Glucose and galactose are sugars made of a single sugar unit, also referred to as monosaccharides. They are found naturally in the food supply, or they result from the digestion of larger chains of sugar units in the human gastrointestinal tract. Glucose is the main sugar in the bloodstream and is also the form that our body stores in chains called glycogen. It is used to fuel daily activity and exercise.
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Structure and Taste
Glucose and galactose are both simple structures made of a six-carbon ring. They are almost identical, but galactose differs slightly in the orientation of functional groups around the fourth carbon. Galactose has a higher melting point than glucose as a result of the structural differences. The main difference between these two sugars from a sensory perspective is that glucose is substantially sweeter, though neither is as sweet as table sugar.
Very few foods naturally contain glucose and galactose, with the exception of sweeteners like honey. However, longer chains of carbohydrates that contain glucose and galactose are very common in the food supply. The main sources of glucose are fruits and dried fruit such as raisins and apricots; fruit juices; and sweeteners like honey. The main source of galactose is foods containing lactose, such as dairy products, though it does occur naturally in honey and beets. Lactose is broken down to generate galactose and glucose before it is absorbed in the intestine.
Digestion, Absorption and Metabolism
Starches, which are long strings of sugars, and big sugars like lactose are composed of smaller building block sugars like glucose and galactose. During the digestion of these larger sugar structures, glucose and galactose are released. Once they have been released, they do not undergo further digestion and are transported from the intestine to the bloodstream. Glucose and galactose are transported into the bloodstream by the same transporter. Once they are absorbed, glucose is used by the body for fuel or is stored in the liver and muscles for future energy needs. Galactose undergoes structural rearrangement so that it can be used in the glucose pathway for fuel or stored. Galactose is also a component of fatty acids in the nervous system.
Too Much Galactose
Although they are not common, high levels of galactose in the blood can lead to galactosemia. Galactosemia is a condition in which galactose is not sufficiently metabolized and is removed from the blood and stored in tissues as galactitol. This can lead to cataracts. Severe galactosemia can occur in infants with a genetic condition that results in a complete lack of the enzymes necessary to metabolize galactose. In these infants, more serious symptoms, such as vomiting, failure to thrive, infection and jaundice, can occur. Newborns are screened for this condition.
Too Much Glucose
High levels of glucose in the blood can also be detrimental, as is demonstrated by type 1 and 2 diabetes. Individuals with diabetes cannot manage their blood sugar levels and must be very conscious of their carbohydrate intake. Complications of diabetes include cardiovascular disease, retinopathy, nephropathy, chronic kidney disease, and in acute instances, hypoglycemia and hyperglycemia, ketoacidosis.