Fiber, total dietary

Dietary fiber is the part of plant carbohydrate the body cannot break down into sugar, so it passes through the digestive system largely intact. It comes in soluble and insoluble forms, found in foods such as oats, lentils, whole grains, and fruit, and most adults fall well short of the 25 to 30 grams a day that health authorities recommend.

What Is Dietary Fiber

Per Harvard T.H. Chan School of Public Health, The Nutrition Source, fiber "cannot be broken down into sugar molecules, and instead it passes through the body undigested," helping regulate the body's use of sugar and supporting satiety and blood glucose control. There are two main types: soluble fiber, which dissolves in water and can "help lower glucose levels as well as help lower blood cholesterol" — found in oatmeal, chia seeds, beans, lentils, and apples — and insoluble fiber, which does not dissolve and instead helps move food through the digestive system, found in whole wheat products, brown rice, legumes, and leafy greens like spinach. Per MedlinePlus, "most Americans don't eat enough dietary fiber," despite it being widely available in oats, nuts, seeds, fruits, and vegetables.

Health Benefits of Dietary Fiber

Per Harvard's Nutrition Source, higher fiber intake is associated with a range of documented benefits:

• Cholesterol and heart disease: A meta-analysis found "a modest benefit of dietary fiber in lowering total and LDL cholesterol," and high fiber intake is associated with reduced cardiovascular disease risk.
• Blood sugar and type 2 diabetes: Research summarized by Harvard's Nutrition Source shows "diets low in fiber, especially insoluble types, may increase the risk of type 2 diabetes," with whole-grain fiber showing the strongest protective association.
• Colorectal cancer: Meta-analyses reviewed by Harvard's Nutrition Source found fiber from fruits, vegetables, and legumes associated with lower risk, with cereal fiber showing the strongest association.
• Diverticular disease: A large study cited by Harvard's Nutrition Source found women consuming 25 or more grams of fiber daily had a 13% lower risk of diverticulitis compared with those consuming less than 18 grams.
• Breast cancer: The same source reports a 25% reduced risk of breast cancer among premenopausal women comparing highest to lowest fiber intake.
• Digestive regularity: Per MedlinePlus, fiber "adds bulk to your diet and makes you feel full faster, helping you control your weight," while supporting regularity and helping prevent constipation.

How Much Fiber You Need

No nutrient_rda row currently exists in FoodData Explorer's database for Fiber, total dietary (nutrient_id 1079). Per Cleveland Clinic, "experts recommend that adults consume 25 to 30 grams (g) of fiber every day." Harvard's Nutrition Source similarly states that "children and adults need at least 25 to 35 grams of fiber per day for good health, but most Americans get only about 15 grams a day." No Tolerable Upper Intake Level has been established for dietary fiber from food in the sources reviewed for this draft; the practical caution at high intakes is digestive discomfort, covered below, rather than toxicity.

Signs of Deficiency and Excess

Consistently low fiber intake is the norm rather than the exception in the United States, per both Harvard's Nutrition Source and MedlinePlus, and is linked to the elevated cardiovascular, diabetes, and colorectal cancer risks described above rather than to an acute deficiency syndrome. Because fiber's effects on the body are gradual and probabilistic rather than the sharp deficiency signs typical of a vitamin, no allowlist source reviewed for this draft describes a distinct "fiber deficiency" symptom picture; low intake is instead framed in terms of the disease-risk associations above.

Excess intake, on the other hand, has a well-documented near-term effect. Per MedlinePlus, "increasing dietary fiber too quickly can lead to gas, bloating, and cramps," and the recommended approach is to add fiber gradually rather than all at once, giving the digestive system's natural bacteria time to adjust.

Warnings & Interactions

MedlinePlus's core practical warning is about the pace of increasing fiber intake, not a fixed upper limit: adding fiber suddenly and in large amounts is what causes gas, bloating, and cramping, and this discomfort typically resolves as intake is increased gradually instead. No specific drug interactions with dietary fiber from whole foods were identified in the sources reviewed for this draft; readers taking a fiber supplement rather than eating fiber from food should be aware that some supplement forms can affect the timing of medication absorption, but that is a supplement-specific consideration outside the scope of the dietary fiber values this database reports.

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