The Disappointing Science Behind Fiber
In their dietary guidelines, the USDA recommends a daily fiber intake of 28g for adult women and 34g for adult men. Certain foods are often touted as beneficial for health based on their fiber content, and headlines everywhere bombard us about not getting enough. With everyone shouting about how great fiber is, we would expect a mountain of science explaining exactly why it’s so good for the body. Fortunately, based on how flimsy the research is, the widespread worry over this “nutrient” is totally unnecessary.
The reason I put “nutrient” in quotes is that calling fiber a nutrient is questionable. The Merriam-Webster definition of nutrient is “a substance or ingredient that promotes growth, provides energy, and maintains life.” Fiber does not fit these criteria because it is completely indigestible by the human body. It may only be considered a nutrient because some energy can be derived from it via bacterial fermentation. Most carbohydrates are absorbed by the small intestine, but fiber leaves the small intestine undigested, entering the large intestine to be excreted as waste. Also worthy to note is that fiber is not a singular substance, but comes in many different forms. The two main subtypes, soluble and insoluble, each come with their own purported benefits. That being said, the health benefits of fiber are usually stated as follows:
Promotes healthy gut bacteria
Alleviates constipation
Reduces risk of heart disease, type 2 diabetes, and diverticular disease
Helps with feeling full after eating
Fiber and Gut Bacteria
Bacterial fermentation of fiber in the gut leads to increased production of short-chain fatty acids (SCFAs). SCFAs are fats with carbon chain lengths between 2 and 6, compared to more typical dietary saturated fats like palmitic or stearic acid, which have carbon chain lengths of 16 and 18, respectively. SCFAs are essential sources of energy for gorillas and other large herbivores. A gorilla’s colon is designed as a giant fermentation tank to break down fibrous plant matter and use it as fuel. That’s why an estimated 57% of the energy they use is created from the fermentation of fiber into SCFAs. In this human study, 50-60g of fiber intake yielded between 6-9% of daily energy intake. For a typical woman on a 2000 calorie diet meeting the recommended 28g of fiber, that equates to between 3.6% and 4.5% of energy intake. So, the amount of energy that fiber yields for most humans is quite small. Furthermore, only soluble fibers are fermented in any significant amount into SCFAs, while insoluble fibers are not. The aforementioned percentages assume that 100% of fiber intake is from soluble fiber, which is not the case in realistic scenarios. Even so, do SCFAs offer any additional benefits over longer-chain fats? There is some evidence that SCFAs may have some therapeutic effects. That being said, this review gives a summary of how flimsy the current evidence is:
A wide range of pre-clinical evidence supports a role for SCFA as a modulator of colonic and overall health. However, fundamental differences between animal and human physiology dictate caution in extrapolating animal effects to human diseases and create a need to conduct clinical trials to validate these effects in humans.
In short, more research is needed to determine the mechanisms behind SCFA contributions to health in humans, if they even exist. Research on the gut health of people following a zero-fiber carnivore diet would also be useful, because their anecdotal reports often involve improvements in gut health. Anecdotally, I experienced the same with my temporary carnivore diet experiment in the past compared with a more vegetable-rich diet. A noteworthy observation regarding gut health is that independent of food intake, the hormone leptin has also been shown to impact gut health. That means in a typical metabolically unhealthy person who is leptin resistant (see previous posts), the best course of action is to improve overall metabolism instead of solely incorporating more fiber or other prebiotics/probiotics into what is most likely a poor diet.
Fiber and Constipation
Constipation, according to the Mayo Clinic, means having fewer than three bowel movements per week, or having difficulty passing them. Fiber, because it is indigestible, will cause more frequent bowel movements because the body simply excretes it as waste. Naturally, diets lower in fiber will usually mean less frequent passing of stool. But the assumption that fewer than three bowel movements per week is abnormal relies on the idea that most people exceed that number, rather than something actually being wrong. While most of the time in the general population, low fiber intake is due to a high consumption of ultra-processed foods, a nutrient-rich diet with as low as zero fiber is possible. Individuals following such diets, once again using carnivore diets as an example, usually report small, infrequent bowel movements that are quite comfortable. From a common sense standpoint, producing less waste on its own should not be defined as a condition that must be remedied if no other negative effects are present. What about difficulty passing stools, or other negative effects that define constipation? A 2012 meta-analysis concluded that while increased fiber intake did improve bowel movement frequency, as expected, it did not improve any other symptoms. Personally, if I had to deal with discomfort while pooping, I would prefer a lower fiber intake because I would want to be pooping less frequently.
Fiber and Disease Risk
Most of the research showing the benefits of fiber for lowering disease risk are entirely associative with no known mechanistic cause. What’s likely happening is healthy user bias, where people eating more fiber tend to be eating more whole foods and living healthier lives in general, which is the real cause of the lower risk. The one clear mechanistic benefit for fiber is its effect in type 2 diabetics. Strong evidence suggests that fiber improves glycemic control in type 2 diabetics and can be valuable if those individuals choose to eat carbohydrates. The research makes sense because fiber tends to decrease or slow macronutrient absorption, meaning blood sugar spikes are more controlled. Fiber can also decrease cholesterol in the blood by small amounts due to requiring increased use of bile acid to excrete more waste, which theoretically sounds good for preventing heart disease. But cholesterol does not cause heart disease directly; instead, individuals with heart disease may see slower progression of the disease by lowering cholesterol. Even so, fiber on its own makes a small enough difference to likely be negligible in that situation. Finally, although common wisdom would suggest that a low-fiber diet causes diverticular disease, there is no good evidence that this is the case. Once again, diverticular disease is simply associated with a Western-style diet in general, and not fiber in particular.
Fiber and Feeling Full
The argument that fiber is healthy because it helps people feel full is silly and demonstrates a complete misunderstanding of health. It’s based on the old idea that poor health is caused by eating too much, rather than eating too much being caused by poor health. I recommend reading previous posts for more clarity, but the basic idea is that short-term tricks to feel full don’t control the body’s long-term appetite and preferred amount of fat storage; that is dictated by food selection, which is the main controlling factor for metabolic health. In other words, poor food selection leads to worse metabolic health, which then leads to malfunctioning appetite.
Conclusion
Plenty of great foods incidentally have fiber, but the pressure to eat enough fiber or else face consequences has no basis in reality. Focusing on eating real, whole foods is best, whether they happen to have fiber or not.