Table of Contents
In the last 3 lessons we discussed the macronutrients protein, carbohydrate, and fat. While some people consider dietary fiber to be a subset of carbohydrates, it has such different properties that it merits its own discussion. Fiber is a substance typically found in foods (though can be supplemented) that resists digestion and can have a variety of health benefits. In this lesson we will discuss what dietary fiber is, how it generates health benefits, what those benefits are, and how much we should consume.
Definition of dietary fiber
An exact definition is difficult to develop, with a variety of different definitions proposed over the years and currently in use.(Jones, 2014) I have reproduced the Codex Alimentarius Commission (CAC) June, 2009 definition in the note below for those who are curious.
Note: The CAC is an international food standards-setting body originally established by the United Nations Food & Agriculture Organization (FAO) & the World Health Organization (WHO). It is composed of 188 member countries as well as the European Union and over 200 inter-governmental & international non-governmental observer organizations. Individuals from the various participating organizations are able to take part in committees on various food-related topics to attempt to generate a consensus based on the most recent scientific evidence (committees meet annually). For further reading, click here for the primary CAC website and here for the US Codex website.
Note: The June 2009 Codex Alimentarius Commission definition of dietary fiber (paraphrased by me):
Carbohydrate polymersa with 10+ monomeric unitsb which are not hydrolysed by the endogenous enzymes in the small intestine of humans and belong to the following categories:
- edible carbohydrate polymers naturally occurring in foods we consume
- carbohydrate polymers obtained from raw food material by food processing shown to have a beneficial human physiologic effect
- synthetic carbohydrate polymers shown to have a beneficial human physiologic effect
a when derived from plant origin this may include fractions of lignin and/or other compounds associated with polysaccharides in plant cell walls
b whether to include carbohydrates of 3-9 monomeric units is left up to national authorities
Of note regarding footnote b, this is largely due to older methods of characterizing dietary fiber that would have a difficult time actually detecting the smaller polymers. This is less of a problem with more modern technologies and physiologically it makes sense to include the smaller compounds.
Do keep in mind that this is only one definition and different countries and food organizations may utilize different definitions. This makes it harder to standardized nutrition labels across countries as the same exact food may have a different fiber content if a different definition of dietary fiber is used.
Individual properties of various dietary fibers
Even if one exact definition is used it can be difficult to quantify and characterize the dietary fiber in food.(McCleary, 2018) This is due to the various physical properties of the fiber, which also lead to different physiologic effects. Some of these considerations include(Poutanen, 2018):
- polymer structure (length, branching, molecular composition)
- functionality (solubility, gel formation, viscosity formation)
- impact on physiology (stomach distension & emptying, satiety, nutrient absorption, gut transit time, colonic fermentation, impact on blood cholesterol levels, impact on postprandial glucose & insulin levels)
- additional carbohydrate moieties, bioactive phenolic components, purification, and for some types the degree of methylation
In addition to all of this, the same type of fiber can have variable impacts based upon:
- different baking/cooking processes can alter the molecular structure and hence the fiber’s properties
- an individual’s gastrointestinal (GI) microbiome, may ferment the fermentable fibers in different quantities and generate different byproducts relative to others, which can yield different physiologic responses
Note: With regards to baking/cooking, of interest is that one subset of dietary fiber is resistant starch, broadly defined as the portion of starch that resists digestion in the small intestine but may be fermented in the colon.(Ashwar, 2016) Unripe bananas can have ~50% resistant starch. With ripening the resistant starch disappears and the net calories absorbed increases. Of the 5 classifications of resistant starch, type 3 is very interesting with respect to cooking. By cooling food some of the digestible starch present may undergo retrodegradation to form resistant starch type 3; this will decrease the net calories absorbed. Even when reheating after cooling there can be a considerably lower increase in blood glucose after a meal(Robertson, 2020). However, it should be noted a recent review found that resistant starch over longer periods of time only had a small, likely clinically insignificant, impact on fasting blood glucose and no impact on insulin resistance.(Xiong, 2020).
While I do not believe this “diet hack” will be a game changer for anyone, you can consider cooling or cooling/reheating digestible starch if you’re attempting to lower the total calories you consume. This is one of several examples regarding why counting calories is an inexact science; as described in earlier lessons they best we can generally do is derive an informed estimate and then adjust accordingly.
With all of these differences it is not practical to attempt to tailor specific food intake towards obtaining specific fiber types in the diet (one exception is with specific fiber supplements, see below). There is also not a large enough body of research on all of the specific fiber types to advise an “optimal” fiber consumption strategy that recommends exact amounts of specific fiber compounds. Thus, when discussing the health impact of dietary fiber most references speak to the entire class of dietary fibers.
An appreciation of some of the basic structure-function properties can provide insights into why certain fiber types and supplements may benefit us in different ways.(McRorie, 2017)
Note: Some basic fiber type classifications are:
- Soluble – the fiber mixes well with the stomach contents to form a solution
- Insoluble – the fiber does not mix well with the stomach contents
- Viscous – soluble fiber that mixes with stomach contents in such a way to increase the viscosity of the solution (this occurs when fiber does not have significant branching, and the longer the chain the greater the viscosity)
- Gel-forming – soluble fiber that mixes with stomach contents in such a way as to make the solution take on gel-like properties (this occurs when linear fiber polymers form cross-links with adjacent chains)
- Fermentable – fiber that can be fermented by the GI microbiome to promote its growth and to generate byproducts such as short-chain fatty acids
- Non-fermentable – fiber that cannot be fermented by the gut microbiome
Soluble, viscous, gel-forming fiber is going to do the best job of thickening stomach contents, thus slowing the rate of digestion & absorption. This will:
- increase satiety
- decrease the rate of blood glucose absorption and thus lower postprandial glycemia
- bind and sequester bile acids leading them to being excreted in the stool rather than reabsorbed via enterohepatic circulation; the liver then must reabsorb more LDL from the bloodstream to synthesize new bile acids and this lowers LDL cholesterol
Fermentable fibers essentially provide food for the GI microbiome and aid in its growth. When microbes metabolize the fiber short-chain fatty acids are produced, and these provide a myriad of health benefits.
Non-fermentable soluble fibers will bring associated fluid to the stool to add bulk and aid in constipation. Non-fermentable insoluble fibers with large particle sizes can irritate the lining of the colon, which stimulates increased fluid secretion and aids in constipation. At the same time, too much of this insoluble fiber may lead to diarrhea, and non-fermentable insoluble fibers with small particle sizes may add bulk to the stool and can have a constipating effect when consumed in excess.
As seen in the above section fiber can aid in control of blood glucose levels, blood cholesterol levels, satiety, and stool regularity via the way it impacts the flow of nutrients throughout the GI tract. In addition to this, fiber itself can directly impact the GI tract, and the promotion of a variety of gut bacteria also leads to health benefits. A recent review details some of these mechanisms(Gai, 2020) including positive effects on intestinal barrier function and immunity, influencing the microbial gut-brain axis (benefiting mood, cognition, and suppressing appetite), as well as binding to certain toxins and pathogenic microbes to neutralize them.
All of these health benefits additionally lead to beneficial health outcomes. Recently an umbrella review of systematic reviews & meta-analyses was published finding a benefit of increased fiber intake on multiple health outcomes including cardiovascular disease, all-cause mortality, and certain types of cancer.(Veronese, 2018)
A recent systematic review and meta-analysis of randomized controlled trials on the use of viscous fiber supplements found their use (median dose of 8 grams daily) was associated with a 0.33kg decrease in body weight relative to controls, with longer trial lengths associated with a larger degree of weight loss.(Jovanovski, 2020) 0.33kg may not seem like a lot of weight but importantly none of these trials involved caloric restriction. A separate recent review concluded mixed results of fiber consumption on appetite and energy intake, and they attribute some of the variability to different characterizations, quantities, and time frames of fiber used in trials (most being <24 hours duration).(Poutanen, 2017) There is, however, strong epidemiological evidence that dietary fiber intake is associated with improved body weight.(Slavin, 2005)
Note: In general it is best if relationships seen with epidemiological studies can then be further supported by randomized controlled trials. With nutrition this is typically difficult as the time frame of many dietary associations is seen over several years, and it is very resource-intensive to carry out trials over that duration. Regarding fiber specifically, as discussed above, different fiber types have different properties. Thus, to mimic real world scenarios simply supplementing one type of fiber may not yield results that generalize to increasing consumption of a variety of fiber-rich foods. Interventions can attempt to increase intake of fiber rich foods to yield results that can be better generalized, but this will increase confounding of effects related to the other properties of the consumed foods. Therefore, it is very difficult to truly isolate the impact of dietary fiber directly.
The POUNDS Lost trial compared 4 diets over a 2 year time period that varied in protein and fat content.(Miketinas, 2019) All were designed with a 750 kcal deficit and at least 20 grams fiber daily. ~30% of the subjects in the trial stayed within 5% of the macronutrient targets. Adherent subjects averaged 25.2 grams of fiber daily compared with 21.1 grams otherwise. There was no difference in weight loss between the diet groups, however adherent subjects lost 9.3 kg compared to 6.4 kg otherwise. Weight loss increased across each quartile of change in fiber intake from baseline; in fact fiber intake was the strongest predictor of weight loss of all examined variables. Those who met the fiber goals had 2.94x the odds of remaining adherent to their prescribed diet. This suggests greater fiber intake may aid dietary adherence and weight loss.
Recommended fiber intake
In short we do not have strong enough evidence to recommend a specific amount of dietary fiber. This is in part because there are many different types with different properties.
The 2020-2025 Dietary Guidelines for Americans suggests 14 grams of fiber per 1,000 kcal intake. This comes from the Institute of Medicine recommendation set back in 2002. There they examined several data sets and utilized 3 large studies: the Health Professionals Follow-up Study, the Nurses’ Health Study, and a large cohort of Finnish men. They found a trend of decreased risk of coronary heart disease with increased fiber intake, with top quintiles having fiber consumption of 12.9, 14.31, and 14.45 grams per 1,000 kcal intake. They averaged this to 14 grams per 1,000 kcal intake. In lieu of strong data in pediatrics they extend this data to all people age 1 and above. Considering median caloric consumption per age and gender, they recommend 38 grams daily fiber intake in young adult men and 25 grams daily in young adult women.
Note: To be clear, while the above is the official recommendation, that does not mean this is an “optimal” amount. In those 3 studies a greater effect was seen in each increasing quintile of dietary fiber consumption; it is possible even higher levels of consumption would yield a greater beneficial effect. Additionally, the official recommendation was based on data for decreasing the risk of coronary heart disease; it does not consider the quantities needed to have beneficial effects on blood glucose control, cholesterol, cancer, all-cause mortality, appetite suppression, mood, and the other health benefits that can be expected with increased fiber consumption.
There is no proposed upper limit of fiber consumption above which a beneficial effect will no longer be seen. There is a theoretical risk that some types of fiber will bind to nutrients and prevent absorption, but the literature thus far is unclear if this is actually significant and there are studies indicating that fermentable fiber can actually enhance absorption.(Baye, 2017)
A recent study showed continued changes in the gut microbiome and fermentation byproducts for intakes up to 35 grams daily of a specific fiber type and up to 50 grams daily in roughly one-third of studied subjects.(Deehan, 2020) Thus, when eating fiber from a variety of different sources, it is possible there will not be a plateau of beneficial effects at a certain upper limit.
Note: This same study also shows that different dietary fibers will impact the microbiome and fermentation byproducts in different ways, promoting the growth of different types of bacteria that then metabolize fiber in different pathways. Much more research needs to be done but this shows the possibility of tailoring specific fiber preparations with the goal of promoting specific strains of bacteria and may aid in promoting growth of bacterial strains ingested as probiotics or obtained via fecal transplants.
Tip: Even for individuals with no desire to track calories I believe it is helpful to track grams of fiber intake sporadically to ensure adequate intake due to all of the health benefits. Plugging in a day of eating into cronometer.com can help with this. For people who do want to track calories, when fiber is fermented to generate short-chain fatty acids these can be absorbed through the colon and thus lead to caloric consumption, while fiber can also bind to a small amount of nutrients leading to loss in the stool, with both of these factors being impractical to quantify accurately. Thus, I recommend sticking with the calorie counts on nutrition labels rather than attempting to track calories in fiber specifically.
As mentioned above some fiber types can contribute to constipation or diarrhea when consumed in excess, and it is possible to have other forms of GI distress (bloating, flatulence, cramping) at high levels of fiber intake. If this occurs this would be one reason to limit total daily intake.
Overall, dietary fibers are a complex group of compounds providing a plethora of health benefits. If choosing to take a fiber supplement the above structure-function relationships should be considered, but otherwise it seems most practical to consume fiber from a variety of different sources. There seems to be no upper limit (at least to studied intakes of 50 grams daily) above which beneficial effects will not further be seen. Thus, I recommend aiming for at least the standard recommendation of 14 grams per 1000 kcal intake daily, and for those not counting calories at least 30-40 grams daily will be a good target to shoot for. If going above this and GI distress becomes an issue, then I would decrease the fiber intake until this is no longer problematic.
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