Every type of sugar, and what it does in your body

In short

• All sugars are built from a few simple units: glucose, fructose and galactose. Everything else (table sugar, honey, syrups) is a mix or a chain of those.
• Glucose raises blood sugar and insulin quickly. Fructose barely touches blood sugar but is handled almost entirely by the liver, where too much becomes fat and uric acid.
• "Natural" sugars such as honey, coconut sugar, agave and maple syrup behave in the body almost exactly like white sugar. Their extra minerals are tiny.
• The real health line is free sugars: sugar added to food, plus honey, syrups and juice. The WHO advises under 10% of daily energy, ideally under 5% (about 25 g, or six teaspoons).
• Genuine benefits exist, but they are narrow: fast fuel during long exercise, rescuing a low blood sugar, oral rehydration salts, and honey for children's coughs (over one year old).

The three building blocks

Chemists call sugars carbohydrates: carbon, hydrogen and oxygen, arranged in rings. The smallest ones, monosaccharides, are the units everything else is built from.

Unit	Formula	Where you find it	How sweet (sucrose = 1)
Glucose	C6H12O6	Fruit, honey, every starch once digested	0.7
Fructose	C6H12O6	Fruit, honey, agave, half of table sugar	1.2–1.7
Galactose	C6H12O6	Milk (inside lactose)	0.3

All three share the same formula. They differ only in how the atoms are arranged — and that small difference decides which organ handles them.

Two units joined together make a disaccharide: sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose). Long chains of glucose are starch, which your saliva and gut turn back into glucose within minutes.

What happens after you swallow

Glucose: the blood-sugar sugar

Glucose is absorbed in the small intestine through a transporter called SGLT1 and goes straight into the blood. The pancreas responds with insulin, which opens muscle, fat and liver cells so glucose can enter. Every cell can burn it.

The problem is pace. A large, fast dose produces a spike, a big insulin response, then often a dip that brings hunger back. Repeated over years, cells listen to insulin less well (insulin resistance), the first step towards type 2 diabetes.

Fructose: the liver sugar

Fructose enters through a different door (GLUT5) and goes almost entirely to the liver. There it is processed by an enzyme, fructokinase, that skips the main brake on sugar breakdown (phosphofructokinase). In plain words: the liver cannot slow the flow down.

In small amounts, as in a piece of fruit, that is fine. In large amounts, especially in drinks, the liver turns the surplus into fat (de novo lipogenesis) and produces uric acid as a by-product. This is linked to fatty liver disease, raised triglycerides and gout. Because fructose barely raises insulin, it also does little to make you feel full.

Galactose

Galactose from milk is converted to glucose in the liver. It is harmless for nearly everyone, except babies born with galactosaemia, a rare condition screened for at birth.

The chemistry that happens everywhere else

• In your mouth: bacteria such as Streptococcus mutans ferment sugar into lactic acid. Below a pH of about 5.5, tooth enamel starts to dissolve. Frequency matters more than amount: sipping a sweet drink all afternoon is worse than eating dessert once.
• In your blood: glucose slowly sticks to proteins without any enzyme involved. This is glycation, the same reaction doctors measure as HbA1c. The end products (AGEs) stiffen blood vessels, skin collagen and the lens of the eye. Fructose glycates even faster than glucose.
• In the oven: heat makes sugars react with proteins (the Maillard reaction, the brown crust on bread) or with themselves (caramelisation, from around 160 °C for sucrose). Fructose browns earliest. These reactions create flavour; at very high heat they also form small amounts of compounds such as acrylamide.

Every sugar, one by one

Ratings follow the Frogwise traffic light: green = fine in normal amounts, amber = keep it small, red = avoid where you can. GI is the glycaemic index (glucose = 100); lower means a slower rise in blood sugar.

Sugar	What it is	GI	Main health problems	Any upside?	Rating
Glucose / dextrose	Pure glucose, usually from corn or wheat starch	100	Fastest blood-sugar spike	Treats hypoglycaemia; fuel in long exercise	Amber
Fructose (crystalline)	Pure fructose	~20	Fatty liver, triglycerides, uric acid in large doses	Low GI on paper only	Red
Sucrose (white sugar)	Glucose + fructose, from cane or beet	~65	Teeth, weight, insulin resistance, liver fat	None beyond energy	Amber
Brown sugar	Sucrose with a little molasses	~65	Same as white sugar	Trace minerals, negligible	Amber
Lactose	Milk sugar (glucose + galactose)	~46	Bloating if lactose intolerant	Comes packaged with protein and calcium in milk	Green
Maltose	Glucose + glucose, from malted grain	~105	Even faster spike than glucose	None	Amber
Maltodextrin	Short starch chains, a thickener and filler	85–105	Hidden fast sugar in "sugar-free" products	Useful only in sports drinks	Red
Glucose syrup	Broken-down starch, in sweets and sauces	~100	Hidden fast sugar	None	Red
High-fructose corn syrup (HFCS)	Corn syrup with 42–55% fructose	~60	Same as sucrose; easy to over-consume in drinks	None	Red
Agave syrup	Up to ~85% fructose	~15–30	Highest fructose load of any syrup	Low GI is misleading	Red
Honey	~40% fructose, ~30% glucose, water	~55	Same as sugar; botulism risk under age one	Antibacterial compounds; calms children's coughs	Amber
Maple syrup	Mostly sucrose, boiled tree sap	~54	Same as sugar	Some manganese and zinc; minimal processing	Amber
Coconut sugar	Mostly sucrose (70–80%) from palm sap	~54*	Same as sugar	A little inulin and minerals	Amber
Molasses (blackstrap)	What is left after sugar crystals are removed	~55	Still sugar	Real iron, calcium, potassium per spoon	Amber
Panela / rapadura / jaggery	Whole cane juice, dried	~65	Same as sugar	Keeps the molasses minerals	Amber
Brown rice syrup	Glucose and maltose from rice	~98	Very fast spike; arsenic found in several brands	None	Red
Isomaltulose (Palatinose)	Sucrose rearranged by enzymes	~32	Still 4 kcal/g	Slow, even release; kinder to teeth	Green
Trehalose	Glucose + glucose, in mushrooms and processed foods	~70	Possible link to virulent C. difficile strains (debated)	None meaningful	Amber
Fruit juice concentrate	Sugar from fruit, fibre removed	~45–65	Counts as free sugar; same liver load	A few vitamins	Red

*The low GI quoted for coconut sugar (35) comes from a single small test; independent tests give about 54.

"Natural" does not mean different

Honey, maple syrup, coconut sugar and agave are often sold as healthier. Chemically they are still glucose and fructose, and the body treats them as such. To get a meaningful dose of the minerals in maple syrup or coconut sugar, you would have to eat far more sugar than is wise. The exception is blackstrap molasses, which is a genuine source of iron and calcium — and is so strong in taste that few people over-eat it.

What does differ is the fructose share. Agave is the worst choice for the liver. Maple syrup, which is mostly sucrose, sits in the middle.

Where sugar actually helps

• Low blood sugar: glucose tablets rescue a diabetic hypo in minutes. Nothing works faster.
• Long exercise: over 60–90 minutes, mixing glucose and fructose lets muscles take in more fuel, because they use two different transporters.
• Oral rehydration salts: glucose pulls sodium and water through the gut wall via SGLT1. This simple mix has saved millions of lives from diarrhoea.
• Children's coughs: a Cochrane review found honey probably eases cough a little better than no treatment. Never give honey to babies under one year.
• Brain fuel: the brain runs mainly on glucose, but your body makes all it needs from starch, protein and fat. You never need added sugar for it.

How much is too much

• WHO: free sugars under 10% of daily energy, with extra benefit below 5% — about 25 g a day for an adult.
• EFSA (2022): intake of added and free sugars should be "as low as possible" in a nutritionally adequate diet.
• Children: no added sugar for the first two years is a common recommendation across European health services.

Sugar naturally inside whole fruit, vegetables and plain milk does not count towards these limits: fibre, water and protein slow it down.

What to do with this

• Treat every sugar and syrup as the same thing with different flavours, and pick on taste, origin and packaging rather than on health claims.
• Keep sugar out of drinks first; that is where most fructose overload happens.
• Eat sweet things with a meal rather than as constant snacks, for your teeth and your blood sugar.
• Read on: part 2 compares sugar with every sweetener on the market.