Years ago I remember lamenting (and writing somewhere) that I was fairly sick of reading research papers on how eating more fiber was good for people, how it was time for nutritional science to move into relatively more interesting things than a topic that had literally been beaten to death.
Thankfully, soon thereafter leptin was discovered and nutritional researchers could start looking at things more interesting than why eating high-fiber vegetables were good for you (a nutritional tidbit that I file under the ‘Grandma was right’ category).
Even so, there is still some confusion regarding fiber out in the world of nutrition regarding fiber. And boring or not, it’s a topic worth clearing up. So today I want to take a fairly comprehensive look at dietary fiber, what it is, what it does in the body, how it impacts on things like body composition (and health to a lesser degree) and finish by looking at some (admittedly vague recommendations).
What is Fiber?
Generally speaking, fiber is included within the category of dietary carbohydrates (many athletes or bodybuilders divide carbohydrates into starchy and fibrous for example). But fiber is distinct enough to be considered separately from other types of digestible carbohydrates. Perhaps surprisingly, defining what is and isn’t a fiber is actually a more complicated issue than most would think.
Chemistry, botanical and physiology types all sort of want to use different definitions and spend altogether too much time arguing about what is and what isn’t a fiber. Since I’m less interested in chemical or botanical issue than physiological ones, I won’t bore people with the details of those of those definitions and arguments. Rather, I’m interested in the physiological effects and, hence, the physiological definitions.
Even there there are two primary definitions which are used:
- Soluble vs. insoluble (aka viscous vs. non-viscous)
- Fermentable vs. infermentable
I suspect that most are at least passingly familiar with the first definition above. If not, here’s what it means. Soluble fibers go into solution in liquid. A good example is guar gum, if you put a spoonful in water and mix it, it will turn into this gel-like mass; that’s because it’s soluble in fluid. Insoluble fibers, in contrast don’t do this, you can mix them until the cows come home but they won’t ever go into solution.
I suspect that readers are relatively less familiar with the fermentable vs. infermentable definitions. I’ll come back to this below when I talk about the caloric value of fiber but, simply, some fibers can be fermented (specifically by bacteria in the intestine) into other things (e.g. short-chain fatty acids, CO2 or methane) while others are infermentable (they cannot be converted into those other things).
What Does Fiber Do?
Fiber has a number of different effects in the body which are relevant to both health and body composition. It’s worth noting that, strictly speaking, fiber is not an essential nutrient. That is, you won’t die if you don’t eat it regularly (or at all) and there are cultures such as the Alaskan Inuit and the African Masai that subsist on a diet that is essentially devoid of fiber. However, that doesn’t mean that a sufficient intake of dietary fiber isn’t good for you or can’t provide either health or body composition benefits.
Below, I’ve listed a bunch of the major effects of dietary fibers (and note that some of these occur in the upper GI tract, others in the lower but I’m not getting into that much detail) in terms of their physiological effects.
- Promoting fullness/satiety
- Slowing gastric emptying
- Decreasing nutrient absorption
- Improved glycemic control, secondary to delayed gastric emptying and impaired nutrient absorption
- Decreasing blood cholesterol
- Decreasing mineral absorption
- Effects on insulin sensitivity via fermentation to short-chain fatty acids
- A number of effects relevant to colon cancer
- Helps with poopin’
I want to touch on each below although I’m going to focus more on some than others.
One of the myriad signals for fullness during or after a meal has to do with the physical stretching of the stomach. And high-bulk foods are far more likely to do this than low-bulk foods. In this context, meals or foods high in fiber generally contain a lot of bulk in few calories (a topic I discussed in more detail in Energy Density).
Thus they tend to make people feel fuller both in the short- and long-term. In this context, I recall a rather ‘brilliant’ study a few years back which found that people who ate salad first in a meal ate less total calories; the high-bulk, high-fiber items filled them up so that they ate less of the more calorie dense foods. Another one for the ‘Grandma knew best’ file.
In a slightly different context, it’s worth noting that individuals who have trouble meeting their energy requirements (e.g. athletes or ‘hardgainers’) may find it better to save salads for the end of the meal specifically so that they don’t get full too soon before eating the higher calorie part of the meal.
Slowing Gastric Emptying
As I mentioned above, soluble fibers tend to form a gel-like substance in liquids and one consequence of a high soluble fiber intake is that gastric emptying (the rate at which foods empty the stomach) is slowed when they are eaten. Basically, they cause the chyme (the partially digested nutrients in the gut) to form this big gel which empties the stomach more slowly. This, along with the physical stretching of the stomach tends to keep people fuller in the longer term because the food stays in the gut longer.
Impaired Nutrient Absorption
Another effect, again primarily seen with soluble fibers, is an impairment of nutrient absorption, and this holds for carbohydrates, fats and dietary protein. Essentially, due to the gel-like mass that is formed, digestive enzymes can’t get access to the other nutrients so that more is carried out of the body. This means that high-fiber diets will result in less total caloric absorption, basically the left-hand side of the equation discussed in The Energy Balance Equation will be lower when a large amount of soluble fiber is consumed.
I’d note that the effect isn’t massive, fiber may reduce total fat absorption by about 3%, protein by 5%. I can’t find a good value for carbohydrates at the moment. Put more concretely, an increase in dietary fiber from 18 to 36 grams per day might reduce total caloric absorption by 100 calories per day.
Now, depending on how you want to look at this, it can be seen as either a good or bad thing. For individuals trying to lose weight, higher fiber diets will not only have positive effects on fullness and the rest but will result in less total calories being absorbed from the diet. Again, the high-fiber nature will reduce the Energy In side of the equation (which only counts calories which are actually absorbed).
On the other hand, for athletes or bodybuilders, the impact of a high-fiber intake could be seen as detrimental, especially given that soluble fibers impact on protein absorption. While it would be nice if fiber only impacted on carb or fat absorption, that simply isn’t the case. As well, for athletes with very high energy demands, losing digestible energy due to a high fiber intake might not be the best thing. Again, I’d note that the total impact isn’t massive but it is worth considering.
Improved Glycemic Control
One of the most well-known and talked about effects of a high-fiber intake is improvement in blood glucose control. Between the slowing of gastric emptying and impairment of carbohydrate digestion, high soluble fiber intakes tend to improve blood sugar control; rather then seeing larger spikes (due to rapid digestion) which can be followed by crashes, blood sugar levels are balanced out. In that crashing blood glucose can be another stimulus for hunger, this can have an additional impact on hunger control between meals (especially important when dieting).
Decreasing Blood Cholesterol
I’m actually not going to talk about the impact of fiber intake on blood cholesterol levels in great detail. Sufficed to say that high-fiber intakes tend to improve blood lipid levels and do this through a variety of different and inter-related mechanisms. If you want more detail than that, pick up a nutrition textbook.
Impairment of Mineral Absorption
In addition to global impacts on carbohydrate, protein and fat absorption, dietary fibers can also negatively affect mineral absorption especially calcium, magnesium, sodium and potassium. I’d note that, in general, this isn’t really an issue for concern unless the intake of those nutrients is insufficient in the first place.
As well, when fiber intake is increased from foods (as opposed to dietary supplements), there is generally an increase in mineral intake in the first place which should help to offset any issues. For example, the fiber intakes of our evolutionary diet is thought to be massive (some have estimated it at 100-150 grams per day) but nutrient deficiencies aren’t seen; this is most likely due to the fact that the fiber is coming from nutrient dense fruits and vegetables.
However, when people start adding horse-doses of fiber supplements to their diet, problems can start. Older readers may remember the bran craze in the 80’s, when bran was found to lower cholesterol, people starting eating it in massive amounts. But they were often doing it from purified sources rather than whole foods. While this may have improved cholesterol levels, it ended up causing issues with mineral imbalances because the massive fiber intake was not accompanied by an increase in nutrient intake.
Effect on Insulin Sensitivity via Fermentation to Fatty Acids
As I mentioned above, another categorization of fiber is that of fermentable vs. non-fermentable, referring to whether a given fiber can be fermented (via the bacteria in the gut) to other things. The other things that most are familiar with are hydrogen, carbon dioxide and methane; these are what cause the gassiness that can occur with high-fiber intakes. Specifically, methane is what give farts their wonderful smell.
But fiber can also be fermented to short-chain fatty acids such as acetate, propionate and butyrate that are re-absorbed into the body and which have a variety of physiological effects. One of those is to provide calories, a topic I’ll come back to shortly. But the other is to impact on fuel metabolism.
The short-chain fatty acids provided by fiber fermentation impact on both fat cell metabolism and insulin sensitivity. And while these short-chain fatty acids positively impact on insulin sensitivity, they appear to do it by blunting the release of fatty acids from the fat cell. Yes, that says what you think it says: high-fiber intakes may be limiting fatty acid release from fat cells. I’ll come back to this below.
A Number of Effects Relevant to Colon Cancer
Again, not a topic I’m going to get into much detail on. Sufficed to say, high-fiber intakes have a number of physiological effects that reduce the risk of colon cancer. Get a textbook for more.
Helps with Poopin’
And, of course, possibly the most well known effect a high-fiber intake is regularity and comfort in pooping. That’s actually what the title of this piece refers to, I have often commented that fiber is nature’s broom. It helps sweep stuff through the intestines and out the other end. It does this through a number of mechanisms.
First and foremost, fiber speeds the transit time of food from one end of the intestines to the other. So rather than sitting in the intestines, it moves towards the exit more quickly. As well, fiber contributes to fecal bulk, essentially the size of the poo that is produced. This increase in bulk also pulls more water into the fecal mass which makes the poop softer and easier to pass. Both of these latter effects further contribute to the decreased transit time and all of this contributes to better regularity.
And, at the end of the day, who can argue with a good poop?
Fiber and Energy Balance
Relevant to issues of body composition, fiber can contribute in a number of ways to The Energy Balance Equation. As noted above, fiber impacts on caloric absorption (decreasing it, generally) along with fullness (which may cause people to spontaneously eat less) along with blood glucose control and several other mechanisms. In general, the effect is to reduce either total food intake or caloric absorption, facilitating weight loss.
I’d mention again that the effect of fiber on fat cell metabolism via the conversion to short-chain fatty acids is perplexing, one way of looking at this is that high-fiber intakes might hurt with fat loss. This might become more relevant when people get very lean and fatty acid mobilization is becoming more difficult (for reasons discussed in The Stubborn Fat Solution). At the same time, real-world results call the real-world significance of this into question. High-fiber intakes have been part of hardcore diets for decades and folks seem to be doing alright.
Depending on the goal (e.g. weight loss vs. weight gain), this can be seen as good or bad depending on the context. For individuals trying to lose weight, most of the effects of a high-fiber diet could be seen as generally positive. Being fuller with more stable blood sugar and absorbing fewer calories would seem a good thing.
As noted above, for individuals trying to increase their energy intake and/or gain weight, a high-fiber intake could potentially be a negative. Between making the individual fuller at a given meal and/or keeping them fuller longer during the day, along with impairment of caloric absorption, high-fiber intakes might have a negative impact overall for some people.
Newsflash: Fiber Provides Calories to Humans
But there is another effect of fiber on energy balance that often goes unappreciated. Backing up, it’s often stated that fiber provides no calories to the body since humans lack the enzymes necessary to digest it. This has often been taken even further to claim that high-fiber vegetables are ‘negative calorie foods’, that is they take more calories to digest than they provide (assumed to be zero).
Here’s the thing: it’s not true. Not entirely anyhow.
Above I discussed the issue of fermentation of some types of fiber to short-chain fatty acids which are then reabsorbed by the body. Well, those fatty acids provide calories to the body. While there is still some debate in the area, researchers have assigned a caloric value to fiber of 1.5-2 cal/gram (depending on the specific type).
Admittedly this is an average and will depend on the specifics of the diet and the type of fiber but, simply, the idea that fiber provides no calories to the body is not true. While the caloric value of fiber is still lower than starchy carbohydrates (4 cal/g), it is not zero.
How Much and What Kind of Fiber?
So how much fiber do we need? As noted above, strictly speaking fiber is not an essential nutrient; you might be healthier with it but if you never ate another gram you would not die. You might want to die when you tried to poop but you wouldn’t actually die without it.
But due to the non-essentiality of fiber for human survival, it’s hard to make specific recommendations for daily fiber intake.
The American Dietetic Association recommends an intake of 10-13 grams of fiber per 1000 calories consumed. This is roughly 20-30 grams per day for an average day’s diet of 2000-3000 calories per day. It’s worth noting that the average fiber intake in the modern diet is about 10-11 grams/day which is far below this. Most people would probably benefit from eating more fiber but they’d also generally benefit from eating more fruits and vegetables generally.
As I mentioned, our evolutionary diet is thought to have contained absolutely massive amounts of fiber on average, intakes of 100-150 grams/day has been thrown around in some scientific papers. I would note again that this would have come from the intake of massive amounts of fruits and vegetables, providing numerous other nutrients (especially minerals and vitamins) that wouldn’t be found if you tried to get that much fiber from supplements.
In that context, it’s worth mentioning that high-fiber foods, typically fruits and vegetables, contain tons of other nutrients important to health or what have you so looking only at the fiber content can be a bit misleading. Getting adequate amounts of high-fiber fruits and vegetables on a daily basis has benefits far beyond just the fiber content; getting some at each meal would seem to be a good thing.
And yes, I am waffling on this. There is very little hard and fast data on optimal fiber intakes for any goal. Too little is bad, too much is probably bad. Somewhere between those two extremes is about right. People eating the modern diet get too little fiber and should increase it. I’ve seen some meal plans that, frankly, included absurd amounts of fiber (folks with eating disorders often do this type of thing to stave off the gnawing hunger). Find balance, people.
Depending on meal frequency, somewhere between 5-10 grams of fiber per meal would seem a decent place to start. That should provide anywhere from 30-60 grams of fiber per day, covering average recommendations without being excessive.
I would note that if you fiber intake is currently low, do NOT try to increase it drastically in a short period of time. The body needs time to adapt to big increases in any nutrient intake and people who jump their fiber intakes massively often pay a hard price in terms of gas and such.
Finally, on the topic of types of fibers, I don’t get overly concerned with it. The soluble/insoluble fibers can be further subdivided into a whole host of other categories but I consider this nutritional minutiae of little real relevance. If you strive to consume a variety of fruits and high-fiber vegetables on a day to day basis, you’ll get a mix of fibers and cover your bases.
In specific situations, fiber supplements may play a role (for example, soluble fibers such as guar gum can be put into yogurt/protein powder mixtures to thicken it up and/or help with fullness on a diet). And many will use psyllium husks as a form of insoluble fiber if they are having issues with constipation or what have you. But, for the most part, I’d rather see people increase their intakes of high-fiber whole foods rather than use purified supplements.
- Nuts and Bodyweight – Q&A
- Calories Not Matching Macros – Q&A
- Carbohydrate and Fat Controversies: Part 2
- Comparing the Diets: Part 2
- Calories Nutrients or Food?