Paleo? Keto? You’re Likely Deficient in This Nutrient
By Erica Zelfand

From heart health to trim waist lines to healthy bowel movements, fiber’s got you covered.

This is Part II in my three-part series on low-carb diets and fiber. As explored in my previous post, the paleo and ketogenic (“keto”) diets are powerful therapeutic tools. It’s all too easy, however, to forget to eat enough fiber while following a low carb plan. In this post we’ll dive deep into the fantastic world of fiber and explore what this nutrient can do for you. In the next post I’ll share some strategies and recipes for increasing your fiber intake without having to “cheat” on Dr. Atkins.


Why Fiber is Important

Fiber’s healing benefits are numerous. This plant-based nutrient has been shown to:

Reduce the risk of cardiovascular diseases like coronary heart disease (CHD) and stroke.[1] It also lowers the levels of harmful cholesterol while enhancing the levels of HDL, or “good” cholesterol,[2] and has even been shown to help lower blood pressure.[3]

Stabilize blood glucose (aka “blood sugar”), fight insulin resistance, and thus lower the risk of diabetes and other related conditions.[4]

Improve the frequency and consistency of bowel movements, thus reducing both diarrhea and constipation and generally supporting colon health.[5] Fiber has also been shown to alleviate the symptoms of irritable bowel syndrome (IBS).[6]

Reduce the risk of various types of cancer, including cancers of the esophagus,[7] pancreas,[8] colon, rectum,[9] and breast.[10],[11]

Balance hormone levels in the body, in part by increasing the production of sex hormone binding globulin (SHBG). This is key for managing diseases associated with hormone imbalances, such as polycystic ovary syndrome (PCOS).[12],[13]

Control appetite and support a healthy weight: Fiber-rich foods tend to be more filling than foods low in fiber, enabling us to eat less at mealtimes and stay full for longer. This is why people on a low-carb diet tend to snack more between meals. Fiber-rich foods also deliver fewer calories per bite than, say, protein-rich foods do.[14]

Dietary fiber has also been shown to reduce the risk of asthma and allergies, likely by creating positive changes in the bacterial composition of the gut.[15],[16]

So! Fiber is clearly an important part of a healthy diet. But what is it, actually?


What is Fiber, Anyway?

Fiber is a plant-based nutrient that typically passes through the digestive tract intact without getting broken down into its component sugars.

The types of fiber are typically sub-categorized into insoluble and soluble fiber:

Insoluble fiber, which promotes healthy bowel movements by more or less retaining its form while absorbing water, thereby creating soft yet bulky stools. In the process insoluble fibers help decrease the type of bowel pressure that otherwise leads to diverticula (pouches). By speeding up “transit time” (the speed at which food travels down the intestines for elimination), furthermore, insoluble fiber decreases the body’s exposure to the toxic metabolites being carried out in the stool. Sources of insoluble fiber include wheat bran, whole-wheat flour, nuts, beans, cauliflower, green beans, and potatoes.

Lignans, a type of insoluble fiber found in seeds and legumes,[17] have been shown to have not only antioxidant activity, but also noteworthy hormone-balancing effects – namely in reversing estrogen-dominant states. Lignans can thus help regulate the menses, balance the hormones, and in turn prevent certain types of cancer.[18] The richest source of lignans is in flax seeds, which I prescribe (ground up in a coffee grinder) to just about every woman in my practice with hormonal imbalances. Whereas whole flax seeds provide insoluble fiber, the ground seeds contain both soluble and insoluble forms.

Soluble fiber dissolves in water to create a gel-like material that is fermented in the colon. It also slows down the rate of the stomach’s emptying, thereby helping us feel full for longer after we eat. Soluble fiber also helps delay the body’s absorption of glucose (sugar) from food, thus tempering the “spike and crash” some experience after meals and lowering the risk of type 2 diabetes. On its way down the digestive tract, soluble fiber binds to bile acids, thereby helping to lower harmful cholesterol levels. Sources of soluble fiber include oats and oat bran, peas, beans, apples (with their skins), barley, fruits, and berries.

Inulin is a type of soluble, fermentable fiber that withstands digestion by the upper digestive tract and supports the growth of beneficial bacterial. Whereas probiotic foods (like plain yogurt and sauerkraut) contain live organisms that (theoretically) populate in our guts upon consumption, inulin is a prebiotic – a substance that feeds the healthy bacteria already residing in our digestive tracts. A common type of inulin is fructo-oligosaccharide, or FOS, which is sometimes added to probiotic supplements. Because it has a slightly sweet flavor, is low in calories, and does not trigger a rise in blood sugar or insulin levels, FOS is commonly used to enhance the flavor of low-calorie/low fat foods. Dietary sources of inulin include artichokes, garlic, leeks, onions, chicory, and soy products, and grains such as barley, flax, oat, and wheat.[19] (Note: inulin and FOS may aggravate some individuals with untreated small intestine bacterial overgrowth, or SIBO.[20])


Take the Fiber Challenge

Are you ready to take the fiber challenge? The fiber challenge consists of eating 30 to 50 grams of (gluten-free) fiber per day. The average American consumes 8 to 12 grams of fiber per day (including gluten), so chances are it’s time to up your fiber game.

How much is 30 to 50 grams? Explore this handy chart (PDF) of gluten free foods and how much fiber they contain, courtesy of Riley Wimminger, MScN at Bridgetown Nutrition.

And be sure to check out Part III of this series to see a list of common foods’ fiber content and try some of my favorite fiber-rich recipes.





[1] Mayor S. Eating more fibre linked to reduced risk of non-communicable diseases and death, review finds. BMJ 2019; 364:l159. doi:

[2] Bazzano LA, Hu T, Reynolds K, Yao L, Bunol C, Liu Y, Chen CS, Klag MJ, Whelton PK, He J. Effects of low-carbohydrate and low-fat diets: a randomized trial. Ann Intern Med. 2014 Sep 2;161(5):309-18. doi: 10.7326/M14-0180. PMID: 25178568.

[3] Aleixandre A, Miguel M. Dietary fiber and blood pressure control. Food Funct. 2016 Apr;7(4):1864-71. doi: 10.1039/c5fo00950b. PMID: 26923351.

[4] Post RE, Mainous AG 3rd, King DE, Simpson KN. Dietary fiber for the treatment of type 2 diabetes mellitus: a meta-analysis. J Am Board Fam Med. 2012 Jan-Feb;25(1):16-23. doi: 10.3122/jabfm.2012.01.110148. PMID: 22218620.

[5] Yang J, Wang HP, Zhou L, Xu CF. Effect of dietary fiber on constipation: a meta analysis. World J Gastroenterol. 2012 Dec 28;18(48):7378-83. doi: 10.3748/wjg.v18.i48.7378. PMID: 23326148.

[6] Okawa Y, Fukudo S, Sanada H. Specific foods can reduce symptoms of irritable bowel syndrome and functional constipation: a review. Biopsychosoc Med. 2019 May 8;13:10. doi: 10.1186/s13030-019-0152-5. PMID: 31080496.

[7] Sun L, Zhang Z, Xu J, Xu G, Liu X1. Dietary fiber intake reduces risk for Barrett’s esophagus and esophageal cancer. Crit Rev Food Sci Nutr. 2017 Sep 2;57(13):2749-2757. doi: 10.1080/10408398.2015.1067596. PMID: 26462851.

[8] Mao QQ, Lin YW, Chen H, Qin J, Zheng XY, Xu X, Xie LP. Dietary fiber intake is inversely associated with risk of pancreatic cancer: a meta-analysis. Asia Pac J Clin Nutr. 2017 Jan;26(1):89-96. doi: 10.6133/apjcn.102015.03. PMID: 28049267.

[9] Encarnação JC, Abrantes AM, Pires AS, Botelho MF. Revisit dietary fiber on colorectal cancer: butyrate and its role on prevention and treatment. Cancer Metastasis Rev. 2015 Sep;34(3):465-78. doi: 10.1007/s10555-015-9578-9. PMID: 26224132.

[10] Ferrari P, Rinaldi S, Jenab M, Lukanova A, Olsen A, Tjønneland A, Overvad K, Clavel-Chapelon F, Fagherazzi G, Touillaud M, Kaaks R, von Rüsten A, Boeing H, Trichopoulou A, Lagiou P, Benetou V, Grioni S, Panico S, Masala G, Tumino R, Polidoro S, Bakker MF, van Gils CH, Ros MM, Bueno-de-Mesquita HB, Krum-Hansen S, Engeset D, Skeie G, Pilar A, Sánchez MJ, Buckland G, Ardanaz E, Chirlaque D, Rodriguez L, Travis R, Key T, Khaw KT, Wareham NJ, Sund M, Lenner P, Slimani N, Norat T, Aune D, Riboli E, Romieu I. Dietary fiber intake and risk of hormonal receptor-defined breast cancer in the European Prospective Investigation into Cancer and Nutrition study. Am J Clin Nutr. 2013 Feb;97(2):344-53. doi: 10.3945/ajcn.112.034025. PMID: 23269820.

[11] Aune D, Chan DS, Greenwood DC, Vieira AR, Rosenblatt DA, Vieira R, Norat T. Dietary fiber and breast cancer risk: a systematic review and meta-analysis of prospective studies. Ann Oncol. 2012 Jun;23(6):1394-402. doi: 10.1093/annonc/mdr589. PMID: 22234738.

[12] Cutler DA, Pride SM, Cheung AP. Low intakes of dietary fiber and magnesium are associated with insulin resistance and hyperandrogenism in polycystic ovary syndrome: A cohort study. Food Sci Nutr. 2019 Feb 27;7(4):1426-1437. doi: 10.1002/fsn3.977. PMID: 31024716.

[13] Liepa GU, Sengupta A, Karsies D. Polycystic ovary syndrome (PCOS) and other androgen excess-related conditions: can changes in dietary intake make a difference? Nutr Clin Pract. 2008 Feb;23(1):63-71. PMID: 18203965.

[14] Miketinas DC, Bray GA, Beyl RA, Ryan DH, Sacks FM, Champagne CM. Fiber Intake Predicts Weight Loss and Dietary Adherence in Adults Consuming Calorie-Restricted Diets: The POUNDS Lost (Preventing Overweight Using Novel Dietary Strategies) Study. J Nutr. 2019 Jun 7. pii: nxz117. doi: 10.1093/jn/nxz117. PMID: 31174214.

[15] Zhang Z, Shi L, Pang W, Liu W, Li J, Wang H, Shi G.Dietary Fiber Intake Regulates Intestinal Microflora and Inhibits Ovalbumin-Induced Allergic Airway Inflammation in a Mouse Model. PLoS One. 2016 Feb 12;11(2):e0147778. doi: 10.1371/journal.pone.0147778. eCollection 2016. PMID: 26872019.

[16] Trompette A, Gollwitzer ES, Yadava K, Sichelstiel AK, Sprenger N2, Ngom-Bru C, Blanchard C, Junt T, Nicod LP, Harris NL, Marsland BJ. Gut microbiota metabolism of dietary fiber influences allergic airway disease and hematopoiesis. Nat Med. 2014 Feb;20(2):159-66. doi: 10.1038/nm.3444. PMID: 24390308.

[17] Types of Fiber and Their Health Benefits. WebMD. Available from:

[18] De Silva SF, Alcorn J. Flaxseed Lignans as Important Dietary Polyphenols for Cancer Prevention and Treatment: Chemistry, Pharmacokinetics, and Molecular Targets. Pharmaceuticals (Basel). 2019 May 5;12(2). pii: E68. doi: 10.3390/ph12020068. PMID: 31060335.

[19] Pandey KR, Naik SR, Vakil BV. Probiotics, prebiotics and synbiotics- a review. J Food Sci Technol. 2015 Dec;52(12):7577-87. doi: 10.1007/s13197-015-1921-1. PMID: 26604335.

[20] Siebecker A. Dietary Treatments [for SIBO]. Available from:

Chart of fiber-containing foods courtesy of Riley Wimminger at Bridgetown Nutrition.

Image courtesy of artur-rutkowski for Unsplash.