By Dr. Peter Lio
For many years, most patients—and doctors—have operated under the assumption that food allergies drive atopic dermatitis, not the other way around. That what you put in your mouth ends up on your skin. Unfortunately, many of my patients underwent very strict dietary changes and did not have much success with their skin, despite what the Internet seems to preach.
A new body of research, however, is accumulating that at least some of the time, we have been barking up the wrong organ. The problem of “leaky skin” may be just as important as “leaky gut,” particularly when you consider that the skin is always exposed to the environment. It is the primary barrier to toxins and antigens of all kinds. The skin is full of mast cells, the first line of effector cells that are mobilized by the presence of an allergen. They are the ones that produce the wheals and flares that show up on a skin-prick test. They are present in the skin to help defend against staph bacteria—germs abundant on the skin in patients with eczema–which when we scratch too hard can result in infection. They are also part of the body’s defenses against intestinal parasites. Why do we need protection against intestinal parasites in our skin? Because some parasites, notably hookworms, use the skin as their point of entry. “Larval invasion of the skin might give rise to intense, local itching, usually on the foot or lower leg, which can be followed by lesions that look like insect bites, can blister (“ground itch”), and last for a week or more.”
If you consider the sequence of the atopic march, it almost invariably begins with eczema, and progresses to seasonal allergies, asthma, and food allergies, although not necessarily in that order. People wonder how an infant who has never eaten peanuts or tree nuts can be sensitized to them. Considering the persistence of peanut residue in the home environment, a weak skin barrier may be like leaving the back door of your house unlocked. The mast cells in the skin are programmed to respond vigorously to hookworms, but when vulnerable skin encounters an otherwise harmless allergen like peanut, it may mistake its proteins for something more sinister.
Research by Raif Geha, MD of Boston Children’s Hospital, and others shows that skin deficient in a protein called filaggrin is particularly vulnerable to exposure. Eric Simpson, MD and colleagues at Oregon Health Science University take this a step further: they suggest that by protecting the barrier function of the skin using moisturizers, one can possibly delay or even prevent eczema and food allergies! This is been incredible to me because now I can tell patients and their families that we are not simply treating the symptoms, but that by protecting their skin barrier function–even though it must be done artificially–we are possibly protecting them from developing worsening eczema, food allergies, and other allergies in the long run.
A recent article called “Food allergy: Insights into etiology, prevention, and treatment provided by murine models” in the Journal of Allergy and Clinical Immunology makes all these points emphatically, and suggests that the skin may actually be a more powerful entry point for sensitizing a child to foods than eating. “Epidemiologic data suggest that sensitization to peanut protein can occur in children through exposure to peanut in oils applied to inflamed skin, whereas early oral exposure to food antigen induces tolerance. Food allergen consumption at home correlates with the incidence of food allergy. Furthermore, loss-of-function mutations in FLG, a gene that encodes the epithelial barrier protein filaggrin, conferred increased risk for AD and other allergies, including peanut allergy. These observations led to the hypothesis that the altered barrier function in AD skin might facilitate cutaneous sensitization to food antigens, potentially leading to the development of food allergies. We have recently used a mouse model of allergic skin inflammation with many features of AD and AD-associated asthma to demonstrate that epicutaneous sensitization with the food antigen results in IgE-dependent expansion of intestinal mast cells and IgE-mediated anaphylaxis on oral challenge.” Avoiding triggers such as allergens and irritants is critical as well, but without a good substrate for the barrier, it can become an impossible task. Just as repair of the digestive system has recently begun to attract attention as a means to treat food allergies, so, too, should repair of the body’s largest and most exposed organ, the skin.
I often compare the situation to a house that is on fire. Even though we’d like to understand what caused the fire in the first place, once the fire is burning, it is critical to put it out and protect the rest of the home. It might not look pretty, but you need to keep animals, bugs, and intruders out of the home, while protecting all that is precious to you on the inside. Thus, we often rely on anti-inflammatory agents like topical steroids to put out that initial fire, but we must also fireproof the structure for the future. Thus, we try to lean heavily on protecting and rebuilding the skin barrier with moisturizers and natural oils and such, chosen carefully to avoid the allergens of course.
These, among other reasons, are why I think that food allergies and eczema are so challenging: they truly are multifactorial and do not have one easy answer. That is also why I am so very suspicious of people preaching about “cures”, and easy solutions. So many patients have this problem and it is so disruptive, that such a solution would literally change the world overnight. Not that we don’t continue to hope!
I think that there is such a powerful mythology around this area, that we need clear thinkers with fresh ideas to help us out of this mess. Outside-in thinking from is one such idea that has really made an impact for me.
Peter A. Lio, MD is an Assistant Professor of Clinical Dermatology & Pediatrics at the Northwestern University Feinberg School of Medicine, and a Diplomate of the American Board of Dermatology. Dr. Lio received his medical degree from Harvard Medical School, completed his internship at Boston Children’s Hospital and his dermatology training at Harvard.
He served as a full-time faculty at Harvard (Beth Israel & Children’s Hospital Boston) from 2005-2008 before returning home to his native Chicago to join Northwestern and Children’s Memorial Hospital. He is also a trained acupuncturist and a leader in the Chicago integrative health care community. This is his second piece for this website. You can read the earlier post here.
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