What is galactose?

The galactose is a monosaccharide sugar found mainly in milk and other dairy products. By binding to glucose, they form the lactose dimer. It functions as a structural component of nerve cell membranes, is essential for lactation in mammals and can serve as a source of energy.

However, its consumption in the diet is not mandatory. Various metabolic problems related to galactose lead to pathologies such as lactose intolerance and galactosemia.


Galactose is a monosaccharide. It is a six-carbon aldose, with the molecular formula C 6 H 12 O 6 . The molecular weight is 180 g / mol. This formula is the same for other sugars, such as glucose or fructose.

It can exist in its open chain form or also present in its cyclical form. It is an epimer of glucose; they only differ at carbon number 4. The term epimer refers to a stereoisomer that only differs in the position of its centers.

On the diet

The main source of galactose in the diet is lactose, which comes from dairy products. It can be used as a source of energy.

However, the contribution in the diet is not essential for the body, since UDP-glucose can be transformed into UDP-galactose and this metabolite can perform its functions in the body as a constituent of a group of glycolipids.

There is no type of study that reveals any pathology associated with low galactose consumption. In contrast, excessive consumption has been reported as toxic in model animals. In fact, excess galactose is associated with cataracts and oxidative damage.

However, in children, lactose contributes 40% of the energy in their diet, while in adults this percentage decreases to 2%.

Structural functions: glycolipids

Galactose is present in a specific group of glycolipids called cerebrosides. Cerebrosides that contain galactose in their structure are called galactocerebrosides or galactolipids.

Cerebrosides are degraded by the enzyme lysosime. When the body is unable to break them down, these compounds build up. This condition is called Krabbe disease.

Lactose synthesis in mammals

Galactose has a fundamental role in the synthesis of lactose. In mammals, the mammary glands produce large amounts of lactose after pregnancy to feed their young.

This process is triggered in females by a series of hormones characteristic of pregnancy. The reaction involves UDP-galactose and glucose. These two sugars are fused by the action of the enzyme lactose synthetase.

This enzyme complex is to some extent chimeric, since its component parts are not related to its function.

One of its parts is made up of a galactosyl transferase; under normal conditions, its function is related to the glycosylation of proteins.

The other part of the complex is made up of α-lactalbumin, which is very similar to lysozyme. This enzyme complex is a fascinating example of evolutionary modifications.

Galactose metabolism

Lactose is a sugar found in milk. It is a disaccharide formed by the monosaccharides glucose and galactose linked together by a β-1,4-glycosidic bond.

Galactose is obtained from the hydrolysis of lactose, this step is catalyzed by lactase. In bacteria there is an analogous enzyme called β-galactosidase.

The enzyme hexokinase, present in the first step of the glycolytic pathway, is capable of recognizing different sugars, such as glucose, fructose and mannose. However, it does not recognize galactose.

That is why the conversion step called epimerization must occur as a step prior to glycolysis. This route is intended to convert galactose into a metabolite that can enter glycolysis, specifically glucose-6-phosphate.

Galactose degradation is only possible in amniotic cells, liver cells, erythrocytes and leukocytes (blood cells). The liver pathway is known as the Leloir pathway in honor of its discoverer, Luis Federico Leloir, an important Argentine scientist.

Galactose is taken up by enterocytes by active transport, via SGLT1, SGC5A1 (sodium-glucose cotransporters) and to a lesser extent by SGLT2.

Steps of metabolism

The steps of metabolism are summarized as follows:

  • Galactose is phosphorylated at the first carbon. This step is catalyzed by the enzyme galactokinase.
  • The uridyl group is transferred to glucose-1-phosphate by galactose-1-phosphate uridyltransferase. The result of this reaction is glucose-1-phosphate and UDP-galactose.
  • UDP-galactose is transformed to UDP-glucose, a step catalyzed by UDP-galactose-4-epimerase.
  • Finally, glucose-1-phosphate is transformed to glucose-6-phosphate. This compound can enter the glycolytic pathway.

These reactions can be summarized as: galactose + ATP -> glucose-1-phosphate + ADP + H +

The regulation of galactose homeostasis is complex and strongly integrated with the regulation of other carbohydrates.

Pathologies associated with galactose metabolism


Galactosemia is a pathology in which the body is unable to metabolize galactose. Its causes are genetic and its treatment includes a galactose-free diet.

It encompasses a series of varied symptoms, such as vomiting, diarrhea, mental retardation, developmental problems, liver problems and cataract formation, among others. In some cases the disease can be fatal and the affected individual dies.

Patients with this condition do not possess the enzyme galactose-1-phosphate uridyltransferase. By not being able to continue the rest of the metabolic reactions, this highly toxic product accumulates in the body.

Lactose intolerance

In some adults there is a deficiency of the enzyme lactase. This condition does not allow the normal metabolism of lactose, so the consumption of dairy products produces alterations in the gastrointestinal tract.

It is worth mentioning that the deficiency of this enzyme occurs naturally as individuals grow older, since the diet of an adult supposes a lesser importance of lactose and dairy products in the diet.

Microorganisms that inhabit the large intestine can use lactose as a carbon source. The end products of this reaction are methane and hydrogen gas.

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