Mike Hopson of St. Louis doesn’t have celiac disease, and he’s not allergic to dairy products. But when January rolled round, he was determined to find a diet that would help him lose weight and “feel better,” he says. That’s when he started the Bulletproof Diet — one that omits gluten, sugar, additives and most dairy.

“It’s helped immensely,” says Hopson, 38. “I’ve lost 15 pounds [and] 5 percent of my body fat, and I just feel better.”

Hopson says he’s never been overweight — just slightly heavier than he’d like to be.

“I probably start a diet every January, and this one just happened to click,” he says, adding that he plans to continue the regimen.

If you’re going gluten-free, vegan or dairy-free simply to drop a few pounds, rather than for a health condition like celiac disease, it’s wise to talk with your doctor first, says Jennifer McDaniel, a registered dietitian and owner of McDaniel Nutrition Therapy in St. Louis. “[Eliminating] several food groups without medical reason warrants consideration of health pros and cons,” McDaniel says.

Though people tend to expect black or white nutrition advice, “Nutrition recommendations come in many shades of gray,” she adds.

A diet that eliminates food groups should be evaluated by a registered dietitian — and not pursued solely based on advice from bloggers or diet books, McDaniel says. She and other nutrition experts share what you should know before changing your diet for reasons other than a medical condition.

Gluten-free

Gluten-free diets require substantial changes to your eating habits.

They’re intended for people who have problems digesting gluten because they either have celiac disease or digestive symptoms — not for weight loss, says Dr. Robert Shmerling, an associate professor of medicine at Harvard Medical School and clinical chief of rheumatology at Beth Israel Deaconess Medical Center in Boston.

“I would encourage someone thinking about that to step back and examine the entire premise because gluten-free diets are not for weight loss. They never have been,” Shmerling says.

Many gluten-free foods lack folic acid, also known as vitamin B9, an important nutrient that can prevent birth defects among women who are pregnant or plan to become pregnant, Shmerling says. Folic acid also helps prevent anemia and supports the growth of hair, skin and nails.

McDaniel adds that gluten-free does not mean calorie-free. “Quite often, gluten-free substitutes are more caloric than their standard counterparts. Manufacturers add extra fat, salt or sugar to make up for the missing flavor and texture of gluten,” she explains.

Vandana Sheth, a registered dietitian in Los Angeles and spokeswoman for the Academy of Nutrition and Dietetics, says many of her clients who have tried gluten-free diets for weight loss have actually gained weight. For those who do shed pounds, the weight loss may be explained by an avoidance of grains and reductions in the amount of food they used to consume.

“People often say, ‘I have less bloating and I feel better,'” she explains. “They might lose weight, but it may not be from the gluten they’re avoiding.”

Avoiding gluten can also lead to a vitamin deficiency, since wheat is rich in thiamine and other B vitamins, McDaniel says. Foods with gluten such as breads, pasta and cereals contain whole grains that are known to combat the risk of heart disease, Type 2 diabetes and even certain cancers, she adds.

For those who must follow a gluten-free diet, she recommends eating foods that are naturally gluten-free such as fresh fruits and vegetables, nuts and beans, fresh meats and gluten-free whole grains, including corn, brown rice and buckwheat.

Vegan

It’s easy to try a vegan diet these days, thanks to better restaurant menu options and recipes available online, but what are the health risks?

“The vegan diet is no magic bullet for weight loss or disease prevention,” McDaniel explains.

The potential health risks of going vegan — which requires dieters to ditch all meat products, fish, dairy and eggs — without consulting a nutritionist first include shortfalls of calcium, protein, iron, zinc, iodine, essential fats and vitamins B12 and D, she says.

“Zinc from plant-based foods is not absorbed as well as zinc found in meat. Vegans should include zinc-rich foods such as whole-grain breads, sprouted legumes and grains,” she says.

Vegans — unlike vegetarians — also exclude fish from their diet. That’s why some who follow a vegan lifestyle have lower levels of omega-3 fatty acids, which are believed to lower the risk of heart disease while improving cholesterol levels. One way to supplement is with nuts, seeds, oils and vitamins. Before going vegan, it’s also smart to consider vitamin D and B12 foods or supplements, McDaniel says.

The National Institutes of Health Office of Dietary Supplements reports that vitamin B12 keeps your blood cells healthy and elevates your energy levels; without it, you can experience tiredness, weakness, constipation, loss of appetite and weight loss.

Some foods containing B12 don’t provide sufficient levels of the nutrients you’ll need each day, and vitamin D is only found in animal-based foods. “While foods such as fermented soy foods, seaweeds, spirulina, and sourdough bread contain B12, they provide only B12 equivalents that have no actual B12 activity,” McDaniel says.

Bone pain and muscle weakness are symptoms of a vitamin D deficiency. To prevent this and supplement their diet, Vegans can incorporate vitamin D-fortified products such as soy or rice milk, orange juice, breakfast cereals, margarines and UV-exposed mushrooms. You should also make sure you’re getting a healthy exposure to sunlight.

Dairy-free

Unless you have a food allergy or intolerance, dairy should remain part of your diet. It’s a “nutrient gold mine” many Americans already lack, McDaniel says. Milk, butter, cheese and yogurt provide calcium, potassium, vitamin D, phosphorus zinc, vitamin B12 and riboflavin.

Without dairy, you’d have to consume 8 servings of plant-based or fortified foods — such as leafy greens, broccoli, chia seeds, flaxseeds, sesame seeds and quinoa — each day to meet recommended daily intake levels of 1,000 to 1,500 mg, according to McDaniel.

If you’re seeking a strict dairy-free diet, be wary of hidden dairy ingredients in energy bars, chocolate, breads and baked goods. Although the Food and Drug Administration requires food containing milk to declare it on labels, it’s still important to carefully examine them.

“However, not all foods and products are covered by FDA allergen labeling laws, so it is still important to know how to read a label for milk ingredients,” McDaniel says.

Some of the most common hidden ingredients include: whey, whey protein, lactose and artificial butter.

Social Considerations

Maybe you’ve mastered the labels. What about the menu at your favorite restaurant? Sheth says there’s a social impact to eliminating food groups from the diet: “It affects how you feel in social situations, family planning, attending social potlucks.”

Although many restaurants now offer alternatives such as gluten-free pizza or vegan entrees, Sheth says these may not provide all your essential nutrients. “As long as you’re OK with that, you can compensate with another meal,” she says.

For those who can’t eat specific foods or ingredients, attending a party or family dinner may require some planning. Sheth says eating before you go to a social event and only choosing to snack while you’re there is one way to satisfy your dietary restriction.

Talk to Your Doctor

The bottom line: A balanced diet that’s portion controlled is going to be more effective for weight loss than a medically unnecessary elimination diet, when combined with exercise, Shmerling says. Needlessly restrictive diets, he says, “may actually harm you because you’re missing nutrients by restricting your diet dramatically,” he says.

If you’re considering one of these diets, talk to a physician or dietitian, he emphasizes. And be wary of diet misconceptions. For instance, many people believe a gluten-free diet is healthier than a regular diet, Shmerling says. “It’s not true for most people.”

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What You Should Know Before Restricting Food Groups originally appeared on usnews.com