Get more fiber

TL;DR. Fiber is the part of plant food your enzymes can't break down. It reaches your large intestine intact, where your gut microbes ferment it into short-chain fatty acids that feed your colon and calm inflammation. It slows blood sugar, lowers LDL cholesterol, keeps you full, and tracks with lower rates of heart disease and colorectal cancer in long-term cohorts. The average American eats about 15 grams a day. The target is 25 to 35. You don't need a powder or a bar. A bowl of beans is the answer.

Why this is the goal most people skip

Three out of four U.S. adults miss the fiber target, and they miss it by half. The gap shows up downstream as blood sugar spikes, stubborn LDL, sluggish digestion, and a starved microbiome (see Your microbiome and you).

No one sells fiber. There's no patent on a black bean. The cheapest, most boring foods on the shelf, beans and lentils and oats, do most of the work.

What fiber actually is

Fiber is plant carbohydrate your small intestine can't digest. It comes in three flavors, and a working diet hits all three:

• Soluble fiber dissolves in water and turns gel-like. It slows how fast sugar enters your blood, and it grabs bile acids on its way through the gut. Your liver has to pull cholesterol out of circulation to make more bile, which is how soluble fiber lowers LDL. Sources: oats, beans, lentils, apples, pears, psyllium, barley.
• Insoluble fiber doesn't dissolve. It bulks up stool and keeps things moving. Sources: wheat bran, whole-grain skins, leafy greens, nuts, seeds, the strings in celery.
• Resistant starch is starch that resists digestion in the small intestine and reaches the colon intact, where microbes ferment it. Sources: cooked-then-cooled potatoes and rice, green bananas, well-cooked beans, oats.

All three end up as food for gut microbes. The waste product is short-chain fatty acids (SCFAs), mostly butyrate, acetate, and propionate. Butyrate is the main fuel for the cells lining your colon. SCFAs calm inflammation, strengthen the gut barrier, and influence appetite and insulin sensitivity. Without fiber, the microbes that should be making butyrate start eating the mucus layer of your gut wall instead. That is the literal finding.

How much, in plain numbers

The official Adequate Intake is 14 grams of fiber per 1,000 calories: around 25 grams a day for women, 38 for men. Most people land at 15. No other nutrient gets missed by this much.

You don't need to count. Look at what's on the plate. If a meal has no beans, no whole grain you can see, no fruit with skin, and no vegetable bulk, fiber is missing. Add one.

The carb-to-fiber rule

Reading a label takes time. Robert Lustig (Metabolical, Ch. 28) offers a fast filter: divide total carbs by fiber. If the ratio is 10 to 1 or lower, the food still has its plant structure. If the ratio is higher, milling stripped the fiber out. Whole wheat berries: about 5:1. Steel-cut oats: about 8:1. Brown rice: about 20:1 (yes, brown rice is borderline). White bread: 30:1 or worse. Sugary cereal: 60:1.

This rule isn't perfect. It rewards added-fiber products that pump in chicory root or inulin to game the number. Pair it with the soft rule below.

The soft rule, in one line

Less than 1 gram of fiber per serving in a carb-bearing food is a refined-grain flag. White bread, instant rice, most crackers, most breakfast cereals, most pretzels. The fiber was milled out. What's left is fast sugar.

The bonus, also in one line: 3 grams of fiber per serving or more is a win. Intact whole grains (you can see the kernel) earn another point.

What the long-term evidence actually shows

Two strands of evidence carry the most weight here.

Cardiovascular disease. The Nurses' Health Study and the Health Professionals Follow-up Study (Willett, Eat, Drink, and Be Healthy, Ch. 6) tracked hundreds of thousands of people for decades. Higher whole-grain and fiber intake tracked with lower rates of heart disease, in a clean dose-response way that survived adjustment for smoking, exercise, and overall diet quality. The 2019 Reynolds Lancet meta-analysis (185 prospective studies, 58 trials) found a 15 to 30 percent lower rate of all-cause and cardiovascular death in the high-fiber groups. Mechanism: soluble fiber lowering LDL through bile-acid sequestration, slower glucose absorption, and the SCFA story.

Colorectal cancer. This is the rare nutrition claim with RCT-grade evidence behind cohort data. The pooled evidence (Aune et al. 2016 BMJ) shows about 10 percent lower colorectal cancer risk per 10 grams of fiber per day, with whole grains carrying most of the signal. Bulkier, faster transit means less contact time with carcinogens, and butyrate appears to suppress tumor growth in colon cells.

The claim is modest. People who hit the fiber target get fewer of the diseases we're trying to prevent, and the mechanism holds up at every step. No one has shown that fiber stops anyone from getting sick.

What to eat (worked examples)

One or two foods do most of the work. Don't fetishize.

• Black beans, 1 cup cooked: 15 g fiber. Half the daily target in one bowl. About 30 cents.
• Lentils, 1 cup cooked: 16 g fiber. Cook in 20 minutes, no soak.
• Steel-cut oats, ½ cup dry: 8 g fiber. Rolled oats: 4 g. Instant oats: 3 g and a sugar spike. The matrix matters. Steel-cut keeps the grain whole, so it digests slowly.
• Frozen broccoli, 1 cup: 5 g fiber. Frozen is fine. Often fresher than "fresh."
• Apple with skin: 4 g fiber. Apple peeled: 2 g. The skin is the point.
• Brown rice, 1 cup cooked: 3 g fiber. Better than white, but the ratio is still borderline. Mix with lentils or beans.
• White bread, 1 slice: 1 g fiber. Refined-grain flag.
• Lara Bar: 4 to 5 g fiber from real dates and nuts. A reasonable bar.
• Quest Bar: lists 14 g "fiber," but most is isolated chicory root or soluble corn fiber added to a low-carb formulation. It is not the same food as a bowl of beans. It can cause gas and doesn't carry the polyphenols or food matrix. Treat it as backup.

A bowl of beans does most of the work. Fiber supplements (psyllium, inulin, partially hydrolyzed guar gum) have real data for specific uses (constipation, IBS, LDL), but they don't replace the variety of fibers you get from eating plants. They run a distant second. Use them when food won't reach.

What good enough looks like

• A serving of beans or lentils most days.
• One intact whole grain a day: oats, barley, farro, brown rice, whole-grain bread where you can see the kernel.
• A fruit with the skin on, and a vegetable at most meals.
• A 30-second label check on carb-bearing foods: less than 1 g fiber per serving means put it back.

That gets most people from 15 to 30 g without a tracker, a powder, or a brand.