Carbohydrates

Carbohydrates supply most of the world's dietary energy, and they do it at a steady 4 kilocalories per gram — the same as protein, and well under fat's 9. But "carbohydrate" is not one thing: it covers the single sugar molecules in fruit, the starch in a potato, and the fiber in a bean that your own enzymes cannot touch at all. Those forms behave very differently once they reach the gut, and the differences explain why two foods with identical carbohydrate grams can have very different effects on digestion, blood glucose, and long-term health. This chapter follows carbohydrate from its chemical structure through digestion and absorption into the bloodstream, then out to the evidence on how much we need and what the research actually supports. Along the way, resist the urge to sort foods into "good" and "bad" — the physiology is far more interesting than the moralizing.

Section 1Types of carbohydrates

One building block, many arrangements

Every dietary carbohydrate is built from small sugar units, and the name is a fossil of nineteenth-century chemistry: carbo (carbon) plus hydrate (water), because these molecules carry carbon, hydrogen, and oxygen in roughly a 1:2:1 ratio. What separates a spoonful of table sugar from a slice of bread is not the atoms but the architecture — how many sugar units are strung together and how the links between them are angled. That architecture decides whether your digestive enzymes can take the molecule apart, and that single fact drives most of this chapter.

Carbohydrates that your small intestine can break down and absorb yield 4 kilocalories per gram. Hold that number next to the others: protein also yields 4 kcal/g, fat yields 9 kcal/g, and alcohol yields 7 kcal/g. Carbohydrate is not an unusually dense fuel; it is simply the most abundant one in most food supplies.

Monosaccharides and disaccharides — the sugars

A monosaccharide is a single sugar unit and cannot be broken down further by digestion. Three matter nutritionally. Glucose is the body's reference fuel, the sugar that circulates in blood and the one the brain relies on most heavily. Fructose, found in fruit and honey, is the sweetest of the common sugars and is handled largely by the liver. Galactose rarely appears free in food; it arrives bound up in milk sugar.

A disaccharide is two monosaccharides joined together. Sucrose (glucose + fructose) is table sugar, and chemically it is identical whether it came from cane, beet, or a fruit. Lactose (glucose + galactose) is the sugar of milk. Maltose (glucose + glucose) appears mostly as an intermediate when starch is being digested or grain is being malted. Notice that all six of these — three monosaccharides and three disaccharides — are what everyday language calls "sugars," and every one of them ends up as monosaccharides before it can cross the intestinal wall.

Polysaccharides — starch, glycogen, and fiber

String hundreds or thousands of glucose units together and you get a polysaccharide. Starch is the plant storage form and comes in two shapes: amylose, a long straight chain, and amylopectin, a branched chain. Branching matters, because enzymes attack chain ends, and a branched molecule offers many more ends at once — which is part of why amylopectin-rich foods digest faster than amylose-rich ones. Glycogen is the animal storage form, even more highly branched, and it is stored in your liver (roughly 100 g) and muscle (roughly 400 g); it is not a meaningful source of carbohydrate in food, because it degrades quickly after slaughter.

Fiber is the third group, and it is defined by what your body cannot do. Cellulose is also a straight chain of glucose — the same monomer as starch — but the bonds joining the units are oriented differently (beta rather than alpha linkages), and human digestive enzymes have no key for that lock. The rule is specific, not blanket: brush-border lactase is a beta-galactosidase and cleaves the beta-1,4 bond of lactose perfectly well. What no human enzyme can cut is the beta-1,4 bond of cellulose, so cellulose passes into the colon intact. That one geometric difference is the entire reason lettuce and bread behave so unlike each other.

Section 2Fiber

Two behaviors: soluble and insoluble

Dietary fiber is carbohydrate (plus the non-carbohydrate polymer lignin) that resists digestion in the small intestine. Nutrition scientists sort it by how it behaves in water, and while the categories overlap in real food, they predict real physiological differences.

Soluble fiber dissolves or disperses in water and often forms a gel. Pectin in apples and citrus, beta-glucan in oats and barley, gums, inulin, and psyllium all belong here. The category is not uniform, though, and the split that matters physiologically is viscosity: beta-glucan, pectin, guar gum, and psyllium thicken, while inulin dissolves without gelling. Where a gel does form, it slows gastric emptying and slows the rate at which glucose is absorbed, which blunts the rise in blood glucose after a meal. Viscous soluble fibers also bind bile acids and carry them out in the stool, forcing the liver to pull cholesterol from circulation to make replacements — the mechanism behind the well-replicated finding that oat beta-glucan and psyllium modestly lower LDL cholesterol. That specific effect is strong evidence, reproduced across many controlled trials.

Insoluble fiber does not dissolve. Cellulose, most hemicelluloses, and lignin are the classic examples, and they concentrate in the bran layers of grains, in vegetable stalks, and in seeds and skins. Insoluble fiber works mechanically: it holds water, adds bulk to stool, and speeds transit through the colon. Low total fiber intake is commonly associated with harder, slower-moving stool. Worth noting, though, that the trial evidence for relieving established constipation is actually strongest for viscous soluble fiber such as psyllium; the neat textbook pairing of "insoluble fiber equals regularity" describes the mechanism better than it describes the outcome data.

What the colon does with it

Fiber is not inert once it reaches the large intestine. Gut bacteria ferment much of the soluble and some of the insoluble fraction, producing gas and short-chain fatty acids (SCFAs) — chiefly acetate, propionate, and butyrate. Butyrate is the preferred fuel of the cells lining the colon, and the SCFAs collectively lower colonic pH and feed the resident microbial community. Because some of this fermentation energy is recovered, fiber is not truly calorie-free; regulatory bodies assign fermentable fiber roughly 2 kcal/g, well below the 4 of digestible carbohydrate.

Resistant starch deserves a mention here, because it is starch that behaves like fiber. Starch trapped inside intact grains and legumes, the high-amylose starch of green bananas, and the retrograded starch that forms when cooked potatoes, rice, or pasta are cooled all escape small-intestinal digestion and are fermented in the colon instead.

How much, and who needs how much

Fiber has an Adequate Intake (AI) rather than an RDA, because the evidence was judged sufficient to set a target but not to define a precise requirement. The AI is anchored at 14 g of total fiber per 1,000 kcal, a level associated in observational studies with lower coronary heart disease risk. Because it scales with energy intake, the gram target differs by age and sex, and it rises in pregnancy and lactation.

Actual intake falls far short of these figures — adults in the United States average roughly 15 g/day, well under half the target for younger men. There is no Tolerable Upper Intake Level for fiber; very high intakes cause bloating, gas, and discomfort rather than toxicity, though extremely high intakes can modestly reduce absorption of some minerals.

Section 3Digestion and absorption

Mouth and stomach: a short start, then a pause

Digestion begins before you swallow. Chewing breaks food apart mechanically, and saliva delivers salivary amylase, which starts cleaving the alpha bonds of starch into shorter chains called dextrins and into maltose. This is why a plain cracker held in the mouth slowly turns sweet. The work is brief, though: as the bolus reaches the stomach, gastric acid drops the pH far below amylase's working range and the enzyme is inactivated. No meaningful carbohydrate digestion occurs in the stomach, and none is absorbed there.

Small intestine: pancreatic amylase and the brush border

The real work happens in the duodenum. The pancreas releases pancreatic amylase into the small intestine, where bicarbonate has neutralized the acid, and starch is rapidly reduced to disaccharides and short oligosaccharides. These fragments are still too large to absorb, so the final cut is made by enzymes anchored in the brush border — the microvilli covering the intestinal cells. Maltase splits maltose into two glucose molecules, sucrase splits sucrose into glucose and fructose, and lactase splits lactose into glucose and galactose. Only now, as single sugars, can carbohydrate be absorbed.

Crossing the wall and reaching the liver

The three monosaccharides do not all enter the same way. Glucose and galactose ride the sodium-glucose cotransporter SGLT1, a secondary active transport system that uses the sodium gradient to pull sugar in against its concentration gradient — the same physiology that oral rehydration solutions exploit when they pair glucose with sodium. Fructose takes a different route, entering by facilitated diffusion through GLUT5, which needs no energy and moves fructose only down its gradient. All three then exit the far side of the cell through GLUT2 into the capillaries of the villus.

From there, blood carries the sugars through the hepatic portal vein straight to the liver — a deliberate piece of anatomy. The liver sees the entire absorbed load first and converts most fructose and galactose into glucose or into liver glycogen before the rest of the body ever meets them. What leaves the liver and enters general circulation is largely glucose.

Section 4Blood glucose regulation

A two-hormone thermostat

Blood glucose is held within a remarkably narrow band. In a person without diabetes, fasting plasma glucose sits below 100 mg/dL (5.6 mmol/L) and rarely exceeds about 140 mg/dL even after a large meal. The control system is a classic negative feedback loop run from the islets of Langerhans in the pancreas.

When glucose rises after a meal, beta cells release insulin. Insulin is the storage signal: it prompts muscle and adipose cells to move GLUT4 transporters to their surfaces so glucose can enter, it drives the liver and muscle to build glycogen, and it switches off the liver's own glucose production. When glucose falls — between meals, overnight, during exercise — alpha cells release glucagon, which does the opposite: it breaks liver glycogen back down (glycogenolysis) and, as that store runs low, drives the liver to manufacture new glucose from amino acids and glycerol (gluconeogenesis). Epinephrine, cortisol, and growth hormone reinforce glucagon's side of the ledger during stress.

Where the glucose goes, and what runs out first

The body's glucose reserves are small. Liver glycogen (roughly 100 g) is the only store that can be released back into the bloodstream, and it is largely depleted after 12–24 hours without food. Muscle glycogen (roughly 400 g) is far larger but is locked in place: muscle lacks the enzyme needed to release free glucose, so muscle glycogen fuels that muscle only and never rescues blood glucose. Beyond those stores, the brain's substantial daily glucose requirement — on the order of 120 g — is met by gluconeogenesis, which consumes body protein, and increasingly by ketone bodies as fasting continues. Adequate dietary carbohydrate spares that protein, which is the physiological argument behind the carbohydrate RDA you will meet in the next section.

Glycemic index and glycemic load

The glycemic index (GI) ranks a food by how much it raises blood glucose over two hours compared with a reference (pure glucose = 100), using portions matched for 50 g of available carbohydrate. Values of 55 or below are called low, 56–69 medium, and 70 or above high. Because GI ignores portion size — watermelon has a high GI but very little carbohydrate per slice — the glycemic load (GL) was introduced: GI multiplied by the grams of available carbohydrate in an actual serving, divided by 100. A GL of 10 or below is low; 20 or above is high.

Both tools are genuinely useful for understanding why foods differ, and low-GI dietary patterns show modest benefits for post-meal glucose in controlled trials. But treat published values cautiously. GI varies substantially between individuals eating the identical food, and within a food it shifts with ripeness, particle size, cooking time, cooling, and whatever else is on the plate: fat, protein, acid, and fiber all lower the glycemic response of a mixed meal. GI also says nothing about nutritional quality — a food can have a low GI and little else to recommend it.

Section 5Carbohydrate and health

The actual numbers

The RDA for carbohydrate is 130 g/day for adults and for children aged 1 year and older, of both sexes. That figure is not about health optimization; it is the amount judged sufficient to supply the brain's glucose needs without forcing reliance on gluconeogenesis. The RDA rises in pregnancy to 175 g/day and in lactation to 210 g/day, reflecting fetal and milk glucose demands. Infants have Adequate Intakes rather than an RDA: 60 g/day at 0–6 months and 95 g/day at 7–12 months.

Separately, the Acceptable Macronutrient Distribution Range (AMDR) for carbohydrate is 45–65% of total energy for adults and children aged 1 year and older. Note what these two numbers are doing: 130 g is a floor for a specific physiological function, while 45–65% describes the range within which populations appear to meet nutrient needs while keeping chronic disease risk low. There is no Tolerable Upper Intake Level for total carbohydrate.

Added sugars

Added sugars are sugars and syrups put into foods during processing or preparation, plus sugars from syrups and honey and from concentrated fruit or vegetable juices. They are distinguished from the sugars intrinsic to intact fruit, vegetables, and milk. Chemically the molecules are the same; the distinction is about the package they arrive in and about what the food displaces. Regulators find the distinction useful enough that added sugars now appear as their own line on the US Nutrition Facts label.

What is the evidence? Two conclusions are solid. First, added sugars supply energy with essentially no vitamins, minerals, or fiber, so at high intakes they crowd out nutrient-dense foods — a straightforward dilution argument. Second, the frequency and amount of free sugars is strongly and consistently linked to dental caries; this is among the best-established diet-disease relationships in nutrition. Sugar-sweetened beverages are the largest single source of added sugars in many countries, and their consumption is consistently associated with weight gain and with type 2 diabetes risk in cohort studies — a relationship attributed partly to the fact that liquid calories seem to trigger less compensatory reduction in later eating. Claims about specific metabolic harms of fructose at ordinary dietary intakes remain preliminary, extrapolated largely from high-dose feeding studies; whole fruit, which also contains fructose, is consistently associated with lower disease risk.

Carbohydrate and diabetes risk

Type 1 diabetes is an autoimmune destruction of pancreatic beta cells producing absolute insulin deficiency. It is not caused by diet, and no eating pattern in childhood causes or prevents it. Type 2 diabetes is different: insulin resistance in muscle, liver, and fat combined with progressive beta-cell failure. Its risk is shaped by genetics, age, physical activity, sleep, body-fat distribution, and dietary pattern together.

The most robust dietary findings point to carbohydrate quality rather than quantity. Higher intakes of whole grains, legumes, and total fiber are consistently associated with lower type 2 diabetes incidence across large cohorts and long follow-up; higher intakes of refined grains and sugar-sweetened beverages track the other way. Randomized lifestyle-intervention trials show that structured programs combining dietary change with physical activity substantially reduce progression from prediabetes to type 2 diabetes — strong evidence, though those programs change many things at once, which makes it impossible to credit any single nutrient. Very-low-carbohydrate patterns can improve glycemic markers over the short term in trials, but long-term comparative evidence is mixed, and adherence, not macronutrient ratio, tends to predict outcomes.

ReferenceWords worth knowing

A quick reference for the terms in this chapter — skim it now, or circle back whenever a word trips you up.

Added sugars

Sugars and syrups added during processing or preparation, plus sugars from honey, syrups, and concentrated juices; distinguished from sugars intrinsic to intact fruit, vegetables, and milk.

Amylase

The enzyme that cleaves the alpha bonds of starch; secreted by the salivary glands and, far more importantly, by the pancreas.

Amylopectin

The branched form of plant starch; its many chain ends allow faster enzymatic digestion than amylose.

Amylose

The straight-chain form of plant starch; digests more slowly and is more likely to retrograde into resistant starch on cooling.

Dietary fiber

Carbohydrate (plus lignin) that resists digestion in the human small intestine and passes into the colon.

Disaccharide

Two monosaccharides joined together: sucrose, lactose, or maltose.

Fructose

A monosaccharide found in fruit and honey and in half of every sucrose molecule; absorbed via GLUT5 and handled largely by the liver.

Glucagon

The pancreatic alpha-cell hormone that raises blood glucose through glycogenolysis and gluconeogenesis.

Gluconeogenesis

The liver's synthesis of new glucose from non-carbohydrate precursors such as amino acids and glycerol.

Glycemic index (GI)

A ranking of how much a 50 g available-carbohydrate portion of a food raises blood glucose over two hours, relative to glucose set at 100.

Glycemic load (GL)

Glycemic index adjusted for the carbohydrate actually present in a realistic serving: GI × available carbohydrate ÷ 100.

Glycogen

The highly branched storage polysaccharide of animals, held in liver (≈100 g) and muscle (≈400 g).

Insulin

The pancreatic beta-cell hormone that lowers blood glucose by promoting cellular uptake via GLUT4 and stimulating storage.

Lactase nonpersistence

The normal, genetically programmed decline in lactase production after early childhood, affecting most of the world's adults; the basis of lactose intolerance.

Lactose

Milk sugar; a disaccharide of glucose and galactose.

Monosaccharide

A single sugar unit — glucose, fructose, or galactose — and the only form of carbohydrate that can be absorbed.

Polysaccharide

A carbohydrate of many sugar units: starch and glycogen (digestible) or fiber such as cellulose (not digestible).

Resistant starch

Starch that escapes small-intestinal digestion — trapped in intact grains, high in amylose, or retrograded on cooling — and is fermented in the colon.

Short-chain fatty acids (SCFAs)

Acetate, propionate, and butyrate, produced when colonic bacteria ferment fiber; butyrate is the preferred fuel of colon cells.

Soluble fiber

Fiber that disperses in water — pectin, beta-glucan, gums, inulin, psyllium. The viscous, gel-forming members slow glucose absorption and modestly lower LDL cholesterol; non-viscous soluble fibers such as inulin do not reliably do either.

Starch

The plant storage polysaccharide, made of amylose and amylopectin and the largest source of dietary carbohydrate worldwide.

Sucrose

Table sugar; a disaccharide of glucose and fructose, chemically identical whatever plant it was refined from.