Fiber: Eat It

Unless you are an especially precocious infant, you are probably among the 94% of Americans over the age of 1 who fail to achieve an “adequate intake” of dietary fiber [1]. Adequate consumption of fiber is linked to reductions in incidence of colorectal cancer (third most common cause of cancer and cancer death in both sexes), cardiovascular disease, and type 2 diabetes [2]. These findings have been corroborated countless times, and were noticed at least as far back as 1976 [3]. Interestingly, there is some debate on what counts as “fiber” from a chemical standpoint, but the National Cancer Institute (NCI) defines it in relation to physiology as “a type of carbohydrate found in plant foods that the body cannot digest” [4]. It is interesting already that the NCI is a source on the definition of fiber and should give the reader a clue as to the magnitude of the implication in colorectal cancer. The Codex Alimentarius of the World Health Organization (WHO), in a technical and esoteric but functionally similar definition, calls fiber “edible plant and animal material not hydrolysed by the endogenous enzymes of the human digestive tract as determined by the agreed upon method” [5].

At any rate, fiber can be further classified into the “soluble” and “insoluble” types, referring to solubility in water. Most sources agree that soluble fiber is found in “apples, bananas, oats, peas, black beans, lima beans, Brussels sprouts, and avocados,” whereas insoluble fiber can be had from “whole wheat flour, bran, nuts, seeds, and the skins of many fruits and vegetables” (both again from the NCI). Whereas insoluble fiber is implicated mostly in a mechanical sense, adding bulk to the stool and thus facilitating regularity of bowel movements, soluble fiber appears to be more metabolically involved. Specifically, soluble fibers may serve as food for many gut bacteria that seem to produce beneficial substances such as short-chain fatty acids (SCFAs), fermentation byproducts from the gut microbiota that are implicated in inflammatory and immune processes [6]. Soluble fiber has been shown to bind bile acids, thus contributing to decrease in free LDL cholesterol (which may partially explain the cardiovascular benefits of fiber). However, the complete mechanism by which fiber lowers cholesterol is poorly characterized [7]. Insoluble fiber did not have a significant relationship with blood cholesterol in rats [8].

For all that the low-carb diets genuinely seem to offer, most ketogenics and certainly carnivores are probably incurring undue risk by reducing their fiber intake to almost nothing.

The recommended daily intake of fiber is 14 g total fiber per 1,000 kcal, or 25 g for adult women and 38 g for adult men. Interestingly, this oft-cited recommendation does not distinguish between soluble and insoluble fibers. It is also interesting to consider how these recommendations are made; most sources will simply appeal to the experts, one source noting that the recommendations are “supported by the Institute of Medicine, the Academy of Nutrition and Dietetics, and major U.S. guidelines” [2].

So, statistically speaking, you should probably eat more fiber. Importantly, it is recommended to increase your fiber intake slowly to allow your gut to adjust; constipation or gastrointestinal upset are common when increasing fiber intake too fast. Here are a few options I like:

1. Bran flakes. Rock the checkout line grandma-style with your bran flakes. Most have very little (or no) sugar added if, like me, you try to avoid that. But, if you must have it sweet, there are berry brans, frosted options, etc. The one in my pantry claims 5g fiber per 1 cup, probably almost all insoluble (so, if we are reading the literature right, might have beneficial GI effects but probably not bringing down our LDL).

2. Chia seed pudding. Takes a little perfecting, but my current recipe works pretty well. Combine 0.25 cups cocoa powder, 0.5 tsp vanilla extract, a pinch of salt, and a splash (or not) of maple syrup, or honey, or whatever may be on hand. Whisk together. The sugar isn’t strictly necessary, as there will be some in the milk, so feel free to omit it if you are a glutton for penance. Speaking of, measure out 1.5 cups of milk and add a little at a time, whisking until a paste forms. Whisk until clumps mostly gone, then add remaining milk and whisk till smooth. Add 0.5 cups chia seeds, give it a stir, and cover and refrigerate. After 30 minutes in the fridge, give another stir, breaking up all the clumps and scraping the bottom. Keep in refrigerator at least overnight before eating. Makes 2-3 servings. 0.5 cups of chia seeds gives about 30g of fiber, so one serving of this stuff makes a huge dent in your daily fiber totals. Also seems to be mostly insoluble fiber [9].

3. For soluble fiber, oats, beans, and many fruits are your friends.

I have only looked into the fiber supplements enough to know that questions like “which one?” or “how much?” quickly become intractably complex. Maybe in a follow-up I will explore supplementation.

If you take nothing else away, just eat more fiber. If you can bear a few more conclusions, know that fiber is not one thing, that its subtypes have different (but all probably positive) effects, and that there is certainly a fair bit about fiber that is beyond human knowledge at this time.

[1] Dietary Guidelines Advisory Committee, “Scientific Report of the 2020 Dietary Guidelines Advisory Committee: Advisory Report to the Secretary of Agriculture and Secretary of Health and Human Services,” U.S. Department of Agriculture, Agricultural Research Service, Jul. 2020. doi: 10.52570/DGAC2020.

[2] S. F. Daley and A. P. Shreenath, “The Role of Dietary Fiber in Health Promotion and Disease Prevention: A Practical Guide for Clinicians,” in StatPearls, Treasure Island (FL): StatPearls Publishing, 2025. Accessed: Jan. 28, 2026. [Online]. Available: http://www.ncbi.nlm.nih.gov/books/NBK559033/

[3] H. Trowell, “Definition of dietary fiber and hypotheses that it is a protective factor in certain diseases,” The American Journal of Clinical Nutrition, vol. 29, no. 4, pp. 417–427, Apr. 1976, doi: 10.1093/ajcn/29.4.417.

[4] “Definition of fiber - NCI Dictionary of Cancer Terms - NCI.” Accessed: Jan. 28, 2026. [Online]. Available: https://www.cancer.gov/publications/dictionaries/cancer-terms/def/fiber

[5] Y. Motarjemi, “Codex Committee on nutrition and foods for special dietary uses,” Food Control, vol. 2, no. 3, p. 185, Jul. 1991, doi: 10.1016/0956-7135(91)90089-F.

[6] E. R. Mann, Y. K. Lam, and H. H. Uhlig, “Short-chain fatty acids: linking diet, the microbiome and immunity,” Nat Rev Immunol, vol. 24, no. 8, pp. 577–595, Aug. 2024, doi: 10.1038/s41577-024-01014-8.

[7] L. Brown, B. Rosner, W. W. Willett, and F. M. Sacks, “Cholesterol-lowering effects of dietary fiber: a meta-analysis*,” The American Journal of Clinical Nutrition, vol. 69, no. 1, pp. 30–42, Jan. 1999, doi: 10.1093/ajcn/69.1.30.

[8] T. Oda, S. Aoe, H. Sanada, and Y. Ayano, “Effects of soluble and insoluble fiber preparations isolated from oat, barley, and wheat on liver cholesterol accumulation in cholesterol-fed rats,” J Nutr Sci Vitaminol (Tokyo), vol. 39, no. 1, pp. 73–79, Feb. 1993, doi: 10.3177/jnsv.39.73.

[9] B. Kulczyński, J. Kobus-Cisowska, M. Taczanowski, D. Kmiecik, and A. Gramza-Michałowska, “The Chemical Composition and Nutritional Value of Chia Seeds—Current State of Knowledge,” Nutrients, vol. 11, no. 6, p. 1242, May 2019, doi: 10.3390/nu11061242.

[10] S. Reiners et al., “Effect of a regular consumption of traditional and roasted oat and barley flakes on blood lipids and glucose metabolism–A randomized crossover trial,” Front Nutr, vol. 10, p. 1095245, Feb. 2023, doi: 10.3389/fnut.2023.1095245.