Introduction:
Adult-type hypolactasia is a physiological and genetically conditioned process, which is manifested by a regular decrease of the intestinal lactase activity from childhood to adulthood. It results in an incomplete digestion process in the small intestine, which can lead to intestinal disorders, and is diagnosed as lactose intolerance (LI). Objectives: The purpose of the study is to determine the type, frequency, and time of intestinal symptoms in patients with hypolactasia.

Material and methods:
The study covered 200 patients aged 10–23 years, who were divided into the following three age groups: 10–14 years old (36 children), 15–19 years old (77 teenagers), and 20–23 years old (87 university students). The protocol included a hydrogen breath test (HBT) with lactose and the monitoring of patient’s complaints during the test.

Results:
In the group of 48 patients with hypolactasia, in 31 (65%) patients, some intestinal problems were reported, and they comprised 47% of teenagers and 85% of university students. Symptoms were most frequently observed in the 60th and 90th minute after lactose had been administered, mainly in the form of abdominal pain and flatulence (19% and 16%, respectively) and – less frequently – as nausea and loose stools (4% and 2%).

Conclusions:
In patients with hypolactasia, the administration of the aqueous lactase solution leads to adverse reactions – abdominal pain and/or flatulence rather than nausea and loose stools. The proportion of people with hypolactasia in whom the symptoms of lactose intolerance manifest themselves is growing as the patients grow older. Symptoms were diagnosed among half of the teenager population and in the majority of university students.