lactose intolerance

Introduction

lactose cannot be absorbed by the gut wall, absorption relies upon it being broken down into glucose and galactose which can be absorbed
mammals lose their ability to break down ingested lactose after weaning as unnecessary production of lactase in the duodenum has an energy cost and evolution generally avoids persistent unnecessary energy costs to the cell
mammals may also become lactose intolerant due to secondary causes such as:
- injury to small bowel - eg. gastroenteritis, giardiasis, chemotherapy for cancers, inflammatory bowel disease (IBD), coeliac disease
congenital lactose intolerance is an extremely rare genetic disorder in which little or no lactase is made from birth
lactose intolerance should not be confused with milk allergy which affects 4% of people

the ability for humans to digest lactose into adulthood evolved in several human populations independently, probably as an adaptation to the domestication of dairy animals 10,000 years ago
2/3rds of the global population are lactase deficient as adults and thus have lactose intolerance
- this varies depending upon ethnicity:
  - < 20% in eastern Europe, Scandinavia, UK, USA, Australia
  - 20-40% in central Europe, Canada
  - 40-60% in Mediterranean Europe and India
  - 60-80% in Mexico, Central America, northern Africa, Middle East, western areas of Sth America
  - > 80% in Greenland, Alaska, Brazil, Argentina, southern half of Africa, China, SE Asia and Indonesia

the LCT gene provides the instructions for making lactase
the specific DNA sequence in the MCM6 gene helps control whether the LCT gene is turned on or off and perhaps around 4,300yrs ago, some humans acquired a mutation in this gene which didn't turn the LCT gene off in adulthood, and given their survival relied upon milk from dairy production, this gene became prevalent in the descendants
he LCT and MCM6 genes are both located on the long arm (q) of chromosome 2 in region 21

these are similar to those which occur when taking lactulose for treatment of constipation and generally occur half an hour to 2hrs after consumption
flatus related due to fermentation within the colon producing hydrogen, carbon dioxide and methane:
- increased flatulence, borborygmi, abdominal cramps, nausea
osmotic-related due to increased osmotic pressure within the colon increasing water content of stool:
- diarrhoea

the lactose hydrogen breath test is considered to be the gold standard for the clinical evaluation of lactose malabsorption
- may give false negative results in hydrogen nonexcretors, in which case, a breath methane measurement or a lactose tolerance test can be used