Glucose

Glucose is the body's main fuel and the sugar in your blood. Almost all digestible carbohydrate — starch, sucrose, lactose and maltose — is broken down to glucose, which every cell can burn for energy and the brain depends on.

What is it?

Glucose is a monosaccharide (a single sugar unit) with the formula C6H12O6, also called dextrose. Plants make it by photosynthesis and store it as starch; the body stores it as glycogen in the liver and muscles. Glucose is sweet but less sweet than sucrose or fructose.

Blood glucose is tightly controlled by the hormones insulin (which lowers it) and glucagon (which raises it). In diabetes, this control fails.

What it does

1. Is the preferred fuel for the brain, which uses roughly 120 g a day
2. Powers muscles, red blood cells and almost every other cell
3. Is stored as glycogen in the liver and muscles for later use
4. Is the reference food for the glycaemic index (glucose = 100)

Food sources

Free glucose occurs in fruit (grapes, dates, bananas), honey, some vegetables, and in glucose syrups, dextrose and sports gels and drinks. Most dietary glucose comes from starch in rice, noodles, bread and potatoes, and from sucrose, lactose and maltose when digested.

Singapore food examples

1. Rice, noodles, bee hoon, roti prata and bread — starch that breaks down to glucose
2. Ripe fruit — banana, grapes, durian, dates and longan
3. Honey, syrups and sweet drinks
4. Glucose sweets or gel for treating a low blood sugar (for people on diabetes medicine)
5. Choose brown or mixed-grain rice and wholegrains for a slower glucose release

How much do I need?

There is no recommended intake for glucose itself. The US National Academies set a minimum of 130 g of carbohydrate a day for adults, based on the brain's glucose needs, much of which comes from starch. Singapore's guidelines advise sufficient grains, at least half as wholegrain, and added sugar under 10 percent of energy — see Singapore's dietary guidelines for adults (HealthHub).

Low intake and deficiency

Low blood glucose (hypoglycaemia) is usually below 4 mmol/L and causes shakiness, sweating, hunger, dizziness, confusion and, if severe, loss of consciousness. It is most often caused by diabetes medicines (insulin and some tablets), and sometimes by heavy alcohol use, prolonged fasting or certain illnesses.

If you have diabetes and your blood glucose is low, the standard advice is to take about 15 g of fast-acting carbohydrate (glucose tablets or gel, half a glass of juice), recheck after 15 minutes, and seek medical help if it does not improve. Follow your own doctor's plan.

Too much and safety

Persistently high blood glucose (hyperglycaemia) is the hallmark of diabetes. A fasting blood glucose of 7.0 mmol/L or higher on repeat tests suggests diabetes, and values just below that (the prediabetes range) are a signal to act — ask your doctor about screening and your own targets. Over time, high glucose damages blood vessels, nerves, kidneys and eyes.

Reducing sweet drinks, choosing wholegrains and fibre, keeping a healthy weight and being active all help keep blood glucose in range.

Absorption and bioavailability

Glucose is absorbed quickly in the small intestine through the SGLT1 transporter, which also carries sodium, and then enters the blood. How fast it rises depends on the meal: fibre, protein, fat, vinegar and intact grains slow it, while liquid sugars and refined starches speed it. Insulin then moves glucose into cells.

Connection to daily life

1. Exercise: glucose, stored as glycogen, fuels moderate and hard exercise; longer sessions may need extra carbohydrate — see Exercise
2. Sleep and stress: poor sleep and stress raise blood glucose and appetite — see Sleep and Stress
3. Healthy ageing: regular screening and a fibre-rich diet help prevent type 2 diabetes
4. Related: Fibre slows glucose absorption

Sources: 1. HealthHub Singapore — Dietary guidelines for adults: https://www.healthhub.sg/well-being-and-lifestyle/food-diet-and-nutrition/dietary_guidelines_adults 2. HealthHub Singapore — Recommended Dietary Allowances: https://www.healthhub.sg/well-being-and-lifestyle/food-diet-and-nutrition/recommended_dietary_allowances 3. Health Promotion Board — Singapore Food Insights Database (SG FoodID): https://www.hpb.gov.sg/healthy-living/food-and-beverage/sgfoodid/ 4. FAO/INFOODS — Food composition tables and databases: https://www.fao.org/food-composition/tables-and-databases/en/ 5. National Academies of Sciences, Engineering, and Medicine — Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005)