The American Fiber Gap, in Numbers

Most nutrition gaps in the United States are arguments. The fiber gap is arithmetic.

The Adequate Intake for dietary fiber is 14 grams per 1,000 kilocalories, which works out to 25 grams a day for adult women and 38 grams a day for adult men. The figure was set on the basis of research relating fiber intake to coronary heart disease risk. Against that target, mean intake in the United States is about 17 grams a day, and only about 5 percent of the population meets the Adequate Intake.¹

That is the whole gap in two sentences. The rest of this piece is about how firm those numbers are, what higher intakes have been associated with, and where the evidence stops.

How the intake figure was measured

The 17 gram figure is not a single reading. An analysis of National Health and Nutrition Examination Survey data covering adults aged 18 and older found mean daily fiber intake of 15.6 grams in 1999 to 2000, 16.1 grams in 2001 to 2002, 15.5 grams in 2003 to 2004, 15.8 grams in 2005 to 2006, and 15.9 grams in 2007 to 2008.²

Ten years, five survey cycles, and a flat line. The authors concluded that daily fiber intake generally had not progressed toward national goals over that decade.²

The same analysis found the shortfall was not evenly distributed. Adults with a body mass index of 30 or above consistently reported lower intake than adults of normal weight or overweight, at 14.6 to 15.4 grams a day against 15.6 to 16.8. Non-Hispanic Black participants reported 12.5 grams a day at baseline, rising modestly to 13.1 grams by 2007 to 2008. Mexican American participants reported significantly higher intake than non-Hispanic White participants in 1999 to 2000, at 18.0 against 16.1 grams, but that intake did not increase over the decade.²

The gap people do not know they have

A summit convened in Washington, DC in January 2014 gathered researchers, educators, and communicators specifically to work on the fiber consumption gap. The proceedings note something that complicates every messaging effort aimed at it: consumer research indicates the public is aware of the benefits of fiber and most people believe they already consume enough.³

The same document identifies the specific misperceptions that keep the gap open. Commonly held beliefs include the assumption that all whole-grain foods are good sources of fiber, and that foods with fiber are expensive, unpalatable, and complicated to prepare. Inadequate intake has been described as a public health concern.³

That is a different problem from ignorance. A person who believes they already meet a target has no reason to change anything.

What higher intake has been associated with

The largest synthesis on this question pooled just under 135 million person-years of data from 185 prospective studies and 58 clinical trials with 4,635 adult participants.⁴

In the observational data, comparing the highest consumers of dietary fiber with the lowest was associated with a 15 to 30 percent lower rate of all-cause and cardiovascular related mortality, and of incidence of coronary heart disease, stroke incidence and mortality, type 2 diabetes, and colorectal cancer. In the clinical trials, higher fiber intakes were associated with significantly lower body weight, systolic blood pressure, and total cholesterol than lower intakes.⁴

The dose detail is the useful part for anyone setting a personal target. Risk reduction across a range of critical outcomes was greatest when daily fiber intake fell between 25 and 29 grams, and the dose-response curves suggested that higher intakes could confer further benefit.⁴ That range brackets the Adequate Intake rather than exceeding it.¹

The authors graded the certainty of evidence for the relationship between dietary fiber and critical outcomes as moderate, and were explicit that their findings apply to risk in the population at large rather than to people who already have chronic disease.⁴ Moderate certainty from observational data is a real finding and not a demonstration of cause.

Fiber is not one substance

The population evidence is about total intake. Symptom evidence is not, and this is where general advice to “eat more fiber” starts to break down.

A systematic review and meta-analysis of 14 randomized controlled trials involving 906 patients with irritable bowel syndrome found an overall benefit of fiber supplementation, with a relative risk of remaining symptomatic of 0.86. Expressed the other way, 10 patients would need to receive fiber for one additional patient to stop being symptomatic. When the trials were separated by fiber type, the benefit was seen only in trials of soluble fiber, at a relative risk of 0.83 and a corresponding figure of 7. Trials of bran showed no significant effect, at a relative risk of 0.90 with a confidence interval crossing 1.⁵

The same analysis found no evidence of harm from bran, despite speculation from uncontrolled data that there might be.⁵ The honest reading is that soluble and insoluble fiber behaved differently in these trials, and that pooling them obscures which one did the work.

Whole foods, in a US trial

A partially randomized comparative effectiveness trial at a US medical center enrolled adults with chronic constipation, defined as three or fewer complete spontaneous bowel movements per week, and assigned them to green kiwifruit at two per day, prunes at 100 grams per day, or psyllium at 12 grams per day, for four weeks. Seventy-nine patients were randomized and complete data were available for 75.⁶

On the primary endpoint, the proportion of complete spontaneous bowel movement responders was similar across the three groups. Comparing intervention weeks three and four with baseline, weekly complete spontaneous bowel movement rate increased significantly in all three groups. Stool consistency improved significantly with kiwifruit and prunes; straining improved significantly with all three. Adverse events were most common with psyllium and least common with kiwifruit, and fewer patients reported dissatisfaction with kiwifruit than with prunes or psyllium.⁶

That is a small exploratory trial and the authors labelled it as such. It is worth citing anyway, because it is one of the few US comparisons in which a whole food, a dried fruit, and a supplement were run against each other rather than each against nothing.

The practical read

The gap is large, it is stable across a decade of survey data, and most people who have it do not think they do.²³ The intake band associated with the greatest risk reduction in the pooled evidence, 25 to 29 grams a day, sits close to the Adequate Intake rather than far above it, which makes the target a normal amount of food rather than an extreme one.¹⁴

For digestive symptoms specifically, the trial evidence separates by fiber type rather than aggregating, and soluble fiber and bran did not perform the same way.⁵ Anyone with ongoing bowel symptoms is better served raising them with a clinician than titrating fiber alone.