In a room full of 10 Americans, how many are eating enough fiber?
Believe it or not, just one. Actually, less than one. Fiber is so egregiously underconsumed that you need at least 20 people in that room to produce a small enough percentage for this example to work. The best available estimates suggest that only 4-5% of Americans eat enough fiber, meaning roughly 19 of 20 Americans are falling short. This article will discuss why that’s a huge problem, but more importantly, how you can strategically leverage dietary fiber to your benefit.
The (Many) Benefits of Fiber
You may have seen recent headlines indicating that gastrointestinal cancers are on the rise in young adults. That’s serious business, and researchers are worried that low dietary fiber intake is playing a role as the world continues shifting toward diets packed with ultraprocessed foods. But reducing the risk of gastrointestinal cancers is far from the only benefit of adequate fiber intake.
As shown in Figure 1, there are many upsides to be enjoyed. Eating enough fiber improves glycemic control, blood lipids, and immune function. It also reduces chronic inflammation and the risk of depression, in addition to feeding your beneficial gut bacteria and keeping your bowel movements more consistent (in terms of both timing and texture). When it comes to reducing disease risk, adequate fiber intake is associated with lower risk for coronary heart disease, stroke, hypertension, diabetes, obesity, and certain gastrointestinal diseases. If you’re more focused on not dying than avoiding disease, there are benefits for you, too: lower risk of cancer mortality, cardiovascular mortality, and all-cause mortality.
Image courtesy of MASS Research Review. massresearchreview.com
Clearly there’s a lot to like about fiber. But if you’re feeling skeptical, I understand where you’re coming from. The effects on the gastrointestinal system seem pretty straightforward. After all, fiber resists digestion in the upper gastrointestinal tract, ultimately making its way to the large intestine. Once there, beneficial gut bacteria break it down into short-chain fatty acids, feeding themselves in the process. It’s quite intuitive that fiber would help move things through your system and keep those bacteria happy and well-fed. But how would fiber have such widespread effects throughout so many tissues and physiological systems?
The answer has many facets. A big part of fiber’s magic relates to increasing fullness or satiety, thereby facilitating appetite management. It goes without saying that attenuating fat gain can bring diverse cardiometabolic benefits. Another huge factor relates to those short-chain fatty acids I mentioned earlier. They don’t just get created – they get absorbed in the large intestine, ultimately entering the bloodstream. These fatty acids seem to play a role in benefits related to immune function and inflammation, among others. Then there are the downstream effects of cultivating a healthy colony of gut bacteria – you may have heard of the gut-brain axis, but researchers are currently exploring gut-organ interactions in just about every organ you can name off the top of your head. All that is to say, fiber’s effects are surprisingly global.
What Is Fiber, Exactly?
Now that you’re sold on the value, we need to settle on an actual definition. An oversimplified definition is that fiber refers to nondigestible soluble or insoluble carbohydrates that, according to the FDA, have satisfactory evidence supporting beneficial effects on health. Some fibers are naturally occurring, while others are synthetic. Due to the “nondigestible” component, they contribute fewer metabolizable calories per gram than a typical carbohydrate. The FDA is constantly updating its list of “officially recognized” fibers – partly because food scientists are actively creating new candidates, and partly because the evidence pertaining to health benefits from existing candidates is always evolving.
The fiber review process is not without drama. You might remember some controversy about the “fiber” content of protein bars several years ago. At the time, companies were labeling isomaltooligosaccharides as fiber. Upon further review, it was determined that isomaltooligosaccharides do not meet the criteria to be considered a fiber; they get digested too quickly, provide too much energy (due to relatively complete digestion and absorption in the upper gastrointestinal tract), and have insufficient evidence regarding direct health benefits. As a result, the protein bar companies had to change their labels or pivot their formulations.
In summary, you might want to make sure your go-to fiber source is actually fiber. But above and beyond that simple distinction, you should think about what type of fiber you’re consuming if you want to make the most of it.
The Main Types of Fiber
The two main types of fiber are soluble and insoluble. They have distinct characteristics, which means they can be leveraged strategically to achieve different outcomes.
Soluble fiber dissolves in water, which is why it’s used in a lot of fiber supplements. Common sources include wheat dextrin, inulin, methylcellulose, and tapioca fiber, among others. In most cases, soluble fiber is extensively fermented by bacteria in the large intestine, producing short-chain fatty acids and gas (as a byproduct). As a result, soluble fiber doesn’t add much bulk to your feces, but it does yield a bunch of beneficial short-chain fatty acids and provide plenty of fuel for your “good bacteria” in your gut. These fibers provide roughly 1.5 to 2 kcal per gram ingested, which is considerably lower than the 4 kcal per gram associated with a typical carbohydrate.
The big caveat to the above paragraph is that viscosity matters. For example, consider psyllium husk (the fiber used in Metamucil). It forms a thick gel when it interacts with water. This makes it harder for gut bacteria to metabolize, so it largely passes through the gastrointestinal tract intact. As a result, it adds more bulk to your feces and provides less fuel for your gut bacteria when compared to a more typical soluble fiber. By extension, it is more likely to help alleviate constipation, less likely to produce flatulence (gas), and contributes fewer calories per gram ingested.
In short, highly viscous soluble fibers behave a lot more like insoluble fibers. Insoluble fiber doesn’t dissolve in water and undergoes little to no fermentation. The upside is that it adds bulk to your feces, helps with constipation symptoms, and doesn’t produce flatulence. The downside is that it’s not feeding your gut bacteria or producing short-chain fatty acids, which are both linked to some of fiber’s multifaceted benefits. Since insoluble fiber isn’t getting fermented to a substantial degree, you’re absorbing virtually zero calories per gram ingested. A general comparison of soluble versus insoluble fiber is presented in Table 1.
Image courtesy of MASS Research Review. massresearchreview.com
So which fiber should you be eating?
In general, the answer is both. You want insoluble fiber to keep things moving through your gastrointestinal tract, and you want soluble fiber to feed your gut bacteria and produce beneficial short-chain fatty acids in your large intestine. But if you’re seeking to address a specific problem, the answer will vary.
If you’re constipated, having relatively infrequent bowel movements, or have poor stool consistency (based on the Bristol Stool Scale), you’ll probably want to increase your intake of insoluble fiber and/or a highly viscous soluble fiber. If you are trying to leverage the benefits of well-fed gut bacteria and the short chain fatty acids they produce, you’ll want to increase your intake of a non-viscous soluble fiber.
How Much Fiber is Enough?
Standard guidelines come in two forms. The sex-based guidelines is that adult males should aim for at least 38 grams per day, while adult females should aim for at least 25 grams per day. As shown in Table 2, these targets change slightly throughout the lifespan. The calorie-based guideline is to aim for at least 14 grams of fiber for every 1,000 calories in your diet (e.g., at least 28 grams per day on a 2,000 calorie diet).
Image courtesy of MASS Research Review. massresearchreview.com
When working with clients, I take the higher of the two recommended targets, and I treat it as a starting point. From there, we gradually increase fiber until we reach our “comfort zone” – the fiber intake at which we have a satisfactory balance of appetite management, bowel movement frequency, stool consistency, and gas/bloating level. This process should be slow and methodical; large and abrupt increases may cause symptoms, whereas a gradual process of increasing 5-10 g/day at a time may allow someone to gradually work up to a high intake without issue. Anecdotally, I’ve found that most people feel best with a high-but-not-too-high intake, often in the ballpark of 40-70 g/day, but this varies considerably from person to person.
How Much Fiber is Too Much?
Needless to say, it’s possible for fiber intake to be too high. If you’re too full to hit your calorie targets, a fiber reduction may be warranted. If you’re constantly bloated or gassy, fiber could be the culprit – but not always.
Before dropping fiber intake due to gas or bloating, take a look at your intake of sugar alcohols and resistant starches. Sugar alcohols, such as erythritol, mannitol, sorbitol, and maltitol, show up in a lot of fitness-adjacent foods and beverages because they are low-calorie sweeteners. They are not harmful to health, but they offer no substantial health benefits (other than replacing sugar calories). It’s hard to find a protein bar without some kind of sugar alcohol these days. That’s not necessarily a problem, but some of them cause significant flatulence, and even osmotic diarrhea, at surprisingly modest intake levels. Some protein bars have high enough sugar alcohol doses that even 2 bars in an afternoon can cause flatulence or diarrhea.
Resistant starch is starch that escapes digestion in the small intestine and reaches the colon intact, where gut bacteria ferment it. The most common sources of resistant starch include legumes, cooked-and-cooled starches, and whole grains. Resistant starches, much like fiber, offer some health benefits of their own. Nonetheless, gas is a byproduct of fermentation, so this is yet another dietary component that can contribute to flatulence. Before slashing fiber, rule out the possibility that you’re overconsuming a bunch of sugar alcohols or resistant starches.
Another potential confounder is your intake of FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides and polyols). FODMAPs are short-chain carbohydrates that are poorly absorbed in the small intestine and rapidly fermented by bacteria in the large intestine. Here’s where it gets complicated: some people are sensitive to FODMAPs in general, while others particularly sensitive to one or more specific FODMAPs. For example, research has suggested that many individuals with self-reported non-celiac gluten sensitivity are actually sensitive to FODMAPs rather than gluten. It just so happens that many gluten-containing foods are high in FODMAP content. And it gets even more complicated – sugar alcohols are a subset of FODMAPs, many foods high in resistant starch happen to contain FODMAPs, and many high-fiber foods contain FODMAPs. As a result, it can be difficult to pinpoint the exact culprit of bloating or flatulence.
Before slashing total fiber intake in response to bloating or flatulence, consider a more nuanced approach. First, cut back on your intake of sugar alcohols and resistant starches to see if symptoms subside. Then, consider temporarily shifting to some lower-FODMAP fiber sources without reducing your overall intake. If the bloating or flatulence persist, try shifting to more insoluble fiber and less soluble fiber (at the same level of total intake). If symptoms persist after all of those adjustments, then it might truly be necessary to reduce fiber intake. If they persist after adopting a low-fiber diet, then medical intervention might be warranted. And it goes without saying – if you ever experience more substantial gastrointestinal symptoms than simple gas (such as blood in feces, indicators of malabsorption, substantial unexplained changes in stool consistency, or narrow ribbon-shaped stools), checking in with a doctor is a good idea.
Conclusions
There’s a lot to like about fiber. If you want to improve your cardiometabolic health, reduce disease risk, or extend your lifespan, adequate fiber intake is linked to all three. But there are fitness-related reasons to manipulate fiber intake as well. If excessive fullness is hindering your bulking goals, a fiber reduction may be warranted. If hunger is sabotaging your fat loss goals, a boost in fiber could come in handy. For most people, the “right” fiber intake is the highest intake that supports optimal appetite regulation and gastrointestinal comfort. But before blaming gastrointestinal symptoms on fiber, take a look at your intake of other fermentable dietary components like resistant starches, sugar alcohols, and other FODMAPs. A diverse intake of fiber sources tends to be the best approach – vegetables, fruits, legumes, whole grains, nuts, seeds, and even supplementary sources as needed.
Find more from Eric on…
Eric Trexler, PhD
IG: @trexlerfitness
Website: massresearchreview.com