Carbohydrates from the atoms up: how carbon, hydrogen and oxygen make a six-carbon sugar that closes into a ring, the single and double sugars drawn as 3D models at one scale, how glucose chains become starch, glycogen and fiber, how you digest them and keep blood sugar steady, and how to read the sugar lines on a label. The figure is on the left and the lesson script on the right; the quiz is at the end.
Sugars, starch and fiber, and how they show up on a label.
Carbohydrate on a food label
Total CarbohydrateEvery gram of digestible carbohydrate gives about 4 kcal. The total includes starch, sugars, fiber and any sugar alcohols. The Daily Value is 275 g.
Dietary FiberCounted inside the total, but mostly not digested, so it gives fewer calories. The Daily Value is 28 g.
Total Sugars, then Added SugarsTotal sugars counts all single and double sugars, including those naturally in fruit and milk. “Includes … Added Sugars” counts only the sugars put in during making; its Daily Value is 50 g.
Carbon, hydrogen and oxygen; a six-carbon chain that closes into a ring.
The atoms
Three atoms, three ways to bond
Carbon makes 4 bondsmethane, CH4
Oxygen makes 2water, H2O
Hydrogen makes 1 bond. Carbohydrates use only these three atoms, carbon, hydrogen and oxygen, in roughly the ratio Cn(H2O)n: one carbon for every water’s worth of H and O. That is where the name comes from: “hydrated carbon”. No water molecules are actually inside.
Six carbons in a row
Glucose as a chain, C6H12O6
123
1Carbon 1: a C=O group at the end of the chain (an aldehyde). This is the reactive end.
2Carbon 5 and its OH group.
3Carbon 6: a CH2OH at the other end.
Six carbons, and an OH on almost every one: that is what makes sugars dissolve so easily in water and taste sweet.
The chain closes into a ring
Chain
21
→
Ring (beta-glucose)
34
1The C=O on carbon 1…
2…is attacked by the OH oxygen on carbon 5.
3That oxygen becomes part of a six-membered ring: five carbons and one oxygen.
4The old C=O oxygen ends up as a new OH on carbon 1. No atoms are added or lost: it is still C6H12O6.
In water, more than 99% of glucose is in the ring form at any moment. The ring models on this page are drawn in 3D, tilted so you can see which groups point up and which point down.
carbon hydrogen oxygen · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it
Glucose, galactose and fructose: same formula, different shapes.
Glucose
Alpha and beta glucose
Alpha-glucose
1
Beta-glucose
2
1Alpha: the OH on carbon 1 points down, below the ring.
2Beta: the same OH points out to the side, roughly level with the ring.
When the ring closes, the new OH can end up either way, and in water the two forms keep switching: about one third alpha, two thirds beta. Once glucose is locked into a chain, the choice is fixed, and it matters: starch is built from alpha links, cellulose from beta links (Figures 5 and 6).
What it is: The main sugar in your blood, and the unit that starch and glycogen are made of.
In your body: Fuel for every cell; the brain uses roughly 120 g a day, and red blood cells run on glucose alone.
Too little: If you eat little carbohydrate, the liver makes glucose from protein and the glycerol of fats, and the body uses more fat-derived ketones for fuel.
Too much: Blood glucose that stays high, as in diabetes, damages blood vessels, nerves, kidneys and eyes over time.
Found in: Free glucose in fruit, honey and some vegetables; as starch in grains, potatoes and beans.
Galactose
Galactose, C6H12O6 glucose with one OH flipped
Beta-glucose
1
Beta-galactose
2
1On glucose, the OH on carbon 4 points out to the side.
2On galactose it points up. That one flip is the only difference.
What it is: A single sugar that is almost never free in food; it comes as half of lactose.
In your body: The liver turns it into glucose; it is also built into the sugar chains on cell surfaces and in brain tissue.
Too much: People with the rare inherited condition galactosemia can’t process it and must avoid milk sugar.
Found in: Milk, yogurt and cheese, as part of lactose.
Fructose
Fructose, C6H12O6 a five-sided ring
12
1Carbon 2: in the chain form, fructose’s C=O is here, on the second carbon rather than the first.
2The ring oxygen. Closing on carbon 2 gives a five-membered ring.
Same formula as glucose, C6H12O6, different arrangement. Fructose is the sweetest natural sugar. The flat drawing here shows every atom but not which way each OH points.
What it is: The main sugar of fruit and honey, and half of table sugar.
In your body: Absorbed more slowly than glucose and handled mainly by the liver, which turns most of it into glucose, glycogen, lactate or fat.
Too much: Large amounts eaten as added sugar are linked to fatty liver and higher blood fats; some people get bloating or loose stools from a lot of fructose at once.
Found in: Fruit, honey, and some vegetables such as onions; whole fruit also brings fiber and water.
carbon hydrogen oxygen · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it
3Carbon 2 of fructose. Both reactive carbons are used up in the link.
What it is: Glucose and fructose joined: ordinary table sugar.
In your body: Split by sucrase on the gut wall; the glucose and fructose are absorbed separately.
Too much: Too much added sugar raises the risk of tooth decay and is linked to weight gain, type 2 diabetes and heart disease.
Found in: Naturally in fruit (bananas, mangoes, peaches), sweet vegetables such as carrots and beets, and maple sap; refined from sugar cane and sugar beets.
1Carbon 1 of galactose, linked beta: the bridge runs out level with the ring.
2The glycosidic bond, beta 1→4.
3Carbon 4 of glucose.
What it is: Galactose and glucose joined by a beta link: the sugar of milk.
In your body: Split by lactase on the gut wall. Babies make plenty; most adults worldwide make much less after childhood.
Too little: If lactase is low, lactose reaches the colon, where bacteria ferment it into gas: bloating, cramps and loose stools (lactose intolerance). Yogurt and hard cheeses have little lactose left and are often fine.
Found in: Milk, yogurt, soft cheeses; human milk has more than cow’s milk.
carbon hydrogen oxygen · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it
Long glucose chains: how plants and you store fuel.
Starch: plants’ glucose store
Amylose: one long chain
Hundreds to thousands of glucoses joined by alpha 1→4 links. The alpha link makes the chain turn, so it coils into a helix. Usually about a quarter of starch.
Amylopectin: a branched chain
The same chain with a branch (an alpha 1→6 link) every 20 to 30 glucoses. Usually about three quarters of starch.
Glycogen: your own store
Animals store glucose as glycogen: like amylopectin but branched about every 8 to 12 glucoses, around a protein core. Many branch ends mean glucose can be released fast.
Cartoons, not to scale: each hexagon is one glucose ring.
Starch in the kitchen
Cooking swells itHeat and water make starch granules soak up water and burst (gelatinizing), which is why rice softens and gravy thickens. Gelatinized starch is much easier to digest.
Cooling firms itWhen cooked starch cools, amylose chains line up again (retrogradation). Some of this becomes resistant starch, which acts like fiber (Figure 6). Day-old rice and potato salad have a little more.
Found inGrains (wheat, rice, oats, corn), potatoes and other root vegetables, beans, lentils and peas, and unripe (green) bananas.
hexagon cartoons: each is one glucose ring · not molecular drawings
Beta links you can’t digest, and the bacteria that can.
Why fiber isn’t digested
Alpha link vs beta link, same scale
Maltose: alpha 1→4 (starch)
1
Cellobiose: beta 1→4 (cellulose)
2
1Alpha link: your enzymes (amylase, maltase) fit this shape and cut it.
2Beta link: the same atoms, but the bridge points the other way. Human enzymes can’t cut it, so cellulose passes through undigested.
Same two glucoses, same formula; only the direction of one bond differs. Cows and termites digest cellulose only because microbes in their guts can cut beta links.
Cellulose: straight chains packed side by side
Beta links keep each chain straight, so chains stack tightly and lock together with hydrogen bonds (dotted). That makes plant cell walls strong and insoluble.
Cartoon, not to scale.
Kinds of fiber
Kind
What it does
Found in
Soluble fiber (beta-glucan, pectin, gums)
Dissolves into a gel; slows digestion, helps lower LDL cholesterol, and is food for gut bacteria
Oats, barley, beans and lentils, apples, citrus, carrots
Holds water and adds bulk, so food moves through the gut more easily
Whole wheat and bran, brown rice, nuts, vegetable and fruit skins
Resistant starch
Starch that escapes digestion and is fermented like soluble fiber
Beans, lentils, green bananas, cooked-and-cooled potatoes and rice
Fiber feeds your gut bacteria
Butyric acid, C4H8O2 what gut bacteria make from fiber
1
1A carboxyl group: in the body it gives up its H and is called butyrate.
Bacteria in the colon ferment fiber into short-chain fatty acids, mainly acetate (2 carbons), propionate (3) and butyrate (4). Fiber gives about 2 kcal per gram this way.
What it is: A four-carbon short-chain fatty acid made by gut bacteria.
In your body: The main fuel of the cells lining the colon; helps keep the gut lining healthy.
Found in: Made in your gut from fiber and resistant starch; small amounts are also in butter.
How much fiber
14 g per 1000 kcalThe US target (from the Institute of Medicine, used by the Dietary Guidelines) is 14 g of fiber per 1000 kcal: about 25 g a day for women and 38 g for men aged 19 to 50. Most Americans eat about half that.
Too littleConstipation, and over the long term a higher risk of heart disease, type 2 diabetes and colon cancer in population studies.
Too much, too fastGas and bloating; raising fiber slowly and drinking enough water helps.
carbon hydrogen oxygen · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it
Cut down to single sugars, absorbed, and kept in balance.
From plate to glucose
Mouthsalivary amylase starts cutting starch into shorter pieces
Stomachacid stops amylase; little carbohydrate digestion here
Small intestinepancreatic amylase cuts starch down to maltose; maltase, sucrase and lactase on the gut wall split double sugars into single ones
Into the bloodglucose, fructose and galactose are absorbed and go to the liver first
Colonfiber and resistant starch are fermented by bacteria into gas and short-chain fatty acids
Only single sugars can be absorbed, so every starch and double sugar has to be cut down first.
Keeping blood sugar steady
After a mealblood glucose rises
Insulinthe pancreas releases insulin, so muscle, fat and liver cells take up glucose
Storedextra glucose becomes glycogen in liver and muscle, and beyond that, fat
Between mealsglucose falls; the pancreas releases glucagon
Releasedthe liver breaks down glycogen and sends glucose back into the blood
Glycogen and the glycemic index
Glycogen storesRoughly 100 g in the liver, which keeps blood glucose steady overnight, and a few hundred grams in muscle, which only that muscle can use.
Glycemic indexHow fast a food raises blood glucose compared with pure glucose. Fiber, fat, protein, intact grains and cooling cooked starch all lower it; the same food can vary with ripeness and cooking.
How much you needThe US RDA is 130 g of carbohydrate a day for adults, set from the brain’s glucose use. The acceptable range is 45 to 65% of calories.
Water and small amounts of other compounds; on a jar of honey its sugars are listed as added sugars
Sweet vegetables (carrots, beets, sweet potatoes)
Sucrose, glucose, fructose, maltose
Fiber, vitamin A precursors
Sprouted and malted grains
Maltose
Starch, fiber
Reading the sugar lines
Total SugarsAll single and double sugars, whether they grew in the food or were added. There is no Daily Value for total sugars.
Includes Added SugarsSugars added during making, plus honey, syrups and concentrated fruit juice. Daily Value: 50 g. The US Dietary Guidelines advise keeping added sugars under 10% of calories.
Sugar alcoholsAn optional line, required only if the package makes a claim about sugars or sugar alcohols. Not counted as sugars, but included in Total Carbohydrate.
Sugar alcohols
Sorbitol, C6H14O6 a sugar alcohol
Glucose (chain)
1
Sorbitol
2
1The C=O on carbon 1 of glucose…
2…gains two hydrogens and becomes CH2OH. Now every carbon carries an OH: a sugar alcohol (polyol).
Sugar alcohols are partly absorbed, so they give fewer calories than sugar (about 2.6 kcal/g for sorbitol), raise blood glucose less, and mouth bacteria can’t ferment them, so they don’t cause cavities.
What it is: The sugar alcohol made from glucose.
In your body: Absorbed slowly and only in part; the rest reaches the colon.
Too much: Large amounts draw water into the gut and ferment, causing gas and diarrhea; this is why prunes and pears can have a laxative effect.
Found in: Prunes, pears, apples, peaches and stone fruit.
carbon hydrogen oxygen · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it
All are almost entirely sucrose. Brown sugar is sucrose with some molasses, which adds color, flavor and tiny amounts of minerals.
Honey vs sugar
Honey is mostly free fructose and glucose plus water; table sugar is the same two joined. The body handles them similarly. Honey should not be given to babies under 1 year because of the risk of infant botulism.
Whole grains vs refined
A whole grain keeps its bran (fiber, B vitamins, minerals) and germ (healthy fats, vitamin E). Refining removes both and leaves mostly starch.
Why bread tastes sweet when you chew it
Amylase in your saliva starts cutting starch into maltose while it’s still in your mouth.
General education, not medical advice. Facts follow mainstream sources (US Dietary Guidelines, NIH Office of Dietary Supplements, standard biochemistry and food-science texts).
Quiz
Tap a question to see the answer
1. How many kcal does a gram of digestible carbohydrate give?
About 4, the same as protein (Figure 1).
2. Which three kinds of atom make up carbohydrates?
Carbon, hydrogen and oxygen (Figure 2).
3. When open-chain glucose closes into a ring, which atom joins the ring?
The oxygen of the OH on carbon 5 (Figure 2).
4. What is the only difference between alpha- and beta-glucose?
Which way the OH on carbon 1 points: down for alpha, out to the side for beta (Figure 3).
5. How does galactose differ from glucose?
Only the OH on carbon 4 points the other way (Figure 3).
6. What two sugars make up sucrose, and lactose?
Sucrose is glucose + fructose; lactose is galactose + glucose (Figure 4).
7. What causes the symptoms of lactose intolerance?
Too little lactase, so lactose reaches the colon and bacteria ferment it into gas (Figure 4).
8. How do amylose and amylopectin differ?
Amylose is a straight chain; amylopectin has branches every 20 to 30 glucoses (Figure 5).
9. Why can’t you digest cellulose?
Its glucoses are joined by beta links, which human enzymes can’t cut (Figure 6).
10. What do gut bacteria make from fiber?
Short-chain fatty acids such as acetate, propionate and butyrate, plus gas (Figure 6).
11. Which hormone lowers blood glucose after a meal, and which raises it between meals?
Insulin lowers it; glucagon raises it, by releasing glucose from liver glycogen (Figure 7).
12. What is the Daily Value for added sugars on a US label?