Lactose intolerance

Clinical significance
In most adults worldwide, the activity of the enzyme lactase (responsible for digesting lactose, the sugar in milk, in the intestine) begins to fall between the ages of 2 and 3 and this is complete between the ages of 5 and 10. This used to be called lactose intolerance, as the condition was considered rare and abnormal, whereas today, since it is recognised as the most common condition, it is called lactase “non-persistence”, as opposed to the “persistence” phenotype common in northern European countries, especially Sweden and Denmark. The prevalence of the C/C genotype (lactase “non-persistence”) is only 15% in Germany, while in Mediterranean countries it reaches 50%. In Italy about 30% of people have the lactase “non-persistence” genotype, being homozygous wild type (C/C), 60% are heterozygous (T/C), while the “tolerance” genotype (homozygous T/T) is rare (10%).
The genetic defect investigated by the test consists of the replacement of a thymine (T) with a cytosine (C) at position -13910 in the regulatory region of the lactase gene. If this variant is present in homozygous form (C/C), there is a total lack of the enzyme lactase, making it impossible to digest lactose and causing gastrointestinal symptoms such as wind, bloating, cramps and diarrhoea, as well as symptoms that are harder to interpret, such as dizziness, insomnia, skin irritation and even, according to recent literature, depression. In heterozygous people (T/C), the 50% reduction in lactase activity is normally enough to ensure lactose digestion (according to some authors, under particular conditions such as stress, heterozygotes also have some tendency to develop intolerance).