Galactose

Galactose is a single sugar that comes mainly from lactose, the sugar in milk. The body converts it to glucose in the liver and also uses it to build parts of cells, including in the brain. It is only a problem for people with galactosaemia, a rare inherited condition.

What is it?

Galactose is a monosaccharide, less sweet than glucose, and rarely found free in food. Mostly it is eaten joined to glucose as lactose and released when lactose is digested by lactase. Smaller amounts are found in some fruit, vegetables, legumes and in raffinose and stachyose sugars in beans.

Inside the body, galactose is converted to glucose by a series of enzymes, and is also built into glycolipids and glycoproteins, which are needed for cell recognition and nerve tissue.

What it does

1. Provides energy after conversion to glucose, at about 4 kilocalories per gram
2. Is used to build glycolipids and glycoproteins in cell membranes and the nervous system
3. Is part of lactose, the main carbohydrate for infants

Food sources

Milk, yoghurt, soft cheese, ice cream and other dairy foods (as lactose); smaller amounts in legumes, some fruit and vegetables, and processed foods that contain milk solids or whey.

Singapore food examples

1. Milk, yoghurt, ice cream and creamy desserts
2. Condensed milk and evaporated milk used in drinks and desserts
3. Beans, dhal and other legumes (as raffinose and stachyose)
4. Check labels for milk, whey and lactose in processed foods if you need to avoid galactose

How much do I need?

There is no recommended intake for galactose and Singapore's RDA table does not list it. It is not an essential nutrient: the body can make the galactose it needs from glucose. Singapore's guidelines on added sugars apply to sweeteners — see Singapore's dietary guidelines for adults (HealthHub).

Low intake and deficiency

There is no deficiency in people eating a normal diet, because the body makes galactose from glucose.

Too much and safety

Galactose from dairy is safe for most people. Infants born with galactosaemia, a rare inherited condition in which the body cannot convert galactose to glucose, can become seriously ill when they drink milk — with vomiting, jaundice, liver damage, cataracts and poor growth — and need a lifelong galactose-restricted diet under specialist care. In many countries it is picked up by newborn screening; tell a doctor at once if an infant becomes unwell after feeding.

Lactose intolerance is different: it is due to low lactase, not galactose, and causes gas and loose stools but is not dangerous (see Lactose).

Absorption and bioavailability

After lactose is split by lactase, galactose is absorbed in the small intestine through the same SGLT1 transporter as glucose, taken to the liver and converted to glucose by the Leloir pathway enzymes. People with galactosaemia lack one of these enzymes.

Connection to daily life

1. Infant feeding: lactose and galactose are normal parts of breast milk and infant formula; specialist formula is used for galactosaemia
2. Dairy tolerance: if milk upsets your stomach, it is usually the lactose, not the galactose
3. Related: see Lactose and Sugars

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)