Dietary fiber

Current research suggests that dietary fiber, especially soluble fiber, may play an important role in maintaining cognitive abilities in old age and in the prevention of dementia.

Dietary fibers are plant components that cannot be digested by the enzymes in our intestines. They are mainly found in plant-based foods such as wholemeal products (e.g. cereal flakes, wholemeal bread, wholemeal pasta, wheat bran), pulses (lentils, beans, chickpeas, peas), vegetables (peppers, broccoli, carrots, potatoes), fruit (apples, pears, citrus fruits, berries), nuts (almonds, hazelnuts) or seeds (linseed, psyllium husks). They are found particularly in the outer layers, i.e. in the skin of fruit, vegetables, pulses and wholegrain products. In addition, fiber can also be ingested in the form of food supplements such as prebiotics.

Dietary fiber is divided into water-soluble and water-insoluble fiber:

• Water-soluble dietary fiber form a gel-like substance in the intestine that helps to stabilize blood sugar levels, lower cholesterol levels and promote the growth of “good” bacteria. This special form of dietary fiber is particularly important for the prevention of dementia. This group includes pectins, ß-glucans, inulin, fructooligosaccharides, galactooligosaccharides, mannanoligosaccharides, xyloooligosaccharides, algal polysaccharides, glucomannans and gum arabic.

• Water-insoluble dietary fiber swell in the intestine, increase stool volume and promote digestion by stimulating intestinal movement. These include cellulose, hemicellulose, lignin, chitin and resistant starch.

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The effect of dietary fiber on the brain

The exact mechanism by which dietary fiber influences the development of dementia diseases such as Alzheimer’s or vascular dementia is not yet fully understood. However, animal and laboratory studies suggest that dietary fiber can have a positive influence on dementia by, among other things, influencing the composition of the gut microbiome. Depending on the change in the gut microbiome, information is then sent to the brain via the gut-brain axis, which can inhibit or promote inflammation there. This in turn can accelerate or inhibit the development of dementia.

Influence on the gut microbiome

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The gut microbiome comprises all the microorganisms in our gut, such as bacteria, yeasts and fungi.

Poor dietary habits, including a low-fiber diet, can lead to dysbiosis, an imbalance in which the “bad” bacteria multiply and the “good” bacteria decrease. The “bad” intestinal bacteria include Bacteroides fragilis, Escherichia coli, Staphylococcus aureus and many Proteobacteria species. They either produce (neuro)toxic metabolic products themselves, release pro-inflammatory substances such as lipopolysaccharides (LPS) or pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1) or interleukin-6 (IL-6). These substances activate the intestinal immune system, which triggers an inflammatory chain reaction and weakens the intestinal barrier. The result is increased permeability of the intestinal wall, which makes it easier for harmful substances to enter the bloodstream and trigger even more inflammation. A vicious circle is created. These inflammations can spread throughout the body and also reach the brain. In the brain, they can increase the deposition of amyloid-β, a protein that plays a key role in Alzheimer’s disease. At the same time, immune cells in the brain, so-called microglial cells, are permanently activated, which can lead to further damage to the nerve cells and ultimately to their death. Such a dysbiosis in the microbiome and the associated effects are frequently observed in dementia patients [4].

A high-fiber diet, on the other hand, serves as food for the “good” bacteria, which keeps the gut microbiome in balance and can have a positive effect on brain health. The “good” bacteria include, for example, Bacteroides, Lactobacillus, Faecalibacterium prausnitzii, Akkermansia muciniphila, Roseburia spp. and Eubacterium spp. These bacteria produce anti-inflammatory substances such as γ-aminobutyric acid (GABA) or short-chain fatty acids (SCFA), which in turn trigger various anti-inflammatory chain reactions and can thus have a positive effect on brain health [4].

γ-Aminobutyric acid (GABA)

γ-Aminobutyric acid (GABA) is a neurotransmitter produced in the gut mainly by bacterial species such as Bacteroides and Lactobacillus. GABA contributes to improving the integrity of the intestinal barrier as well as the Blood-brain barrier which prevents the penetration of harmful substances from the intestine into the bloodstream and from the bloodstream into the brain. In addition, GABA stimulates the vagus nerve, which connects the intestine to the brain, and thus inhibits inflammation in the brain. GABA is also able to inhibit the overactivity of microglial cells and thus prevents the further breakdown of nerve cells [3].

Short-chain fatty acids (SCFA)

Similar to GABA, short-chain fatty acids, especially butyrate but also propionic acid and lactate, strengthen the blood-brain barrier and prevent the penetration of harmful substances. They help to prevent inflammation, calm overactive microglial cells and thus contribute to the reduction of chronic inflammation. Short-chain fatty acids (SCFAs) also prevent the formation of harmful beta-amyloid deposits, which are associated with Alzheimer’s, and in turn promote the production of important neurotransmitters such as GABA. They also provide the brain with additional energy when normal sugar absorption is impaired and can thus help to prevent an Energy crisis in the brain which is also causally linked to the development of dementia [2,7].

Study situation for dietary fiber intake in Alzheimer’s patients

Clinical studies investigating the relationship between dietary fiber and dementia are still limited. However, a few human studies show tendencies for a positive effect of dietary fiber on cognitive functions in older people and thus confirm the results of numerous animal studies.

A recent long-term cohort study with 3,700 Japanese participants aged 40 to 64 years investigated the relationship between dietary fiber intake and the risk of developing severe dementia requiring long-term care over a period of 20 years. The participants’ fiber intake was recorded using a 24-hour recall. At the end of the observation period, 670 of the participants were affected by a severe form of dementia. The results showed that a higher intake of dietary fiber, especially soluble fiber, was associated with a lower risk of dementia. This protective effect was particularly pronounced in people who had not previously suffered a stroke [8].

One further A long-term cohort study with 1,071 Japanese participants over the age of 24 investigated the influence of vegetables, fruit and nutrients on the risk of dementia. During the observation period, 464 people developed dementia, 286 of them Alzheimer’s disease and 144 vascular dementia. Researchers found that a higher vegetable intake was associated with a lower risk of dementia and Alzheimer’s disease, but not vascular dementia. In addition, there was a tendency for a higher fiber intake to be associated with a lower risk of dementia, although this fell just short of statistical significance. Fruit consumption had no demonstrable effect on the risk of dementia in this study [5] .

In another study, scientists investigated how the consumption of dietary fiber affects the mental performance of 1000 older people (over 60 years). Various memory and concentration tests were carried out, including the DSST test and the CERAD test. The DSST test is used to detect cognitive impairment and measures aspects such as processing speed, attention, motor speed and associative learning. The CERAD test, on the other hand, is primarily used to diagnose and monitor the progression of Alzheimer’s dementia and tests memory performance, language comprehension, orientation and concentration. The results showed that a higher fiber intake was associated with better results in the DSST test. However, no additional benefit was found above a daily intake of around 34 grams of fiber. On the other hand, there was no correlation between fiber intake and improved performance in the CERAD test. The study shows that the consumption of dietary fiber can generally have a positive influence on the cognitive abilities of older people, even if no specific correlation was found with an improvement in test results in Alzheimer’s patients [6].

Conclusion

Dietary fiber is an important part of a healthy and balanced diet. They not only promote digestion and general well-being. There is also growing evidence that an adequate intake of fiber also supports cognitive function and can help prevent dementia. To reap the health benefits of fiber-rich foods, it is recommended to consume at least 30 g of fiber daily.

Find out here which foods contain soluble fiber in particular and what additional health benefits they offer.

If you would like to find out more about the influence of the gut microbiome on your mental health, take a look at the download area of “Knowledge stops Dementia” and get the free fact sheet “Dementia and gut health”.

References

1. Barber, T. M., Kabisch, S., Pfeiffer, A. F. H., & Weickert, M. O. (2020). The Health Benefits of Dietary Fiber. Nutrients, 12(10), 3209. https://doi.org/10.3390/nu12103209
2. Berding, K., Carbia, C., & Cryan, J. F. (2021). Going with the grain: Fiber, cognition, and the microbiota-gut-brain-axis. Experimental Biology and Medicine, 246(7), 796-811. https://doi.org/10.1177/1535370221995785
3. Conn, K. A., Borsom, E. M., & Cope, E. K. (2024). Implications of microbe-derived ɣ-aminobutyric acid (GABA) in gut and brain barrier integrity and GABAergic signaling in Alzheimer’s disease. Good Microbes, 16(1), 2371950. https://doi.org/10.1080/19490976.2024.2371950
4. Kang, J. W., & Zivkovic, A. M. (2021). The Potential Utility of Prebiotics to Modulate Alzheimer’s Disease: A Review of the Evidence. Microorganisms, 9(11), 2310. https://doi.org/10.3390/microorganisms9112310
5. Kimura, Y., Yoshida, D., Ohara, T., Hata, J., Honda, T., Hirakawa, Y., Shibata, M., Oishi, E., Sakata, S., Furuta, Y., Chen, S., Uchida, K., Nakao, T., Kitazono, T., & Ninomiya, T. (2022). Long-term association of vegetable and fruit intake with risk of dementia in Japanese older adults: The Hisayama study. BMC Geriatrics, 22(1), 257. https://doi.org/10.1186/s12877-022-02939-2
6. Prokopidis, K., Giannos, P., Ispoglou, T., Witard, O. C., & Isanejad, M. (2022). Dietary Fiber Intake is Associated with Cognitive Function in Older Adults: Data from the National Health and Nutrition Examination Survey. The American Journal of Medicine, 135(8), e257-e262. https://doi.org/10.1016/j.amjmed.2022.03.022
7. Ticinesi, A., Mancabelli, L., Carnevali, L., Nouvenne, A., Meschi, T., Del Rio, D., Ventura, M., Sgoifo, A., & Angelino, D. (2022). Interaction Between Diet and Microbiota in the Pathophysiology of Alzheimer’s Disease: Focus on Polyphenols and Dietary Fibers. Journal of Alzheimer’s Disease, 86(3), 961-982. https://doi.org/10.3233/JAD-215493
8. Yamagishi, K., Maruyama, K., Ikeda, A., Nagao, M., Noda, H., Umesawa, M., Hayama-Terada, M., Muraki, I., Okada, C., Tanaka, M., Kishida, R., Kihara, T., Ohira, T., Imano, H., Brunner, E. J., Sankai, T., Okada, T., Tanigawa, T., Kitamura, A., … Iso, H. (2023). Dietary fiber intake and risk of incident disabling dementia: The Circulatory Risk in Communities Study. Nutritional Neuroscience, 26(2), 148-155. https://doi.org/10.1080/1028415X.2022.2027592

Image1 from: Shutterstock by Tijana Moraca

Image2 from: (Kang & Zivkovic, 2021)