This is a question that I've been asking myself based on moms and dads coming to the clinic with kids reporting if they are sensitive, intolerant or allergic to some foods.
Serious food allergies, particularly peanut, seem to be increasing over time however, I'm unaware of the reason behind it.
There’s been speculation, at least, that it relates to the rise of autoimmune/atopic illness (asthma, eczema, allergic rhinitis) and has to do with insufficient early life exposure to antigens (meaning dirt and germs).
In Israel, where a common baby food is Bamba (think Cheetos but with peanut dust instead of cheese dust), peanut allergy is unknown. Makes me think we may be doing more harm than good in banning nuts from schools -- if parents are buying less nut products cause their school-aged kids can't bring them for lunch, it means less exposure for the school-aged kids.
The globalization of food is making kids to be less exposed to new foods as they're having the same exposure to the same ingredients. You have the same nuts, fruits, vegetables, etc during the whole year, atrophying their immune system as they don't try anything new.
Delaying ingestion of certain things such as peanuts can increase chances of small exposures through broken skin or some other route which can cause an allergy to develop. However Science didn't prove it, yet.
There also seems to be an increase in self-reported allergies and sensitivities without medical basis, where we acupuncturist suggest that allergies happen due to an internal deficiency more than an "allergic" food.
As stronger your immune system is, less chances of developing sensitivities, allergies and intolerance.
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