Instead, they can occur for a variety of reasons such as an enzyme deficiency as in lactose intolerance, the inability to digest milk sugar due to a lack of the enzyme, lactase. Or they can occur because the food contains a toxin, such as bacteria or poison. Another adverse reaction is chemical in origin and is called a pharmacologic food reaction. An example would be caffeine. Some people are more 'sensitive' to caffeine and can drink only one cup of coffee without getting the jitters, while others can drink several cups and not be affected.
Babies are more likely to suffer from food allergies.
Most are outgrown by the age of three, perhaps because of a maturing immune system. The food allergies that seem to be an exception to that, and tend to last a lifetime are allergies to peanuts, fish and nuts. She should nurse exclusively until at least 4 to 6 months. The effects can be long lasting. Another effective tactic in preventing food allergies is to delay introduction of the common allergens.