Why are food allergies on the rise? We took a look at the latest science around the top food allergens.

Rachael Moeller Gorman

More than 4 million Americans have food allergies, and the trend is only going up. Allergy rates have jumped from 3.4 percent of kids in 1999 to 5.7 percent in 2015. One reason is that our previous guidelines-to avoid allergens early in life-may have been totally backwards. (Find out how science is reversing its thinking.) Here are some of the other reasons experts say rates have kept climbing.

The Hygiene Hypothesis

This prominent theory suggests that an infant's immune system needs to be exposed to an assortment of germs to train itself not to overreact to harmless things like food. That doesn't happen as often in today's sanitized world, where antibiotics wipe out infections in babies, hand sanitizers kill bugs on grimy hands and urban areas are devoid of microbe-laden soil, animals and farms. As a result, our immune systems have become uneducated and make mistakes distinguishing the good guys from the bad. Studies in humans are few, but associations have been drawn between infants living with pets and a reduced likelihood of allergy and asthma; the same is true of kids raised on farms.

Related: Is Gut Health Linked to Food Allergies?

Early Skin Contact

Many scientists believe infants can be "sensitized" to foods through the skin, priming them for allergies. What that means: when a child's first exposure to a food-like peanut residue from someone's hands-is through an opening in the skin (a cut or eczema rash), they become sensitive to the proteins in that food. When they later eat it, their immune system attacks those proteins full force, thinking they're an enemy. However, when are initially exposed to a food by eating it, the opposite happens and they become tolerant-their bodies recognize the proteins as harmless. Hence the new recommendations to feed infants a variety of foods at a young age. Indeed, some experts think eczema causes some food allergies rather than the other way around. Moisturize your babies' skin from day one to keep eczema-and possibly food allergies-from developing.

Vitamin D Deficiency

In one study, the incidence of peanut or egg allergy was three times higher in vitamin D-deficient infants than in those with healthy levels of it. (And 70 percent of American kids get insufficient amounts of the vitamin, which is linked to healthy immune system development.) But this research is preliminary, so talk to your pediatrician before giving your child a supplement.

Related: Recipes to Get More Vitamin D

Genetics

A recent study examining patterns of small genetic mutations pinpointed five regions of DNA more highly mutated in kids with food allergies than those without them. These anomalies-which are tied in part to immune regulation-account for about 24 percent of food allergy cases.

The Top 8 Allergens

While some kids have allergies to random foods like kiwi, the foods on this list are far and away the biggest offenders. Among kids with food allergies, here's the percentage affected by each of these everyday foods. (A third of the kids are allergic to more than one.)

1. Peanuts 25%
2. Milk 21%
3. Shellfish 17%
4. Tree nuts 13%
5. Eggs 10%
6. Fish 6%
7. Wheat 5%
8. Soy 5%

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