With swimwear season in full swing, many would-be beach bodies are hitting the gym — and the books. Diet books are on many bestseller lists, and that’s not surprising, says physician David Katz, director of the Yale University Prevention Research Center.

“We know what to feed our aquarium fish, but we don’t know how to feed ourselves,” Katz says, so we can’t resist the lure of an easy answer. But do these top diets deliver what they promise? Here’s what the experts had to say about some of the most popular diets and the books behind them.

‘Wheat Belly’: Cut the gluten

In the bestselling book “Wheat Belly,” cardiologist William Davis writes that modern, genetically modified strains of wheat are the cause of most Americans’ health problems, including expanding waistlines, arthritis and hypertension. He blames gluten, a protein found in wheat and related grains, such as barley and rye, that can cause an autoimmune response in people with celiac disease.

According to Davis, all people fare poorly on gluten, whether they have celiac disease or not, and swapping gluten-loaded breads and pastas for vegetables, meats and other wheat-free foods will lead to weight loss and better overall health.

The problem with this premise is that there’s little evidence to support it, says Lawrence Cheskin, director of the Johns Hopkins Weight Management Center and associate professor of health, behavior and society at the Johns Hopkins Bloomberg School of Public Health.

“It’s really a small group of people who have a pathological response to gluten,” Cheskin says. “And for them it’s absolutely essential to eat a gluten-free diet. Everyone else may be limiting their choices unnecessarily.”

Limiting those choices may not always be a bad thing, however, Katz says. “If you cut out crackers and cookies and cakes, you’re taking in a lot fewer calories, and you may lose weight,” he says, “but it has nothing to do with the gluten.”

Katz urges readers to approach Davis’s popular anti-wheat polemic with caution, and not trade one set of unhealthful habits for another. “It’s entirely possible to eat gluten-free junk food, too,” he says. “Now that it’s caught on, there’s a proliferation of highly processed gluten-free foods. You can definitely cut gluten and still get fatter and sicker.”

‘The Paleo Solution’: Stone-age cuisine

The paleo diet also goes against the grain — literally — in its recommendations, which emphasize the foods that humans’ Paleolithic ancestors ate: meats, preferably wild or grass-fed, vegetables, fruits, nuts and seeds. Robb Wolf, a former biochemist, didn’t start the paleo trend, but he presents its scientific case in “The Paleo Solution.” In it, he details the anthropological and biological evidence behind paleo claims that humans haven’t evolved to digest grains and other foods that became widespread after the birth of agriculture and that people can find optimal fitness and health on pre-agricultural fare instead.

Wolf’s argument — along with the paleo diet itself — has its merits, Katz says.

“The paleo diet is a contender for the best diet out there, if you do it right,” he says. That means getting plenty of fiber-rich vegetables and eating game such as wild-caught fish and venison. “But many people use paleo as an excuse to eat hamburgers or hot dogs, and we know that there were enormous differences between the meat our ancestors ate and the meat we have now.”

Doing paleo the right way is also difficult because of its very structure, says Marion Nestle, a professor of nutrition at New York University.

“Any diet that excludes one or more entire categories of foods is difficult for many people to follow,” Nestle says. “For some people, it’s easier to exclude whole categories — wheat, meat, dairy, carbohydrates, et cetera — than to just eat less and eat better. But the more food categories excluded, the more people are likely to give up on the diet.”

‘Clean’: Drinks that detoxify

Exclusion is at the heart of cleansing diets, including the “Clean Program” popularized by celebrities such as Gwy­neth Paltrow. The Clean Program, a 21-day detox based on the book “Clean” by physician Alejandro Junger, requires giving up caffeine, sugar, wheat, soy, red meat, raw fish, alcohol and an assortment of other foods, and replacing breakfast and dinner with homemade smoothies, juices or soups. Junger claims that exposure to toxins in everyday life, through poor dietary choices (think junk food) takes a toll on the body and that his liquid-centric three-week regimen helps the body heal itself.

But the strategy may not be effective or safe, says Rebecca Scritchfield, a D.C.-based registered dietitian and fitness expert. “I really see cleanses as starvation strategies,” she says. “They tend to be not enough calories, they tend to be low [in] or empty of protein, and they tend to be very low in fiber. You don’t need to stop eating food to be healthy.”

Beyond their nutritional deficiencies, cleanse diets such as the Clean Program are simply unnecessary, Katz says. “This concept of cleanses is a totally manufactured bit of pop culture,” he says. “We have extraordinary resources to detoxify ourselves, and if we take care of them, they’ll take care of us. There’s not a shred of evidence that we need any of these cleanse programs.”

‘The Fast Diet’: The hunger game

Like cleanses, periodic fasting is focused on sacrifice. Based on “The Fast Diet,” by British physician Michael Mosley and journalist Mimi Spencer, this diet calls for intermittent restriction: You eat what you want five days a week, but twice a week you semi-fast, keeping yourself to 500 calories a day for women or 600 for men. Mosley and Spencer claim that the occasional deprivation won’t just melt away pounds but can also protect against cardiovascular disease and cancer.

Scritchfield is skeptical. “Fasting can be dangerous,” she says, especially for people with an underlying health problem. “There has been research on very-low-calorie diets and longevity, but the studies weren’t large enough or long enough to draw any realistic conclusions for the average person — and I don’t believe the research was looking at two days of fasting and five days of whatever you want. I think this is one of the worst [diets].”

Katz isn’t convinced, either. “The one potential upside to occasional fasting is the mindfulness it imposes. . . . It offers a dose of concentration and discipline about your food. But it’s an awkward way to live and hard to share with a family.”

‘The Mediterranean Prescription’: New take on an old favorite

The Mediterranean diet is a veteran of the diet scene and not quite as trendy or weight-loss-focused as some of its siblings. But it’s a perennial favorite of doctors and dietitians, buoyed by studies that suggest it can lower the risk of diabetes and cardiovascular disease. In “The Mediterranean Prescription” by physician Angelo Acquista, the heart-friendly diet gets a weight-loss makeover: The book starts readers off with a two-week introduction to the diet designed to encourage weight loss, then transitions to meal plans geared toward long-term weight maintenance and optimal health.

The combination is accessible and easy to follow, Nestle says. “People love Mediterranean diet plans. They are healthy and may advise eating less of certain foods, but they exclude nothing.”

Katz agrees. “It’s a very strong contender for best diet, and the very best thing about it is it’s not a radical concept,” he says. “It’s mixed, and that makes it more familiar and easier for people to adopt.”

But like any other diet, it holds potential pitfalls. Followers may forgo the recommended fish and nuts for bread and pasta, which are elements of the Mediterranean diet that can contain more sugars and fewer nutrients, Katz says. And the diet’s emphasis on whole grains can lead its followers to less-than-desirable processed foods, Scritchfield says.

“If it comes from a box, have your radar on, even if it says ‘multigrain’ or ‘whole wheat,’ ” she says. “If you look at the ingredients list and it says ‘enriched’ anything, it’s not whole grain.”

Ultimately, building a better beach body isn’t about short-term diets or fads; it’s about long-term lifestyle changes that make your body as healthy as possible, Katz says. And those are fairly simple: eating minimally processed, whole foods, eating only when you’re hungry, and getting more exercise.

“Diets are, almost by definition, things you get on and get off,” he says. “It really needs to be about your whole dietary pattern. If you wouldn’t put your 4-year-old child or your 80-year-old parent on this diet with you, it’s a gimmicky short-term fix and not a way of eating better for a lifetime.”

Telis is a freelance science and health writer.