling up on fiber seems like logical solution.
In truth, we all could stand to eat more fiber, period. "There'll be some vegans and vegetarians that would definitely be getting adequate amounts of fiber, but less than 5% of the U.S. population achieves what we have as recommended adequate intake for fiber," Dr. Dahl says. The average intake is around half of the recommended 25 grams per day for women.
Ideally, you'd get your dietary fiber from legumes, nuts, fruits, vegetables, and whole grains rather than processed foods like protein bars, Dr. Dahl says. It's not that these high-fiber packaged foods are dangerous, but isolated fiber doesn't have exactly the same important effect on your gut, she says. "If you're consuming multiple processed food bars with some added [fiber], they're super fermentable," she says. In excess amounts, isolated fiber can lead to uncomfortable gas and bloating.
So, basically dietary fiber is great for you, and eating more of it is great. "There's no upper limit for dietary fiber — fiber that comes from fruits, vegetables, nuts, legumes," Dr. Dahl says. "No amount is too much for that." But if you're only eating fiber foods as a way to lose weight, that might backfire on you. Eating fibrous bars or crackers might make you feel full, but you may feel deprived of other important nutrients. "Fiber is an important component of fullness, but so is protein and fat," Alissa Rumsey, MS, RD, an anti-diet registered dietitian nutritionist and intuitive eating coach in New York City told Refinery29. "It's not just about fullness either — satisfaction is even more important."
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