Losing Weight with Fiber-Rich Foods

We now know why fiber-rich foods can have such a powerful effect on our weight. The evidence for the role of fiber in weight control started with so-called ecological studies. These involve comparing population averages, and researchers noted that populations with extraordinary fiber intakes tend to have negligible obesity rates. For example, the average Pima Indian in Mexico eating their traditional high-fiber diet centered around the “three sisters” (corn, beans, and squash) is normal weight, but the average Pima on U.S. reservations has obesity. The problem with dealing with population averages is that we don’t know if the individuals eating the higher fiber diets are themselves necessarily the ones protected from obesity.

What do we see in cohort studies, where individuals and their diets are followed over time? A cohort study of overweight youth found that the amount of fiber found in a single half-cup daily serving of beans, about six grams, over about a two-year period, was associated with a profound 25% difference in abdominal obesity. In about the same timeframe, in middle-aged women, each two-gram increase in daily fiber was associated with a weight decrease of about a pound (0.45 kg). The postpartum period seems to be a critical time for women to be at risk of retaining the weight put on during pregnancy. A study of hundreds of new moms followed for the first five months found that inadequate fiber intake appeared to increase obesity risk by 24%. And it’s not just women. A cohort that included tens of thousands of men followed for years concluded that an increase in fiber consumption of just 10 grams a day may prevent about 10% of weight gain within the population.

Overall, the evidence from these kinds of observational studies is strong: “Increasing consumption of dietary fiber with fruits, vegetables, whole grains, and legumes across the life cycle is a critical step in stemming the epidemic of obesity.” These studies can control for nondietary influences such as physical activity by equipping people with gadgets to measure their movement. But there may be uncontrolled confounding dietary factors. Think about the list of high-fiber foods—fruits, vegetables, whole grains, and beans. Maybe fiber intake is just a marker for the intake of healthy foods, and there are dozens of reasons why eating whole plant foods could lead to weight loss that have nothing to do with fiber. To know if there’s a cause-and-effect relationship between fiber and weight loss, you need to put it to the test in interventional trials.

That’s where colonic infusions can come in handy.

In a randomized, double-blind, placebo-controlled crossover study, researchers showed that people’s metabolism can be boosted within 30 minutes of an infusion into their rectums of short-chain fatty acids––the molecules our gut bacteria make when we feed them fiber. The study used the amounts we’d expect to create ourselves just from eating a high-fiber diet. Not only did the study participants get an increase in their resting metabolic rate (the amount of calories burned just by existing), but specifically their fat oxidation shot up as well, increasing the amount of fat they were burning by more than 25%. This translates to about an extra third of a pat of butter’s worth of fat burned off their body within two hours of the infusion.

Colonic catheters aside, we can get short-chain fatty acids directly, and get the same little bump in resting metabolic rate and whole-body fat breakdown, in addition to a decrease in appetite. So again, fiber may work on both sides of the energy balance equation. But does that decreased appetite actually translate into eating less? We’ll find out next.

How does the fiber we eat in foods impact our appetites and our resting metabolic rate––the amount of calories burned just by existing? What happens when we eat fiber-filled foods like beans?

Researchers in Sweden gave people beans for supper, and by the next morning, after their friendly flora had a chance to eat them too, their satiety hormones like PYY were up, their hunger hormone ghrelin was down, and they reported feeling less hungry. The researchers didn’t measure subsequent food intake, but a similar study with whole-grain rye for supper did and found a decrease in food intake at lunch the next day. Those who had eaten the fiber-rich food the night before felt fully satiated, with about a hundred fewer calories at a meal more than 12 hours later. So, by eating fiber-rich foods, you’re setting yourself up for success.

Reduced caloric intake at a single meal, or even over a whole day, doesn’t necessarily translate into long-term weight loss though, as our body may find ways of compensating. Experimentally delivering short-chain fatty acids directly to the colon every day for months does reduce abdominal fat, liver fat, and overall weight gain. But what about getting them the old-fashioned way: eating fiber through our mouth?

A 2022 systematic review and meta-analysis compilation of more than two dozen randomized controlled trials of various straight fiber supplements versus placebo found that those study participants in the groups taking the actual fiber lost an average of about two and a half pounds (1.15 kg) more than those in the control groups. These were studies only averaging 17 weeks long, though there have been fiber supplementation trials that have lasted up to a year, and they showed significantly more weight loss.

The fiber supplement findings were inconsistent, however, presumably because dozens of different fiber types were tested. How much money can you make selling beans? So, there are all sorts of fancy new fibers on the market with names like “IQP G-002AS.” (The IQP stands for InQpharm, the drug company that came up with it.) Ironically, it was found to be quite effective—according, of course, to the employees of said drug company who published the study.

Using isolated fiber extracted from plants or made in a lab can be helpful in experimentally proving fiber’s effectiveness apart from all the other healthy components in whole foods. But if anything, one might expect even greater benefits from eating fiber the way nature intended, in intact plant foods. It turns out that telling people to increase their intake of fiber-rich foods may actually be one of the most effective single pieces of advice for weight loss. Published in the prestigious Annals of Internal Medicine, a study entitled “Single-Component Versus Multicomponent Dietary Goal” randomly assigned hundreds of people into one of two weight loss regimens. One simply encouraged people to get at least 30 grams of fiber each day, which is about the recommended minimum adequate intake. The other group was advised to follow a far more complex weight-loss program recommended by the American Heart Association. In addition to also being advised to hit the 30 daily grams of fiber target, study participants were prescribed carefully calculated calorie intake goals, told to switch from red meat to white, cut down on sugary beverages, reduce sugar and sodium intake across the board, and moderate their alcohol intake.

Even though both groups were told to reach the same fiber target, the group whose sole focus was fiber intake ended up eating more than twice as much extra fiber as the multicomponent intervention––and, surprisingly, similarly improved the quality of their diets. So, the fiber-only group ended up reducing their saturated fat intake as much as the group who was explicitly instructed to do so. Just telling people to eat more whole plant foods seems by default to naturally crowd out some of the less healthy options that may have ended up on their plates.

With similar dietary improvements, both groups lost similar amounts of weight, suggesting if you were able to give only one piece of weight loss advice, eating more fiber might not be a bad choice. Of course, it only works if you actually do it. One low-carb study reported favorable results compared to a “high-fiber” diet. How high was this so-called high-fiber diet? The study participants started out at a terribly low 17.4 grams a day. Unfortunately, that’s pretty typical for the United States. That’s only about half the average recommended minimum daily intake of 31.5 grams. The human body was designed to consume more than 100 grams of fiber a day! The participants started out at 17.4 grams a day, and on their “high-fiber” diet shot up to 19.7 grams a day. That is in no way a high-fiber diet, yet the researchers used their findings to conclude “we believe previous claims of the benefits of fiber for weight loss may have been overstated.”

Less than 3% of Americans reach even the recommended minimum daily adequate intake of fiber. There’s so much fuss about protein, but for that the stats are reversed: more than 97% of Americans do get enough protein, and more than 97% of Americans do not get enough fiber. Almost everyone is eating fiber-deficient diets, and that’s just based on the piddly U.S. dietary recommendations of 14 grams per 1,000 calories, which comes out to be about 25 g/day for women and 38 g/day for men.