In most mammals, production of lactase declines to undetectable levels after the weaning stage.
Persistence of lactase production beyond weaning is a phenomenon seen only in humans and even then occurs in only approximately 30% of the population -usually in people of Northern European descent. This is thought to be linked to the regular consumption of milk and dairy products typical of the native diet in these areas, unlike other parts of the world where intake of milk stops when breastfeeding ends.
Approximately 70% of the world's population produce low levels of lactase but they do not all suffer with lactose intolerance. This is because there are many nutritional and genetic factors involved in the ability to tolerate lactose which will cause variation between different individuals.
The frequency of lactose intolerance varies considerably between different ethnic groups and populations.
The lowest rates of lactose intolerance are seen in white North Europeans, North Americans and Australasians ranging from 4.7% in a British population to 17% in Finland and Northern France.
The highest rates tend to be found in South America, Africa and Asia with approximately 50% of the population affected and almost 100% in some Asian countries.
Ethnic groups also tend to lose lactase activity differently e.g. Chinese and Japanese lose 80-90% of lactase activity within 3-4 years after weaning, Jews and Asians lose 60-70% over several years post weaning and white Northern Europeans may take up to 18-20 years for lactase activity to reach its minimal expression.
The symptoms of lactose intolerance may include flatulence (production of excess gas in the gut), abdominal bloating, diarrhoea and stomach cramps. In extreme cases nausea and vomiting may also occur.
The symptoms can occur anytime from 15 minutes to 2 hours after consumption of lactose-containing food or drink and will vary depending upon the amount of lactose consumed and the ability of the specific individual to tolerate lactose.
As mentioned earlier, not everyone who has difficulty digesting lactose will suffer the symptoms of lactose intolerance.
Lactose intolerance can be diagnosed using several techniques:
The lactose intolerance test
This test involves consumption of approximately 50g of lactose followed by blood glucose tests after a period of 30 minutes.
The blood tests measure glucose concentration in the blood and will indicate whether sufficient lactase is present to digest the lactose into its absorbable components -glucose and galactose.
If blood glucose levels are raised this indicates that sufficient lactase is being produced to tolerate lactose. If they are not raised it suggests that lactose is not being digested and that insufficient amounts of lactase are being produced.
Hydrogen breath test
This test also involves consumption of approximately 50g of lactose, followed by measurement of hydrogen levels in breath over the following 3-6 hours.
Hydrogen is produced as a by-product from the bacterial fermentation of lactose that has escaped digestion and has subsequently passed into the large intestine.
Elevated breath hydrogen levels will indicate that levels of lactose reaching the large intestine are also elevated which suggests that the individual is not digesting lactose and is therefore intolerant.
This test is thought to be the cheapest, most economical, non-invasive and reliable test to measure lactose intolerance.
In certain cases, patients will undergo a minor operation to remove a small sample of the intestine in order to study the cells which produce lactase.
This technique is usually avoided as other less invasive and more reliable tests are now available.