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
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 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.
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.
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
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