You or someone you know may have been told that they are lactose intolerant, and thus avoid drinking milk and eating dairy products.
The second is a genetic mutation causing Lactase Deficency. This means that your cells cannot produce enough lactase to breakdown the lactose from your diet.
Third, the topic of this post, is a diet induced reduction in lactase production. In individuals with the genetic ability to form lactase, the enzyme levels may drop when they have not been consuming dairy on a regular basis.
Simply put, the more lactose you consume – the more lactase you produce. If you eat milk products on a regular basis you will produce more lactase. (Here is a fun video that explains why: http://youtu.be/oBwtxdI1zvk)
However, if you are allergic to milk or have a genetic deficency this method will be of no help and could be dangerous; this is for those who have the machinery to produce lactase, but have lowered their production by lowering their lactose ingestion.
One study provides evidence for including yogurt as a way to improve symptoms, since the bacteria in yogurt will help you break lactose down.