LACTOSE, GALACTOSE and CATARACTS

Lactose and lactose intolerance

Lactose is a carbohydrate composed of equal parts of glucose and galactose and only occurs naturally in milk.  In eutherian milk, lactose can range from 0% of milk solids in sea lions to 76% of milk solids in the rhinoceros.  In marsupial milk, lactose is combined with larger carbohydrate molecules, which are prevalent in the early and mid stages of lactation.  For lactose to be digested it must first be converted into its component sugars glucose and galactose by the enzyme lactase.  In eutherian young, lactase is an extracellular enzyme that acts on lactose within the lumen of the intestine.  In marsupial young, lactase is an intracellular enzyme and the large carbohydrate molecules in the milk must first enter the intestinal cells before the lactose can be digested.  This limits the rate at which these molecules are digested and places a threshold on the amount of lactose that can be tolerated by marsupial young.  Animals, such as sea lions, that do not produce lactase cannot digest lactose and its presence in their diet usually causes diarrhoea.  Animals, such as marsupials, that have a threshold for lactose usually suffer diarrhoea if that threshold for lactose is exceeded.

Lactose intolerance occurs when either the ingestion of lactose exceeds its assimilation or the production of intestinal lactase ceases or is diminished.  Lactose intolerance in the newborn is usually due to a deficiency in the gene that produces lactase. It should be noted that except for some people of particular ethnic origin many mammals lose the ability to produce the enzyme lactase once they are weaned of milk.  Many adult animals display intolerance to even small amounts of lactose in their diet.

Galactose and galactose intolerance

Galactose is a sugar that occurs naturally in other carbohydrates apart from lactose.  Galactose is present in plant carbohydrates such as pectins and gums, plant secretions such as manna and insect secretions such as honeydew.

Galactose is readily absorbed from the intestine and is metabolised in the main by the liver.  It is converted to glucose before entering the normal pathway of glucose metabolism.  Galactose is firstly phosphorylated by the enzyme galactokinase and then transformed to glucose by two enzymes, uridyltransferase and UDP-glucose epimerase.

Galactose intolerance (galactosemia) occurs when there is a deficiency of any of the enzymes that convert galactose to glucose.  Galactosemia in the newborn is due to deficiencies in the genes that produce either galactokinase or uridyltransferase.  The incidence of galactosemia in children is reported at about 0.06%.

Cataracts

Cataracts are a clouding of the lens in the eye.  The causes of cataracts are many and include ageing, eye trauma, UV and other radiation, viral infections, galactosemia and intolerance to other simple sugars.

Ageing appears to be the main cause of cataracts in humans and it has been proposed as the main cause of cataracts in other animals.  Mature age cataracts have been reported in many animal species including dogs, cats, primates and marsupials.  There is anecdotal evidence that free radicals associated with stress may cause cataracts in juvenile marsupials.  It has been suggested that using moderate doses of antioxidants such as Vitamin E may reverse the onset of cataracts.

In galactosemia, any ingested galactose is metabolised to galactitol instead of glucose and damaging amounts of the less soluble galactitol accumulate in the eye causing cataracts.  There is no published evidence of galactosemia in marsupials.

It is considered by some that lactose intolerance causes cataracts.  Although there may well be animals with cataracts that are also lactose intolerant, there is no evidence to support lactose intolerance as a cause of their cataracts.