Lactose intolerance manifests itself by one or many of the following symptoms: bloating, diarrhea, and flatulence, which occur after lactose consumption.
Lactose, the sugar naturally found in milk and dairy, is usually transformed in the intestine by lactase, an enzyme, into glucose and galactose, both simpler sugars used by our body for energy and various functions. The activity of lactase is high during infancy and slowly decline after weaning. In some individuals, for whom the activity of lactase is reduced, undigested lactose consequently enters the colon where it is fermented by the resident microbiota (the microorganism population that lives in the digestive tract). Bacterial fermentation leads to the formation of gas (hydrogen, carbon dioxide, methane), lactic and acetic acids, which increases gut transit time and intracolonic pressure, resulting possibly in bloating, diarrhea, and flatulence.
The amount of lactose that triggers these symptoms differs among individuals. Most of individuals with difficulty to digest lactose (lactose maldigestion) can consume lactose in dairy foods in modest amount, up to 12 g of lactose in one intake or up to 24 g in small amounts across the day during or at the end of a meal, without experiencing the symptoms mentioned above.
For that matter, the European Food Safety Authority has issued a scientific opinion that claims that the consumption of live yogurt cultures in yogurt, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, improves digestion of lactose in yogurt in individuals with lactose maldigestion.