Dietary fibre is the name we use for complex plant based carbohydrate molecules that our intestines are not able to digest. Since we can’t digest fibre, it goes straight through the intestine after we eat it, and comes out the other end. And since it fills up the intestine, it contributes to a feeling a fullness. It can also bind up certain substances, so that they travel through the intestine and come out the other end, rather than being absorbed by the body. In theory, these effects should mean that increased intake of dietary fiber results in weight loss, at least if you’re overweight.
Official guidelines generally recommend a dietary fiber intake of 25-30 grams per day. The average person on a standard western diet consumes about 15-20 grams per day. Personally, I don’t really understand how that’s possible, because I don’t make any particular effort to stuff myself with fiber, and yet according to calculations I did at paleotrack.com, I eat about 50 grams per day of dietary fiber (mainly thanks to the fact that I eat a lot of berries and nuts).
I guess at this point it would be good to mention that dietary fiber is actually a large number of different molecules with different properties. Some are water soluble, while most aren’t. Some are fermentable, which means that they can be digested by bacteria in the colon. This results in the production of short chain fatty acids, which can be absorbed by our bodies and have various beneficial effects, and also in the production of gas (the reason we produce more gas after eating beans). Some of the water soluble fibers are also “viscous”, which means that they form a thick gel in the intestine .
A systematic review was published in Obesity Reviews in 2011. The review was co-funded by the Dutch government and Kellogg’s (yes, that well known champion of nutrition, which has given us Frosties, Rice Krispies, Coco pops, pop tarts, and other equally healthy food products). It included 66 randomized controlled trials, with a total of 2,486 participants, that looked at the effect of increased dietary fiber intake on body weight. The average extra dose of dietary fiber given was 11 grams, and participants were followed for an average of 11 weeks.
So, what were the results?
The intervention group lost an average of 0,72 kilograms more weight than the placebo group. The review authors didn’t report whether this result was statistically significant, so I’m going to assume that it wasn’t. 0,72 kilograms may sound pitiful, but considering that the intervention period was only 11 weeks, I don’t think it’s too bad, if the effect is real (without information about statistical significance it’s impossible to know how likely it is that the effect is real).
Let’s look at another study.
A systematic review and meta-analysis was published in The American Journal of Clinical Nutrition in 2017 that looked specifically at soluble fiber. It identified 12 randomized controlled trials, most of which were also double-blind, with a total of 609 overweight and obese participants. Study durations ranged from two to 17 weeks. The daily dose of soluble fiber given varied from three grams per day to 34 grams per day. The authors reported no conflicts of interest.
One thing I like about this review is that it only included studies in which participants were receiving dietary fiber supplements, rather than being asked to modify their diets in ways that resulted in increased fiber intake. Why do I like this? Because diet modification studies have lots of unintended consequences, which introduce a lot of confounding effects. When people change their diet in one way, it unintentionally changes in a lot of other ways, since eating more of certain things also means eating more of certain other things, while eating less of yet other things. Giving people a supplement instead of telling them to change their diet is an effective way to get around this problem.
So, what were the results?
In the group receiving a soluble fiber supplement, weight decreased by 2,5 kilograms more than in the group receiving placebo. The difference was highly statistically significant. Considering the short duration of the included trials, that is a huge effect. As a side note, although it’s not the topic of this article, the dietary fiber group also had a reduction in fasting glucose that was small but highly statistically significant. So for type 2 diabetics, a high fiber diet might be a way to avoid or at least cut down on medications.
One strange thing about the results was that there was no clear dose-response relationship. The authors suggested explanation for this is that different types of fiber were used in the different studies. For example, one study gave three grams of glucomannan while another study gave 34 grams of dextrin, and the authors propose that glucomannan has a much bigger effect on weight than dextrin.
If that is the case, then there is clearly a need for much more research to determine which specific types of fiber are inducing beneficial weight loss effects. The lack of dose-response is however also a reason to be cautious about believing the results – the trials involved are after all small, with an average of 50 participants each, so the scope for chance to cause seemingly significant effects is quite big.
Apart from that, we know that studies with significant positive results are much more likely to be published. Maybe there are a further fifty similar trials that didn’t show a benefit and which no-one bothered to publish. That could certainly explain the perplexing lack of a dose-response relationship.
We’re going to look at one final systematic review before we get to the conclusions. This one was published in the American Journal of Clinical Nutrition in February 2020. It looked at randomized trials that examined the effect of viscous dietary fibers on weight. The review included 52 trials with a total of 2,761 participants. The average age of the participants was 51 years, and the average BMI was 27 (marginally overweight). Participants were followed for an average of ten weeks, and the average dose of viscous fiber given to the intervention group was 7 grams per day.
The review only included studies that allowed people to eat as much as they want. In other words, no studies that used conscious calorie restrictions were included. This is good, because calorie restricted diets have been clearly shown to be ineffective over the long term, and because we want to know if increasing dietary fiber can be part of an effortless weight loss strategy (a weight loss strategy that isn’t effortless might work over the short term, but probably won’t work over the long term).
One thing to be aware of with this review, is that some of the studies that were included weren’t comparing viscous fiber with an inert placebo, they were comparing viscous fiber with non-viscous fiber. If we think that the weight loss benefits connected with fiber are not just due to viscous fibers, but that other types of dietary fiber can also have positive effects, then this will obviously make the effects appear weaker than they really are.
So, what were the results?
Overall, the intervention group lost 0,33 kilograms more than the placebo group. However, this result included people who were normal weight (whose weight didn’t change at all with a higher fiber diet). When only the studies looking at people who were overweight or obese to start with were included, the reduction was 0,46 kilograms. Both results were highly statistically significant.
As in the first review, this effect seems small, but if you consider that there was no conscious calorie restriction in place, and that the intervention period was on average only ten weeks, then that’s not too bad. If the relationship were linear, that would mean about 2,5 kilograms of weight lost over a year. And since weight gain is a slow process happening over decades, even something that has a small effect over a short time period, could have a very big effect over the course of a few decades.
What can we conclude from all this?
Dietary fiber does seem to have a role as part of an effortless weight loss or weight maintenance strategy. All three reviews found a reduction in weight with an increase in fiber intake, and although the effect was generally small, it was in at least two of the reviews highly statistically significant. And, as mentioned, a small effect over a short time period can mean a very big effect over a long time period.
However, I think these reviews also show that the available evidence is not of great quality. The intervention periods were short, and the individual trials included in the reviews were for the most part small. What we need is bigger trials, with at least a couple of hundred participants, that run for at least a year. And we need more trials that look at different types of fiber. Dietary fiber is a broad concept that includes a large number of different molecules, with different properties. From these three systematic reviews, soluble non-viscous soluble fiber seems to have the biggest effect on weight. But that is a tentative conclusion and more evidence is needed.
Having said that, we can’t constantly be analyzing our food in-depth to figure out which specific dietary fibers it contains. Dietary advice needs to be easy to follow. Which is why I think it’s better to focus on dietary fiber as a whole, rather than going in to detail on different fiber types, at least for the vast majority of people who just want simple advice on what foods to eat and what foods to avoid, in order to lose weight.
So, how much fiber should you be aiming for in your diet?
Like I wrote above, the standard recommendation is usually around 25 to 35 grams of dietary fiber per day. Personally, I think that is low, and it would be sensible to aim for at least 35 grams per day. It’s always worth thinking in terms of what we are biologically adapted to eat, which means thinking about what our ancestors were eating for millions of years back when they were hunting and gathering. That is the diet we are physiologically adapted to. And from what we know, our paleolithic ancestors were averaging a fiber intake of around 100 grams per day, roughly triple the highest currently recommended intake.
Unfortunately, there is as yet no study that I know of looking at what happens if you push people’s fiber intake up to 100 grams per day and follow them for a year, although it would be exciting to see what would happen.
One word of caution if you intend to try increasing your intake of dietary fiber: fiber intake needs to be increased gradually, over weeks or even months. If someone who is adapted to a low fiber diet suddenly increases their fiber intake a lot, they are likely to experience stomach cramping and diarrhea. So you need to go slow.
Which foods are good sources of dietary fiber?
Beans are good, but they contain a lot of fermentable fiber, so they cause gassiness. Green peas are a type of bean that doesn’t cause too much gassiness, however. Fruit is good, but has a high carbohydrate content, so you may want to limit intake for that reason. That also goes for whole-grain cereals – they may have a high fiber content, but they also have an enormously high carbohydrate content and they are low in nutrients, so I think they should be avoided. Berries, nuts, vegetables, and seeds are great – high in fiber, high in nutrients, low in carbohydrates.
You might also be interested in my article about whether exercise is effective for weight loss, or my article about whether time restricted eating is good for weight loss.