in the movie World's Greatest Dad, we learned the fact that Andrew has asthma, then Robin Williams said: "Then he shouldn't be having milk products." Is this true or is it just a joke?
Maybe it's a joke because "Is milk good or bad for you?" is question open for polemic like many other food products.
If you aren't allergic milk can only help, but if you are, like 2% of children then it's bad for you.
A recent study published in the medical journal Thorax suggests that full fat milk and butter might be the key to preventing asthma. In a study of 3000 three year olds, it was found that those who consumed most dairy products could be up to 4 times less likely to develop asthma symptoms. This may be attributed to the fatty acids found in full fat dairy products.
Asthma attacks (worsening of asthma symptoms) can be triggered by allergies. There are some food additives or preservatives that people have reported as triggering asthma symptoms.
However many people with asthma also have allergies, and allergic reactions to food can cause symptoms similar to asthma. It is known that around 2% of babies are allergic to cow's milk.
Some parents have concerns about the amount of mucus created when their children drink milk, and its affects on the child's asthma.
There's an article from the Canadian Family Physician Journal that directly addresses this question:
Many parents of children with asthma are becoming increasingly reluctant to add milk to their children’s diet because they believe it will worsen their children’s asthma owing to increased mucus secretion. Recognizing the importance of milk as part of a healthy diet in supporting growth and calcium consumption, is it advisable to restrict milk in the diet?