There are several types of reactions your body can have to food. Some are digestive responses and some are immune system reactions to the proteins in the food.
There is a lot of confusion around this, so I'm going to break down a few of the most common and misconstrued reactions.
Food Intolerance
In the case of lactose intolerance, you lack the enzyme (lactase) that helps you digest the sugar in dairy (lactose) so you eat ice cream and have a pretty immediate and obvious digestive response that leads you running to the bathroom.
The symptoms are pretty obvious to you but you can mitigate this discomfort by avoiding foods containing lactose and choosing lactose-free dairy products or in some cases taking a digestive enzyme to help you digest the food a little better.
Food Allergy - IgE reactions
Another type of reaction is what I call a full blown food allergy where your immune system has somehow misidentified a food as an invader to your bod... causing an anaphylactic or otherwise severe reaction to a food typically within 2-4 hours of eating it.
Again, this type of reaction is usually quite obvious and easily identified after the first or second time the potentially life threatening reaction occurs.
Food Sensitivity - IgG reactions
There is yet another type of reaction that still involves the immune system attacking a food as if it's invading the body, but it's more delayed and less severe.
The reactions might be days later and could present as joint pain, headache, skin issues, digestive discomfort, congestion or a host of other sometimes bizarre symptoms.
Since these don't happen right away, and are somewhat mild, these are the hardest to pinpoint on your own and often get "shrugged off" as just part of who you are.
These are hidden food sensitivities or IgG reactions in your body.