In short, not necessarily; but it’s a bit nuanced. In recent years fiber has been claimed as “zero calorie” and superflorous terms such as “net carbs” have been used as a marketing strategy geared towards the health/weight-conscious individual. As such, the desire for low carb anything has taken over the supplement and food industry. Most of you have probably seen an item advertised as “only 2g Net Carbs!” only to look at the nutrition facts and see that there are 14g of carbohydrates, 12g of which are classified as “dietary fiber”. So, what does this mean? Are we supposed to track this particular item mentioned above as 14g of carbs or 2g of carbs? Let’s first start by understanding what fiber is and how it is processed by the body.
What is fiber?
Fiber is a non-digestible form of carbohydrates. But, there are different types of fiber. These include soluble and insoluble fibers. Both are inherently non-digestible and tend to be good for overall health but there are distinct differences between the two.
Soluble fiber dissolves in water and has been shown to be beneficial in lowering blood cholesterol. Soluble fiber can exert this effect by binding to bile acids (which are found in your stomach, secreted by your gallbladder, and needed to make cholesterol) and removing them from the body.
Insoluble fiber, found mainly in plants (cellulose), is largely associated with removing potentially harmful compounds from the digestive tract and can contribute to healthy digestion. Like starches, fiber is made up of several glucose units linked together. However, unlike starches, fiber stays relatively intact (resistant to digestive enzymes) when moving through the digestive tract. According to an article by Joanne Slavin and Justin Carlson on carbohydrates, the relatively intact fiber can make its way to the large intestine to be fermented by “colonic microflora” (bacteria found in the gut and digestive tract to help facilitate the breakdown of food). Or, it can pass through the digestive tract, bind to water, and increase stool weight. In regard to the fermentation of fiber mentioned above, I am referring to enteric fermentation which is the body’s ability to breakdown carbohydrates with the help of microorganisms into simpler molecules for absorption into the bloodstream.
When fiber is fermented, these microflora don’t produce glucose but rather short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate (which contain fewer than 6 carbons and thus provide relatively little energy to cells). These can be absorbed into the blood from the gastrointestinal tract and utilized for energy in cells to a limited extent. These SCFAs have been implicated as important for the regulation of metabolic and cardiovascular health (Chambers et al., 2018). But, the degree to which these SCFAs derived from fiber fermentation contribute to the net calorie load seems miniscule.
Only an estimated ~1-2 calories/g of fiber tends to be absorbed and utilized by the body. Because of this, some suggestions state that you should count carbs with dietary fiber as only 2 calories per gram (as opposed to 4kcal/g). My suggestion would be to make it even more simple and count all carbohydrates (dietary fiber, sugars, everything that contains either) as the full carbohydrate amount that is shown on the nutrition facts label. In the same article mentioned above, the Institute Of Medicine (IOC) recommends 14g of fiber per 1000kcal, with slightly lower recommendations for women and the elderly. This coincides with other works and I tend to recommend anywhere between 10-14g fiber per 1000kcal consumed. These recommendations are based on the data on the “relation of fiber consumption and coronary heart disease risk,..” which mentions a decreased risk of chronic disease and other health related issues at that level of fiber/day. For example, for someone consuming 2500kcal/day they should likely consume around 25g fiber. If we assume that the calories consumed include a contribution from fiber of 2kcal/g at 25g that would equate to 50 calories. If we assume fiber grams contributed 4kcal/g this would only equate to an additional 50kcal (less than 2% of total calories). Counting fiber as a normal carb will help ensure there is no under estimation. 2kcal/g is an average estimate but it could be slightly higher or lower depending upon a host of factors related to fermentation in the gut and cellular uptake of SCFAs. Thus, it probably makes sense for cautious individuals to use the standard 4 cal/g for precision, but the value is likely closer to 1-2 cal/g.
Why consume fiber?
Fiber is great to consume for a variety of reasons, from bowel movements, weight loss, and its role in heart health. Nonetheless, by utilizing such a stringent approach, I believe we are missing the overall big picture in regard to exercise, nutrition, and its effect on your health and wellness goals.
As an easy and practical takeaway (regardless of what type of dieting style you choose), I’d recommend that you use a standardized system for just counting all carbs as 4kcal/g while consuming the recommended 10-14g of fiber/1000kcal. This will allow for a safety net of sorts to ensure you don’t go over your allotted caloric goal and as such, this will allow your focus to go toward places of more importance along your fitness journey including forming the habit of going to the gym daily or performing cardio to create a caloric deficit.