Sugars belong to the carbohydrates. A distinction is made between simple sugars, double sugars made of two simple sugars, and multiple sugars made of many. The drawings show their structure as a ring of carbon and one oxygen atom (red); the hydrogen atoms are left out for clarity.
Simple sugars with 5 carbon atoms (here in a 5-membered ring)
Ribose
- Chemistry: Simple sugar with five carbon atoms (C₅H₁₀O₅), a pentose.
- Special feature: Without one of its oxygen atoms it is called deoxyribose and forms the backbone of the genetic material DNA. In experiments, ribose replenished the energy reserves of the heart muscle more quickly after a lack of oxygen.
- Found in: A building block of ribonucleic acid (RNA) and of the energy carrier ATP in every cell. It hardly occurs freely in food; the body makes it itself from glucose.
- Benefits: No ATP without ribose. As a supplement (D-ribose) it is used for exhaustion and heart failure; the evidence for this is still thin.
- Risks: In large amounts it can lower blood sugar and cause digestive complaints.
Simple sugars with 6 carbon atoms (here in a 6-membered ring)
Fructose (fruit sugar)
- Chemistry: Simple sugar with six carbon atoms (C₆H₁₂O₆), the same formula as glucose but built differently; usually forms a 5-membered ring.
- Special feature: Enters the cells without insulin and, in the liver, bypasses the main control step of sugar breakdown; excess fructose is therefore converted into fat particularly easily.
- Found in: Fruit, honey, agave syrup; half of table sugar and a large part of the glucose-fructose syrup in soft drinks and processed foods.
- Benefits: Sweeter than table sugar and barely raises blood sugar. In whole fruit, together with fibre, unproblematic.
- Risks: Processed almost only in the liver. In large amounts, above all from drinks, it promotes fatty liver, raised blood lipids and uric acid (gout). With fructose malabsorption it causes bloating and diarrhoea.
Galactose (mucus sugar)
- Chemistry: Simple sugar with six carbon atoms (C₆H₁₂O₆); differs from glucose only in the position of one OH group.
- Special feature: Enters the cells independently of insulin and barely raises blood sugar; it can therefore supply the cell with energy without straining insulin levels.
- Found in: Above all as half of milk sugar in milk and dairy products, in small amounts also in pulses.
- Benefits: A building block of mucous substances and of sugar chains on cells and in the nervous system; the liver can convert it into glucose.
- Risks: In the rare inborn galactosaemia the enzyme needed to break it down is missing; galactose must then be strictly avoided.
Glucose (grape sugar)
- Chemistry: Simple sugar with six carbon atoms (C₆H₁₂O₆), the most common sugar in nature; measured in the blood as “blood sugar”.
- Special feature: It enters muscle and fat cells only with the help of insulin; the brain, liver and red blood cells take it up independently of insulin.
- Found in: Grapes, fruit, honey; above all, however, it is formed during digestion from starch and table sugar.
- Benefits: The body’s most important source of energy; the brain and red blood cells depend on it.
- Risks: Enters the blood very quickly. Frequent blood sugar peaks strain insulin regulation and promote overweight, insulin resistance and type 2 diabetes; excess glucose glycates proteins (AGEs).
Mannose (epimer of glucose)
- Chemistry: Simple sugar with six carbon atoms (C₆H₁₂O₆); differs from glucose only in the position of one OH group.
- Special feature: Absorbed only slowly in the gut and largely excreted unchanged in the urine; that is why it acts where bladder infections arise.
- Found in: Small amounts in cranberries, apples, peaches and pulses.
- Benefits: A building block of sugar chains on proteins. As a supplement (D-mannose) it is used to prevent recurrent bladder infections because it binds bacteria (E. coli) before they attach to the bladder wall; the studies on this are contradictory.
- Risks: In high amounts bloating and loose stools.
Mannitol (sugar alcohol of mannose)
- Chemistry: Sugar alcohol (C₆H₁₄O₆), formed from mannose; an open chain instead of a ring.
- Special feature: Given as an infusion, mannitol draws water out of the tissues; this is why hospitals use it to lower pressure in the brain.
- Found in: Mushrooms, algae, celery; as the sweetener E 421 in sugar-free chewing gum and sweets.
- Benefits: Hardly any calories, does not cause tooth decay. Mannitol is used in the small-intestine permeability test: how much of it appears in the urine shows the absorptive capacity of the intestinal lining.
- Risks: Only partly absorbed in the gut and laxative in larger amounts; often poorly tolerated with irritable bowel.
Double sugars
Lactose (milk sugar)
- Chemistry: Double sugar of galactose and glucose (C₁₂H₂₂O₁₁); the enzyme lactase splits it in the small intestine.
- Special feature: Being able to digest milk sugar as an adult is an inherited trait that is common above all in northern Europe; worldwide, most people lose this ability after infancy.
- Found in: Milk, cream, yoghurt, quark and cream cheese; long-matured hard cheese contains hardly any lactose.
- Benefits: A source of energy for infants; helps the absorption of calcium.
- Risks: If lactase is missing (lactose intolerance), the milk sugar reaches the large intestine and causes bloating, abdominal pain and diarrhoea.
Lactulose (derivative of lactose)
- Chemistry: Double sugar of galactose and fructose (C₁₂H₂₂O₁₁), made from milk sugar.
- Special feature: Practically does not occur in nature. The gut bacteria ferment it into short-chain fatty acids; this acidifies the gut and promotes beneficial bacteria such as bifidobacteria.
- Found in: Traces in strongly heated milk; otherwise as a medicine.
- Benefits: Not absorbed in the small intestine and fermented by the gut bacteria. It therefore serves as a mild laxative (bowel regulator), to lower ammonia in liver disease, and as a test substance in the small-intestine permeability test and the breath test.
- Risks: Bloating, abdominal cramps and diarrhoea if the dose is too high.
Maltose (malt sugar)
- Chemistry: Double sugar of two glucose units (C₁₂H₂₂O₁₁).
- Special feature: Split into two glucose units straight away in the small intestine, it therefore raises blood sugar even slightly faster than glucose itself.
- Found in: Malt, malt extract, beer and sprouting grain; formed during the digestion of starch.
- Benefits: Quickly available energy.
- Risks: Raises blood sugar as quickly as glucose and hides as malt extract or maltodextrin in many processed foods.
Sucrose (cane sugar, table sugar)
- Chemistry: Double sugar of glucose and fructose (C₁₂H₂₂O₁₁), here with the fructose in a 5-membered ring.
- Special feature: It is the yardstick for sweetness. Fructose is about one and a half times as sweet, glucose only about two thirds as sweet.
- Found in: Sugar cane and sugar beet; in sweets, baked goods, drinks and many processed foods, and naturally in fruit.
- Benefits: Taste and shelf life; quick energy.
- Risks: Tooth decay, overweight, fatty liver and type 2 diabetes. The World Health Organization recommends keeping free sugars below 10 per cent of daily energy, better below 5 per cent (about 25 g).
Multiple sugars
Starch
- Chemistry: Multiple sugar of hundreds to thousands of glucose units; amylose is an unbranched chain, amylopectin is branched.
- Special feature: When cooked starch cools, its chains rearrange and become partly indigestible; this is how resistant starch forms.
- Found in: Grain, bread and pasta, rice, potatoes, maize and pulses.
- Benefits: The most important source of carbohydrate in the diet. From wholegrain and pulses it is digested slowly; cooled potatoes and cooled rice contain “resistant starch”, which feeds the gut bacteria.
- Risks: White flour, white bread and mashed potato raise blood sugar almost as quickly as glucose; unfavourable with insulin resistance.