There are many controversial topics in nutrition. You can hear people swear by one diet, that a specific one is superior to all other. However, what you usually don’t hear is something negative about fiber. When getting nutritional advice, you often hear that you should limit the intake of sugar and try to eat food high in fiber, but what exactly is fiber and why should you eat it?
What happens to the fiber when we eat it?
The previous installment we learned that the part of carbohydrates that we can’t digest is called fiber. So, if the digestive enzymes that we have in our gastrointestinal tract can´t digest fiber, what happens to the fiber when we eat it? The observant reader might have noticed that in the example in part 1 I write that we actually can extract calories from the fiber oligofructose. How is that possible if we don’t have the enzymes to digest fiber?In our gastro intestinal
In our gastro intestinal tract, we have a microbiota with many different bacteria. Some of these bacteria work in symbiosis with us and can ferment parts of the undigested fiber that escape digestion in the upper gut to short-chain fatty acids (SCFA) in the cecum and colon (Koh et al., 2016). These SCFA can then be absorbed and metabolized by us. It is estimated that fiber provides around half the amount of energy that regular carbohydrates provide (Flint et al., 2012), i.e. around 2 kcal/gram (normal carbohydrate provides ~4kcal/g). However, this is dependent on the type of fiber.
Therefore, since our enzymes can´t break down fiber they provide bulk and viscosity in our gastro intestinal tract, and some fiber like mentioned above provide a substrate for the microbiota.
Health effects of fiber
Dietary fiber has been associated with numerous health benefits including:
- Reduced LDL cholesterol (mainly soluble fiber)
- Reduced risk for cardiovascular disease (Otles and Ozgoz, 2014)
- Reduced risk of diabetes (insoluble fiber) (Otles and Ozgoz, 2014)
- Acting as a substrate for gut microbiota (pre biotic function) (Slavin, 2013) (mainly soluble fiber)
- Lowers blood pressure (Otles and Ozgoz, 2014)
- Decreases blood glucose and insulin after a meal by decreasing glycemic index of the meal (effect of soluble fiber possibly due to slowed absorption) (Otles and Ozgoz, 2014)
Soluble fiber usually delays gastric emptying and promotes a slow transit time of the food in the small intestine (due to its increased viscosity, density, and large particle size) while insoluble fiber speeds up the transit time in the small intestine (Otles and Ozgoz, 2014). Food high in insoluble or poorly fermented fiber, for example, cereal brans, could, therefore, be used to prevent and manage constipation in some cases. Additional effects include the ability to bind bile acids (acids helping to emulsify/”solve” fats in the intestines) and carcinogens (substances promoting cancer growth) (Otles and Ozgoz, 2014). It could possibly have protective effects against colon cancer, however, this is not established.
Usually, food rich in fiber also contains other nutrients that may provide health benefits and it is not always easy to determine if the fiber per se is causing the health benefits or if the combination of nutrients and the fiber are what provides the health benefits.
Fiber – Satiety and weight loss
Fiber has been implicated in improving satiety and aid in weight loss. Dietary fiber is associated with lower body weight in epidemiological studies (Clark and Slavin, 2013).
However, a meta-analysis (an analysis of many studies) from 2013 showed no relationship between fiber intake and satiety or food intake overall. Though, some type of fiber showed decreased appetite more frequently, including β-glucan (found in oats and barley for example), rye bran, whole grain rye, and a mixed high-fiber diet. The analysis didn´t find any clear relation between fiber properties (eg viscosity and fermentability) and appetite (Clark and Slavin, 2013).
To summarise, some fibers can possibly have some beneficial effects on appetite and body weight but the evidence is ambiguous. The practical implications are that increased fiber intake in the form of oats and other high fiber grains could aid in curbing your hunger to a larger extent than other food. If not, you can at least reap the other health benefits of a higher fiber diet.
In the next, and last, installment I will give some recommendations or fiber intake and also explain what happens if you eat too much fiber.
Written by: Fredrik Wernstål
 Epidemiological studies investigate associations and it´s therefore not certain that the fiber causes the lower body weight, only that they are associated. Hence, we don’t know if fiber is the cause of the lower body weight or if, for example, normal weight people tend to eat more fiber because they eat less calories and healthier in general.
Clark, M.J., Slavin, J.L., 2013. The effect of fiber on satiety and food intake: a systematic review. J. Am. Coll. Nutr. 32, 200–211. doi:10.1080/07315724.2013.791194
Flint, H.J., Scott, K.P., Duncan, S.H., Louis, P., Forano, E., 2012. Microbial degradation of complex carbohydrates in the gut. Gut Microbes 3, 289–306. doi:10.4161/gmic.19897
Koh, A., De Vadder, F., Kovatcheva-Datchary, P., Bäckhed, F., 2016. From Dietary Fiber to Host Physiology: Short-Chain Fatty Acids as Key Bacterial Metabolites. Cell 165, 1332–1345. doi:10.1016/j.cell.2016.05.041
Otles, S., Ozgoz, S., 2014. Health effects of dietary fiber. Acta Sci. Pol. Technol. Aliment. 13, 191–202.
Slavin, J., 2013. Fiber and prebiotics: mechanisms and health benefits. Nutrients 5, 1417–1435. doi:10.3390/nu5041417