Researchers started messing with the food chain via growth hormones in meats and genetically altering foods. That seems like a possibility. Artificial food additives is another possibility. When I was a kid bologna tasted good and never upset my stomach. Now one piece of it and within minutes I'm sick as a dog.
Nuts in various forms have been with us and eaten safely ever since. Many kids were raised on Carvers discovery of peanut butter. It has always been considered healthy. But when man started messing with our food chain {improving it as they call it} the problems seemed to become more profound.
Now even a loaf of bread can last two weeks and taste just as horrible & stale two weeks later as it did the day you bought it. Remember the term day old bread? It would be highly delicious and far more fresh tasting compared to the preservative filled loafs now being sold in stores. Some persons tolerance to these altered foods is much lower than others. MSG has to be one of the most unhealthiest food additives created by man IMO. I think food allergies for the most part are a result of mans tampering with what was once good food.
I think that's a very good explanation.