Fiber: Is it everything we thought it was?

What Exactly Is Fiber?

First, fiber itself is a carbohydrate. The term was first coined in 1953 and later evolved by the cereal industry. The current scientific definition is:

“Carbohydrate polymers with ten or more monomer units that are not hydrolyzed by endogenous enzymes in the human small intestine.”1

In simple terms, fiber is a carbohydrate we cannot digest with our own enzymes. Instead, it either passes through the body unchanged or is broken down by our gut bacteria.

Oatmeal is considered a good source of fiber but it is also a processed food.

Beans are a souce of soluable fiber.

Wheat is a souce of insoluable fiber but it is also a processed food.

Types of Fiber

There are two main types of fiber: soluble and insoluble.

Soluble Fiber

• Dissolves in water and may be digested by gut bacteria.

• Components include pectins, gums, inulin-type fructans, and mucilages, which are completely fermented by gut flora.

• Because of this fermentation, soluble fibers tend to produce more gas.

• Examples: beans, legumes, modified citrus pectin, vegetable fibers, psyllium (Metamucil).

Insoluble Fiber

• Passes through the body largely unchanged.

• Components include cellulose, waxes, beta-glucans, and lignins, found primarily in plant cell walls and only slightly fermented.

• Examples:

  • Wheat: ~90% insoluble, 10% soluble

  • Oats: ~50% insoluble, 50% soluble

  • Psyllium: ~10% insoluble, 90% soluble

  • Cellulose (a classic insoluble fiber)

Fiber and Blood Glucose Levels

Historically, foods high in dietary fiber have been recommended for their touted health benefits. One of these supposed benefits is their ability to “counteract” carbohydrates. For example, when reading a food label that lists 30 g of carbohydrates and 15 g of fiber, the common math has been to subtract the fiber from the carbs—leaving 15 g of “net carbs” thought to be recognized by the body.

This math has given many people confidence to eat higher amounts of carbohydrates, believing they were only “really” consuming the difference. But it’s not exactly true. While fiber does have benefits, much of its value comes from slowing the effect of carbohydrates on blood sugar. This means that much of the benefit occurs if a person is eating a high-carb diet. This means that the sugars and carbs consumed alongside fiber are absorbed more gradually, producing a slower and more prolonged rise in blood glucose, rather than a sharp spike.

Scientific evidence supports this effect, but it would be cavalier to use fiber to justify high-carb intake. In 2012, a large meta-analysis of 35 randomized controlled trials, conducted over three decades across three continents, demonstrated that individuals with type 2 diabetes who supplemented with fiber saw an average reduction in HbA1c of 0.97% and a drop in fasting blood glucose of 37 mg/dL.2

Fiber and Cholesterol

Many studies over the years have shown that dietary fiber can lower LDL cholesterol and total cholesterol. Unfortunately, fiber does not have significant effects on triglycerides or HDL cholesterol—but importantly, it also does not make these numbers worse.

A large 1999 meta-analysis of 67 controlled trials found that 2–10 g/day of fiber produced a small but statistically significant reduction in total cholesterol and LDL.3 However, 2–10 g/day is considered a relatively low intake.

Many ultra-processed foods piggyback off the touted LDL-lowering benefits of fiber. Do the models in the photo appear healthy, even for their ages?

A 2014 meta-analysis examined studies using ≥3 g/day of oat β-glucan as the fiber source. Across 28 randomized controlled trials, LDL cholesterol decreased by approximately 10 mg/dL.4 For example, an LDL level of 150 mg/dL could be reduced to 140 mg/dL. As in prior analyses, no effects were observed on HDL cholesterol or triglyceride levels.

Interestingly, this meta-analysis confirmed LDL lowering but found it to be greater than previously recognized.4 The study also noted that the effect was not dose-dependent—meaning that consuming three grams per day produced the same benefit as higher amounts.4 Effects on total cholesterol and LDL were also more pronounced in people with type 2 diabetes or those with higher baseline LDL levels.4

Fiber and GI Symptoms

Fiber plays several roles in the gut. It can slow transit time in the small bowel, increase stool bulk, hold water to soften stools, form gels, bind minerals and organic substances, stimulate bacterial growth, and be metabolized into short-chain fatty acids (SCFAs). Fiber is also associated with potential anti-inflammatory activity.

When it comes to inflammatory bowel disease (IBD)—including Crohn’s disease and ulcerative colitis—the evidence on dietary fiber intake is mixed. One observational study found that a high-fiber diet (≥24 g/day) lowered the odds of developing Crohn’s disease by 40%, but had no effect on ulcerative colitis.5 Because this was an observational study, the association could be influenced by confounding factors or bias. Remember, observational dietary studies are among the weakest types of studies and are often subject to bias. Clinically, many physicians recommend a low-fiber diet during an acute Crohn’s flare, when intestinal stenosis is present, if small intestinal bacterial overgrowth (SIBO) is suspected, or as a low-residue diet following surgery.

It’s also important to recognize that high fiber intake can have drawbacks. For some, it may lead to constipation, intestinal blockage, bloating, or diarrhea. In cases of constipation, fiber acts like a double-edged sword: on one hand, it can make stools bulkier and softer, promoting easier passage; but on the other, if insufficient water is consumed, the added bulk can actually worsen constipation instead of relieving it.

"A small study of 63 participants with idiopathic constipation, who averaged one bowel movement every 3.75 days, were instructed to follow a zero-fiber diet for two weeks and then adjust their fiber intake to a personally acceptable level. Of the group, 41 chose to remain on a no-fiber diet, 16 reintroduced fiber but at a lower amount than they had previously consumed, and 6 returned to a high-fiber diet.

The study found that when all fiber intake was stopped, bowel frequency improved to one movement per day. In contrast, all 6 participants who resumed a high-fiber diet experienced constipation, straining, and bloating.

Participants who eliminated fiber reported 0% bloating (0 of 41). By comparison, among those who reduced—but did not eliminate—fiber, 31.3% (5 of 16) continued to experience bloating.6 Furthermore, all participants who eliminated fiber reported complete resolution of abdominal pain (0 of 41), whereas those who continued fiber—at either high or low levels—had no improvement in pain."6

The n value represents the number of participants following a particular dietary fiber intake. The whole numbers in the columns represent the number of participants with a symptom. Table from Kok-Sun Ho, et al.

Fiber... a lot to think about.

At present, there is no universally accepted standard for dietary fiber intake. The U.S. Preventive Services Task Force (USPSTF) does not provide a specific recommendation for fiber, offering only broad dietary guidance instead. Various organizations set their own targets—for example, the American Heart Association (AHA) recommends 25–30 grams of dietary fiber per day.

Interestingly, a quick review of the AHA website’s recipe section highlights a potential disconnect. For instance, choosing one one-dish meal (Pollo con Repollo Chino y Fideos de Arroz) and one dessert (Peach Bread Pudding with Bourbon Sauce) and calculating intake based on three servings of the meal plus one dessert, the daily total would be only 13 grams of fiber, yet a staggering 137 grams of total carbohydrates in a day.

Adding to the complexity, many widely promoted sources of fiber come from grains and cereals—industries heavily influenced by “big food.” This reliance risks skewing public perception of what truly constitutes beneficial fiber.

In light of studies showing significant resolution of gastrointestinal symptoms when fiber is removed, it is worth questioning whether we need to rethink fiber intake, which is often promoted as a one-size-fits-all solution for general health. Could some of its supposed benefits be overstated?

References

1. Codex Alimentarius, Guidelines on Nutrition Labelling CAC/GL 2-1985 as Last Amended 2010. Joint FAO/WHO Food Standards Programme, Secretariat of the Codex Alimentarius Commission. Rome: FAO, 2010. [(accessed on 24 September 2022)

2. Gibb RD, McRorie JW Jr, Russell DA, Hasselblad V, D'Alessio DA. Psyllium fiber improves glycemic control proportional to loss of glycemic control: a meta-analysis of data in euglycemic subjects, patients at risk of type 2 diabetes mellitus, and patients being treated for type 2 diabetes mellitus. Am J Clin Nutr. 2015 Dec;102(6):1604-14. doi: 10.3945/ajcn.115.106989. Epub 2015 Nov 11. PMID: 26561625.

   
   

3. 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. doi: 10.1093/ajcn/69.1.30. PMID: 9925120.

   
   

4. Whitehead A, Beck EJ, Tosh S, Wolever TM. Cholesterol-lowering effects of oat β-glucan: a meta-analysis of randomized controlled trials. Am J Clin Nutr. 2014 Dec;100(6):1413-21. doi: 10.3945/ajcn.114.086108. Epub 2014 Oct 15. PMID: 25411276; PMCID: PMC5394769.

   
   

5. Reduced cronhs: Ananthakrishnan A.N., Khalili H., Konijeti G.G., Higuchi L.M., de Silva P., Korzenik J.R., Fuchs C.S., Willett W.C., Rich-ter J.M., Chan A.T. A prospective study of long-term intake of dietary fiber and risk of Crohn’s disease and ulcerative colitis. Gastroenterology. 2013;145:970–977. doi: 10.1053/j.gastro.2013.07.050.

   
   

6. Ho, S., Mei Tan, C. Y., Mohd Daud, M. A., & Seow-Choen, F. (2012). Stopping or reducing dietary fiber intake reduces constipation and its associated symptoms. World Journal of Gastroenterology : WJG, 18(33), 4593. https://doi.org/10.3748/wjg.v18.i33.4593