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Top facts that will change the way you look at fiber

1. There are few drugs as powerful as fiber
2. Most of us are deficient in fiber by 50% and we over-consume all other nutrients by 20-300%
3. For every 10 grams of fiber you reduce all-cause mortality by about 10%
4. Fiber is not a nutrient for you, but for your gut bacteria
5. Your gut bacteria once fed, play a central role in your hormonal, immune, digestive, cardiovascular, metabolic, and nervous systems
6. If you don't feed them sufficient fiber, you get sick and die younger 
7. It is the ratio of sugar to fiber that matters.  A ratio of 4 sugars to 1 fiber is healthy.  Higher bad, lower good 
8. A lower sugar to fiber ratio correlates with the heath of individuals and nations.  It correlates with lower obesity and avoidance of many diseases.
9. Everything that grows out of the ground has some fiber in it.  The more processed, the less fiber
10. By eating more fiber, you automatically eat less of other stuff



Our gut has 500 million neurons surrounding it.  Same as the brain of an octopus (like the little cute one on the left).  Two-thirds of the octopus' neurons are also spread across its body.  They are highly intelligent animals and so is your gut.

The surface area of our gut is 15 times the surface of our skin which means there is a lot that can happen across this surface.

100 trillion microorganisms live in our gut.  That is the same number as there are stars in our galaxy.

These microorganisms have around 2 million genes, of which we have studied about a third.  In comparison, humans have about 25 000 genes.  

The total weight of these microorganisms is around 1-2 kg.  Even though there are as many microorganisms in our gut as there are humans cells in our body their weight is

less because, each is much smaller than a human cell.

The gut is linked to all other body systems and our brains and is the largest endocrine (hormonal) organ in the body.

It is, therefore, no surprise that our gut with its trillions of helpers has a fundamental effect on our health.

Fiber is the main nutrient for your gut microorganisms and if you don't feed the octopus it gets sick and so do you.



Recent important evidence

The book assembles much of the key evidence around the benefits of fiber.  In general, I quoted the most conservative studies or meta-analyses to ensure I do not overstate any claim.  Also, I tend to exclude studies where there are conflicts of interest (for example the study is funded by a company that benefits from its conclusions).  Below you find additional evidence from the past year that further supports the conclusions of the book.  The main topic of each study is underlined for ease of search.


Observational Studies

Observational studies are generally studies where humans are grouped by different intake of a certain nutrient.  What is good about observational studies is that it is as close to reality as you can get as it looks at natural behavior.  The disadvantage is that what you observe may be a correlation and not causation.  Thus it is important to check for concentration dependencies (i.e., more fiber means more of an effect) and confirm it in intervention studies (i.e., where you give people additional fiber), see below

This very comprehensive meta-analysis shows the impact of fiber on all-cause mortality, cardiovascular disease, and diabetes.  More importantly, they show what is called a dose-response relationship (i.e., the more fiber, the better the effect).  Increasing intake of fiber improves glycaemic control and risk factors for cardiovascular disease, e.g., cholesterol levels and body weight.  The study shows that increasing fiber intake from 19g to 35g per day reduces All-cause mortality by 35%.  It also investigated the impact of fiber on a range of cardiovascular parameters (glycated haemoglobin, fasting plasma glucose, insulin, total cholesterol, low-density lipoprotein (LDL), triglycerides, body weight, Body Mass Index, and C-reactive protein.  All of which showed improvements with higher fiber consumption.

Reynolds AN, Akerman AP, Mann J (2020) Dietary fiber and whole grains in diabetes management: Systematic review and meta-analyses. PLoS Med 17(3): e1003053.

This meta-analysis shows a strong correlation between higher dietary fiber consumption and lower risk of IBD (Inflammatory Bowel Disease).  More specifically they found higher consumption of fruit to reduce Ulcerative colitis and Crohn's disease by 31% and 44% respectively and an even higher impact from higher consumption of vegetables, 53%, and 48% respectively.

Milajerdi A, Ebrahimi-Daryani N, Dieleman LA, Larijani B, Esmaillzadeh A. Association of Dietary Fiber, Fruit, and Vegetable Consumption with Risk of Inflammatory Bowel Disease: A Systematic Review and Meta-Analysis. Adv Nutr. 2020 Nov 13:nmaa145. 

A more focused meta-analysis (over 800,000 participants) shows that individuals consuming 30 g of fiber per day have a 41% reduction in risk of diverticular disease compared to persons with a low fiber intake.

Aune D, Sen A, Norat T, Riboli E. (2020) Dietary fibre intake and the risk of diverticular disease: a systematic review and meta-analysis of prospective studies. Eur J Nutr.; 59(2):421-432.

This meta-analysis shows a strong correlation between higher dietary fiber consumption and lower risk of depression (by 24%).  This is a space to watch.  I have been cautious about promoting this correlation too strongly as the underlying mechanisms are not well understood and it may be the result of other factors than fiber (i.e., people who are happier for some reason eat more fiber or something else in fiber-containing foods acts as an anti-depressant).

Fatahi S, Matin SS, Sohouli MH, Găman MA, Raee P, Olang B, Kathirgamathamby V, Santos HO, Guimarães NS, Shidfar F. Association of dietary fiber and depression symptom: A systematic review and meta-analysis of observational studies. Complement Ther Med. 2020 Nov 18;56:102621.

This meta-analysis shows a strong correlation between higher dietary fiber consumption and lower risk of depression (by 10% in adults and by 57% in adolescents).  What is important in the findings is that the effect showed a dose-response relationship:  For every additional 5g of fiber, there was a reduction of 5% in the risk of depression.  Furthermore, the effect was most pronounced for vegetable and soluble fiber consumption.

Faezeh Saghafian, Maryam Hajishafiee, Parisa Rouhani & Parvane Saneei (2023) Dietary fiber intake, depression, and anxiety: a systematic review and meta-analysis of epidemiologic studies, Nutritional Neuroscience, 26:2, 108-126

A major meta-analysis shows a correlation between whole grain (higher fiber) intake and a range of digestive tract cancers (esophagus 46% reduction, colorectal 11% reduction, gastric 36% reduction).

Zhang, XF., Wang, XK., Tang, YJ. et al. Association of whole grains intake and the risk of digestive tract cancer: a systematic review and meta-analysis. Nutr J 19, 52 (2020).
See also: Oh, H., Kim, H., Lee, D., Lee, A., Giovannucci, E., Kang, S., & Keum, N. (2019). Different dietary fibre sources and risks of
colorectal cancer and adenoma: A dose–response meta-analysis of prospective studies. British Journal of Nutrition, 122(6), 605-615.

This recent meta-analysis shows that both soluble and insoluble fibre have similar protective impact on colorectal cancer (third most common cancer worldwide).

Arayici ME, Mert-Ozupek N, Yalcin F, Basbinar Y, Ellidokuz H. Soluble and Insoluble Dietary Fiber Consumption and Colorectal Cancer Risk: A Systematic Review and Meta-Analysis. Nutrition and Cancer. 2022;74(7):2412-2425

A meta-analysis on the impact of dietary fiber on both premenopausal and postmenopausal breast cancers (18% and 9% reduction).

Farvid, M.S., Spence, N.D., Holmes, M.D. and Barnett, J.B. (2020), Fiber consumption and breast cancer incidence: A systematic review and meta‐analysis of prospective studies. Cancer, 126: 3061-3075.  A second meta-analysis investigated the impact of fiber on women with breast cancer and found that the mortality was reduced by 28%.

Ahmad Jayedi, Alireza Emadi, Tauseef A. Khan, Anna Abdolshahi & Sakineh Shab-Bidar (2020) Dietary Fiber and Survival in Women with Breast Cancer: A Dose-Response Meta-Analysis of Prospective Cohort Studies, Nutrition and Cancer

A third meta-analysis shows that dietary fiber intake was negatively associated with breast cancer. In dose-response analysis, the risk of breast cancer showed a statistically significant linear trend with increasing dietary fiber dose: when adding 10 g per day, the risk decreased by 4.7%.

Xu K, Sun Q, Shi Z, et al. A Dose-Response Meta-Analysis of Dietary Fiber Intake and Breast Cancer Risk. Asia Pacific Journal of Public Health. 2022;34(4):331-337

This meta-analysis looked at the reduction of body weight, waist circumference, and BMI from higher consumption of viscous fiber.  Importantly, they looked at diets that were not restricted in any way, and yet they could detect moderate reduction.

Jovanovski E, Mazhar N, Komishon A, Khayyat R, Li D, Blanco Mejia S, Khan T, L Jenkins A, Smircic-Duvnjak L, L Sievenpiper J, Vuksan V. Can dietary viscous fiber affect body weight independently of an energy-restrictive diet? A systematic review and meta-analysis of randomized controlled trials. Am J Clin Nutr. 2020 Feb 1;111(2):471-485. 

There have been many meta-analyses on fiber content and reduction in all-cause and cardiovascular mortality.  All the important ones are listed in the book or more recent ones, above.  In the real world, most people eat their fiber from a variety of sources and together with a variety of other nutrients.  A recent study across 21 countries (almost 150 000 participants over 9.5 years) has tried to evaluate the extent of grain refinement (i.e., the more refined the grain, the less fiber it has) and all-cause mortality and cardiovascular mortality.  The results show that refined grains correlate with higher mortality.  Interestingly, rice consumption had no negative effect (and a range of other meta-analyses show on average a small positive effect).  One hypothesis is that the consumption of rice displaces other less healthy options and may be one contributor to why the Japanese have among the highest healthy life expectancies.

Swaminathan S., Dehghan M., Raj John M., Thomas T., Rangarajan S., Jenkins D. et al. Associations of cereal grains intake with cardiovascular disease and mortality across 21 countries in Prospective Urban and Rural Epidemiology study: prospective cohort study BMJ 2021; 372 :m4948

Intervention studies

Many studies look at the overall intake of fiber through the diet, but the effect observed may theoretically be due to something else in the diet or the profile of the participants that is the ultimate cause of the positive impact.  One way to resolve this is to look at intervention studies.  This is roughly the same way new drugs are tested.  You have two groups of individuals: One group that receives additional fiber and one group that does not (they receive something neutral).  The studies then look at the health outcomes for the two groups.  If the intervention studies show similar results as the observational studies, then the case is quite strong that fiber is the causative agent for healthy longevity.  In short, fiber is!

Also, note the absence of studies on cancer.  Generally, intervention studies with cancer are more difficult as patients are obviously already receiving various therapies that may confuse the results.  Here the observational studies are probably a more appropriate approach and they demonstrate a significant preventative impact but not necessarily a therapeutic impact (i.e., once the disease has taken hold). 

This umbrella review (across many meta-analyses) looks at all key parameters affecting cardiovascular disease (CVD) and the positive impact from fiber intake.  It combines observational and intervention studies.

McRae M. P. (2017). Dietary Fiber Is Beneficial for the Prevention of Cardiovascular Disease: An Umbrella Review of Meta-analyses. Journal of chiropractic medicine, 16(4), 289–299.

This meta-analysis investigates the impact of fiber supplementation on blood pressure.  The results show fiber supplementation (average dose, 11.5 grams per day) changed systolic BP by –1.13 mm Hg and diastolic BP by –1.26 mm Hg.  Interestingly, reductions in blood pressure were larger in older (>40 years) and in hypertensive populations than in younger individuals.

Streppel MT, Arends LR, van ’t Veer P, Grobbee DE, Geleijnse JM. (2005) Dietary Fiber and Blood Pressure: A Meta-analysis of Randomized Placebo-Controlled Trials. Arch Intern Med.;165(2):150–156.  See also two more recent meta analyses:   Evans CE, Greenwood DC, Threapleton DE. (2015) Effects of dietary fibre type on blood pressure: a systematic review and meta-analysis of randomized controlled trials of healthy individuals. J Hypertens;33(5):897–911.  And: Khan K, Jovanovski E, Ho HVT, Marques ACR, Zurbau A, Mejia SB, Sievenpiper JL, Vuksan V. (2018);The effect of viscous soluble fiber on blood pressure: A systematic review and meta-analysis of randomized controlled trials. Nutr Metab Cardiovasc Dis. ;28(1):3-13.

This meta-analysis demonstrated the impact of soluble fiber supplementation on blood lipids.  For each gram increase in dietary fiber, the concentration of blood low-density lipoprotein cholesterol was lowered by about 2 mg/dL (about 0.052 mmol/L).  Interestingly, they did not find a difference between the different fibers used in the different studies.

Brown L, Rosner B, Willett WW, Sacks FM. Cholesterol-lowering effects of dietary fiber: a meta-analysis. Am J Clin Nutr. 1999 Jan;69(1):30-42.

See also a more recent meta analysis:  Hartley, L., May, M. D., Loveman, E., Colquitt, J. L., & Rees, K. (2016). Dietary fibre for the primary prevention of cardiovascular disease. The Cochrane database of systematic reviews, 2016(1), CD011472. 

This analysis looked at the impact of supplementation with soluble fiber on BMI, body weight, body fat, fasting glucose, and insulin.  The interventions ranged between 2 weeks and 17 weeks.  The results show a reduction of BMI by 0.84, body weight by 2.52 kg, body fat by 0.41%, fasting glucose by 0.17 mmol/L, and fasting insulin by 15.88 pmol/L.  

Sharon V Thompson, Bridget A Hannon, Ruopeng An, Hannah D Holscher, Effects of isolated soluble fiber supplementation on body weight, glycemia, and insulinemia in adults with overweight and obesity: a systematic review and meta-analysis of randomized controlled trials, The American Journal of Clinical Nutrition, Volume 106, Issue 6, December 2017, Pages 1514–1528.

This meta-analysis demonstrated a significant positive impact of soluble fiber on Irritable Bowel Syndrom (IBS). There was a 14% risk reduction for those with soluble fiber supplementation.  No statistically significant effect was seen for bran (10% risk reduction, but with a very wide confidence interval).

Moayyedi, Paul et al. (2014). The Effect of Fiber Supplementation on Irritable Bowel Syndrome: A Systematic Review and Meta-analysis, American Journal of Gastroenterology; Volume 109 - Issue 9 - p 1367-1374

This analysis investigated the impact of fiber supplementation on patients with Chronic Kidney Disease.  It shows that fiber supplementation can significantly reduce the levels of uremic toxins in patients, especially for patients on dialysis and without diabetes.  The meta-analysis shows a significant reduction in indoxyl sulfate, p-cresyl sulfate, blood urea nitrogen, and uric acid.
Yang HL, Feng P, Xu Y, Hou YY, Ojo O, Wang XH. The Role of Dietary Fiber Supplementation in Regulating Uremic Toxins in Patients With Chronic Kidney Disease: A Meta-Analysis of Randomized Controlled Trials. J Ren Nutr. 2021 Sep;31(5):438-447.

The focus of this meta-analysis was on the impact of fiber supplementation on chronic constipation.  It shows that around 10g/day and a treatment duration of 4 weeks is optimal.  The study looked at psyllium and pectin.

Alice van der SchootCandice DrysdaleKevin WhelanEirini Dimidi,  The Effect of Fiber Supplementation on Chronic Constipation in Adults: An Updated Systematic Review and Meta-Analysis of Randomized Controlled Trials, Am J Clin Nutr, 2022 Oct 6;116(4):953-969

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