Some people use the words “allergy”, “intolerance” and “sensitivity” interchangeably. Often, these people are the people who do not experience any of those things, so there is no reason for them to have deeper understanding of the terms apart from “thing or substance that one avoids”. Other people have very strict and specific definitions about which word means what. Usually, these people are the people experiencing one or more of these conditions. Because ALL people tend to feel strongly about all things health, and all things nutrition, internet wars often break out.
Keep in mind these are not strict medical diagnoses, but umbrella terms, so there is no “official right answer”. Thus, even when explaining/exploring what’s what, we have to include a lot of caveats around “most”, “usually” and “some”. Also, if your conversation partner does not differentiate between these terms, and you do, and you are in the mood to educate, you’d better be prepared to be nice about it. Otherwise, all they will hear is angry sounds.
The term “allergy” implies an immune system reaction. The symptoms vary in severity from mild to moderate to severe (and can include death). Those allergic to a particular food avoid ALL exposure to that food. Because “allergy” implies a more severe reaction, and can be life threatening, it is often “taken more seriously”. As a result, some folks use that term when eating out or talking to others, whether or not they have an allergy, as that term just elicits less questioning.
The term “intolerance” implies a digestive system reaction, but can be used for nonspecific symptoms like headaches, or any unpleasant physical sensation. The symptoms are less severe than those of an allergy (think “peanut allergy” vs. “dairy intolerance”), and are often delayed. So, while those with an intolerance aim to minimize exposure to that food, they may choose to consume certain offending foods as an exception and “pay the price”.
The term “sensitivity” can refer to any bodily system, and any substance (e.g. fragrance sensitivity, caffeine sensitivity). The symptoms are usually mild, but still unpleasant. [Note that some save the term “sensitivity” for something that can be detected in blood (e.g. IgE test).]
Personally, I prefer the term “sensitivity” to the term “intolerance”, as it strikes me as more accurate. Most folks with an “intolerance” CAN actually tolerate the food in question in small amounts. For example, the vast majority of human adults have a reduced ability to digest lactose after infancy. That means ingesting large amounts of dairy on an empty stomach would not “sit well” with most of us. Yet, most also do not have an issue with SOME milk in their coffee, or SOME cheese on their salad.
Now, onto the “SO WHAT” part of this discussion.
If you are at a family picnic, and your brother says he is allergic to eggs, when you KNOW that “OMG, he is SO not allergic to eggs, he just doesn’t like eggs!”, AND you feel like getting into it, then… grab a beer, and off you go. Enjoy.
If you are working with someone in a coaching capacity, the “let-me-set-you-straight” conversation is almost never worth it. Whether your client is allergic to eggs, or just hates eggs does not matter. It just means this client won’t be slamming egg whites as their main protein source. But, you, the coach, have many other ideas for protein sources, don’t you?