According to one lab that tests for "toxic food syndrome," eating green peppers may cause bloating or lethargy. Lemons might trigger headaches. Other common foods like corn, soy, egg whites, whey and chicken "may act like a poison in your body," the website warns.

This company and others promise to detect such hidden problems with blood tests that can range in cost from hundreds to thousands of dollars, depending on how many foods are tested for.

Other health practitioners may say they can diagnose food sensitivities by assessing muscle strength, by analyzing hair, gastric juice or body tissue, or by reading the body's "energy pathways." Consumers are told that dietary triggers can cause gastrointestinal complaints such as heartburn or irritable bowel syndrome as well as fatigue, attention deficit problems, autoimmune diseases and arthritis.

But allergists and gastroenterologists say that although food intolerance does occur — most of it involving specific food sugars like lactose or fructose — the tests being marketed to consumers have no scientific basis. Blood tests for food sensitivities are prone to false positives that can lead people to eliminate harmless foods from their diets, they say.

The best way to test for the problem is to eliminate various foods from the diet until the symptoms clear, then reintroduce them one at a time, experts say. None of the other tests is recommended by U.S. or European allergy or immunology societies or the National Institutes of Health.

"Blood testing is confusing to patients," said Elana Lavine, a pediatric immunologist in Toronto who now spends part of her time counseling anxious parents whose children have undergone food sensitivity testing. Armed with their itemized results, which list dozens of forbidden foods, they ask her what to feed their children.

Part of the confusion lies in the difference between food intolerances and allergies, terms that are often used interchangeably by testing companies, health practitioners and even in peer-reviewed medical journals.

In an allergic reaction, the immune system overreacts to a food by producing an antibody called Immunoglobulin E that causes hives, vomiting, diarrhea and respiratory problems, among other symptoms. To diagnose an allergy, allergists use a blood test that checks for IgE, skin prick testing and other methods. The gold standard is an oral food challenge, which involves eating small doses of the suspect food under medical supervision.

Food intolerances are unpleasant reactions that do not involve the immune system, according to the National Institute of Allergy and Infectious Diseases. They can be caused by enzyme deficiencies, sensitivities to food additives such as sulfites and monosodium glutamate, or reactions to naturally occurring chemicals.

For example, people who lack an enzyme needed to digest sugar in milk have lactose intolerance. Sulfites used to preserve dried fruit, canned goods and wine trigger asthma attacks in sensitive people.

Adverse reactions to wheat or the protein gluten come in several forms. Celiac disease is an immune system reaction to gluten that causes inflammation in the small intestine. A wheat allergy is an allergic reaction to wheat, almost always caused by the gluten. And gluten sensitivity means a person has symptoms after ingesting gluten but doesn't have either of the other conditions, said Stefano Guandalini, founder and medical director of the University of Chicago's Celiac Disease Center.

Lavine and other critics say the food sensitivity tests being marketed are muddying the waters for people with legitimate allergies, which are on the rise. There's a big difference between a child who has a true milk allergy and another who has been labeled with a milk "sensitivity," said Lavine.

Blood tests are a common way to test for sensitivity or intolerance; many of these also involve an antibody, this one called Immunoglobulin G, or IgG.

The tests often purport to check for sensitivities to hundreds of common foods, many of which rarely trigger food allergies, such as sugar or yeast. Blood is exposed to a panel of food proteins, and the labs measure the degree of IgG antibody that binds to each food.

But while IgE can indicate the presence of an allergen, IgG hasn't been shown to be a similar marker for intolerance. Instead, IgG is believed to indicate exposure to food and possibly even tolerance, Lavine wrote in the Canadian Medical Association Journal.

"There is no IgG testing of value," said Robert Wood, a professor of pediatrics and chief of pediatric allergy and immunology at Johns Hopkins University in Maryland. "All of us make IgG to the foods we eat, and they are not related to disease, including food intolerance."

Meanwhile, nearly everyone who takes these tests is told he or she has some kind of intolerance. One Florida lab boasts that "95 percent of the people we've tested show that one or more foods they regularly eat cause a toxic reaction in their body."

Proponents of the testing, primarily integrative physicians or alternative health practitioners, argue that the tests can be useful even if they are imperfect. IgG-based testing "showed promise, with clinically meaningful results," according to a 2010 review published in the journal Nutrition in Clinical Practice.

Some suggest that the test results could be used to guide which foods are chosen for testing through elimination from the diet, a trial-and-error process that can be time-consuming and difficult.

Peter Whorwell, a professor of medicine and gastroenterology at the University Hospital of South Manchester in Britain, found in 2004 that using IgG antibodies to guide food elimination diets may be effective in reducing the symptoms of irritable bowel syndrome.

The tests have particular appeal to people who have been coping with chronic symptoms but repeatedly hear: "We can't find anything wrong." When Jeffrey Sesol, of Homer Glen, started feeling strangely fatigued and achy, he went to his internist, a gastroenterologist, a neurologist and a rheumatologist, but no one had an answer.

"Grasping for straws," he went to see chiropractor Nicholas LeRoy at the Illinois Center for Progressive Health in Chicago.

LeRoy took Sesol off the acid reflux medicine he'd been taking for years — something no other doctor suggested — and ordered food allergy testing. Out of 150 foods tested, 33 registered positive, including yeast, wheat, eggs, milk, beans, cheese, garlic, ginger, nuts, lemon, mushrooms, rice, sesame, sugar and squash.