An adult's ability to produce the lactase enzyme is determined almost entirely by his or her genes. Because of this, simply knowing your genotype is almost enough to know whether you continue to produce the lactase enzyme. However, whether you are actually lactose tolerant or intolerant can depend on non-genetic factors, such as the composition of your gut bacteria. And as with many traits, how lactose intolerant you are depends on your individual circumstances—you might have different outcomes from someone else with the same genotype.

For healthy individuals with secondary lactose intolerance, it may be possible in some cases for the bacteria in the large intestine to adapt to an altered diet and break down small quantities of lactose more effectively‏⁵¹ by habitually consuming small amounts of dairy products several times a day over a period of time. This is not suitable for people who have an underlying or chronic illness which may have damaged the intestinal tract in a way which prevents the lactase enzyme from being expressed.
Some studies indicate that environmental factors—more specifically, the consumption of lactose—may "play a more important role than genetic factors in the etio-pathogenesis of milk intolerance",.‏⁵²