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Mark Hyman, MD, leans over his lunch of tofu, soba noodles, and beet-carrot-spinach-ginger juice and searches my face. "You have a little bit of—if you don't mind me saying—you have a little fluid and, you know, puffy eyes, and that is often allergy related," he says. He peers at me for another beat, nodding. "A little redness around the lids. I don't usually tell people I see," he adds with a laugh. "But that is often a sign of low-grade inflammation, and it's often gut-related."

Hyman, a practitioner of "functional medicine" (a holistic branch of medicine), is the founder of the UltraWellness Center in Lenox, Massachusetts, and the guru behind the series of best-selling "Ultra" self-help books (UltraPrevention, UltraMetabolism, The UltraSimple Diet, and The UltraMetabolism Cookbook, among others). He used to be the co–medical director of Canyon Ranch and is a staple on the morning-talk-shows-andladies'- magazines circuit. Unlike my puffy self, Hyman looks freshly polished and pressed, with a light tan and a neat khaki suit. Since I was on maternity leave, Hyman—and his publicist—had graciously trekked up to a vegan restaurant near my apartment in New York City to talk about his latest tome, The UltraMind Solution.

The premise of UltraMind is that unhealthy lifestyles and our environment are not only making us fat and diabetic but mentally ill. "Your brain is broken. You know it. You feel it. You hide it. You fear it," Hyman writes ominously. "We refer to our `broken brains' by many names— depression, anxiety, memory loss, brain fog, attention deficit disorder...autism, and dementia to name a few."

The culprit and cure for most psychiatric disorders lies in the gut, Hyman says. Allergies and toxins in food, the environment, and drugs damage it, causing it to become inflamed and to "leak," allowing undigested food and bacteria to slip into our bloodstreams. This leads to autoimmune disorders, malnutrition, and brain damage. To heal, he recommends taking supplements, discontinuing nonessential drugs, and embarking on an abstemious diet often called the gluten-free, casein-free diet (or GFCF), which eliminates all foods containing wheat or dairy, "the most important allergens that lead to brain problems," he writes. (But to get rid of gluten, you also have to cut out barley, oats, rye, spelt, Kamut, and just about any other grain you could make into a decent slice of bread.)

I first came across GFCF when researching a story about autism. Many of the same people who believe autism is the result of overvaccination also believe putting kids on the GFCF can help cure it. Most mainstream scientists say the theory lacks biological plausibility. When I note that Hyman seems to be recommending the controversial treatment for the general populace, he agrees, telling me that children with autism are the "canaries in the coal mine" and that we're all being damaged, just with less noticeable effects. For example, misplacing your keys isn't normal, he claims, and, if not addressed, one day small lapses may become a more serious disease, such as Alzheimer's. "They're the extreme example of brain dysfunction, but the point is the lessons learned in autism can be generalized to everybody who's got a brain," he says. "All of us are affected in some way."

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Hyman isn't the only one who's fingered wheat and dairy as brain-damage-on-a-bagel. A slew of recent books have made similar claims, and several alternative health-conscious friends of mine swear they've cured various ailments with some version of GFCF. After Hyman's off-the-cuff diagnosis, I couldn't help but worry: Were my bleary eyes and "brain fog" from mothering a newborn or signs of diet-induced dementia? I had to admit: I'd tried GFCF a few times for past stories, and it did put a certain skip in my step.

To sort out the claims, I first called Michael D. Gershon, MD, professor of pathology and cell biology at Columbia University College of Physicians and Surgeons, and author of The Second Brain: A Groundbreaking New Understanding of Nervous Disorders of the Stomach and Intestine. Gershon's research, which Hyman cites in UltraMind, focuses on irritable bowel syndrome and has shown how the gut has its own nervous system that operates independently of the brain. Gershon pulls no punches when I ask him about the leaky gut theory. "What you're talking about is largely myth," he says. "They're talking nonsense, and I'm talking reality." While people with inflammatory bowel diseases (Crohn's, ulcerative colitis) can have, in medical lingo, "increased intestinal permeability," he says, IBD is a relatively rare disorder (affecting less than one percent of the population).

One of the ways that wheat and dairy befuddle people, adherents of the GFCF say, is that when these foods are incorrectly digested, high numbers of certain types of protein fragments, or peptides, are created. The peptides, called gluteomorphins and caseomorphins, can supposedly get into the bloodstream through a leaky gut and have an opiate-like effect on the brain. But Gershon describes this as magical thinking, arguing that digestion is too crude to form these peptides in high numbers and, besides, there are several safeguards that prevent the peptides from entering the bloodstream, at which point they'd only be broken down by the liver anyway. Even if they miraculously survived, he says, they "couldn't make their way into the brain, because peptides don't go through the blood-brain barrier. The barrier would have to part like the Red Sea did for Moses and the Israelites." Finally, he says, there's no evidence that opiates cause the brain disorders in question. "Heroin addicts are disturbed in their behavior, but they don't have autism."

Gershon allows that food allergies do exist (they affect about 4 percent of the population). But unless you have an allergy to wheat and dairy, are lactose intolerant ( you don't make the enzyme to digest a sugar in milk), or suffer from celiac disease (an autoimmune disorder caused by gluten intolerance), he says, the GFCF is "worthless."

How do you know whether you're allergic to gluten and dairy? A friend of mine has food allergies, and his reaction— instantaneous swelling, wheezing, itching, and vomiting—is how I'd always thought of the problem: not something you'd fail to notice. But Hyman says this is a different type of immune response and that many of us are suffering from delayed food allergies that can manifest as GI distress, fatigue, joint pain, rashes, and mental disorders. While my friend's classic symptoms are caused by an immediate autoimmune reaction mediated by an immunoglobulin called IgE, Hyman proposes that many more people suffer from reactions of another immunoglobulin, IgG, which can cause problems up to three days after eating a food. To root out these milder allergies, Hyman recommends blood testing for IgG antibodies or an elimination/challenge diet (like GFCF), where you remove and then reintroduce the foods and watch for symptoms.

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But when I call Anna Nowak-Wegrzyn, MD, assistant professor of pediatrics and a specialist in allergy and immunology at Mount Sinai School of Medicine, she tells me there's no evidence IgG antibodies affect humans. She says our immune systems are constantly interacting with our environment and IgGs simply reflect exposure to food—not a bad reaction to it. "Even classic food allergies have to be confirmed by symptoms on exposure, during an oral food challenge test"—i.e., you get sick immediately (within an hour or two). She adds that allergies are more common in children and something you often grow out of but seldom acquire as you age. When food makes us feel bad, it's usually not because we're allergic, she says. It's more likely food poisoning or indigestion.

Okay, so if leaky gut and hidden food allergies were mythical non-diseases, one question still remained: Why do people (including me) feel so energized on the diet? Gershon and Nowak-Wegrzyn had raised the possibility that the placebo effect could be at play—basically, people expect to feel better, so they do. The placebo effect is most powerful in conditions that have a subjective element, like chronic pain and depression. Or maybe you really do feel better, but for different reasons.

I went on Hyman's program as I was researching this story—no gluten, no dairy, no alcohol, no sugar, no red meat, no processed food, daily vitamins, moderate exercise, and regular sleep—and I felt electrified. And why wouldn't I? You don't need to be secretly allergic to cheese to feel good when you're making time for rest and exercise, cutting out alcohol and all the salty and sugary processed foods in your diet, and losing weight because you end up cutting calories. I couldn't keep it up, of course. All that gluten- and dairy-free cookery and those early bedtimes are exhausting—but it was fun while it lasted.

When I told Hyman about the skeptical reception his beliefs had gotten, he was undaunted. "Reductionist researchers" like Gershon, Hyman said, "sometimes can't see the forest for the trees." And it wasn't like he was recommending surgery or drugs with side effects, he pointed out—it was just a healthy diet. He was getting off a plane from Washington, DC, where he'd met with Senators Hillary Clinton and Tom Harkin about health care reform, and he spoke a bit breathlessly into his cell phone as he made his way through the airport. "It's an exciting time. We're on the edge of being able to understand things in more complexity," he said. "Without a doubt, one of the most powerful things I do is [GFCF]. It's extraordinary. Maybe we don't have the right explanation for it. Maybe I'm wrong; maybe [Gershon] is right. To me, it doesn't matter. People feel better."

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