Walnuts are a key part of the heart-healthy Mediterranean Diet. AP/PRNewsFoto/California Walnut Commission

There was a time when vegetarianism was one thin euphemism away from just plain weird. Subsisting on plants seemed like some kind of kooky California fad or college experimentation and was discussed with an eye roll and hushed tone. Then again, there was a time when Whole Foods was the province of hippies, Bill Clinton ate burgers with bravado (the former commander in chief has since gone vegan to stave off heart disease), and vegetarianism marked the rare dietary restriction at a dinner party. Of course, we weren't quite so hungry then for solutions.

We all know Americans have become fatter but less nourished, and that we face a public health crisis, the likes of which we've never seen before. Our children, one-third of whom are overweight or obese, may, for the first time in our nation's history, be dealt a shorter life span than their parents. Blame it on a sedentary lifestyle coupled with the consumption of cheap, processed food. In any case, the prognosis has us scrambling to literally save our lives by learning how and what to eat.

[See Why We're So Fat: What's Behind the Latest Obesity Rates.]

Meanwhile, food sensitivities, allergies, and intolerances, and about a million other complicated factors, from health concerns to culture and ideology, have created a nation of idiosyncratic diets, often as individual as the dieters themselves.

However, as data increasingly points to the protective and restorative power of so-called plant-based diets, many of us are turning to the whole fruits and vegetables our mothers and their mothers insisted upon. That's not to say Americans are adopting vegetarianism (vegetarians represent just 5 percent of the adult population, and a mere 2 percent identify as vegans, according to a July 2012 Gallup poll). Today, we talk about "plant-based diets." That means—most of the time, but we'll get back to this later—a diet with mostly, not exclusively, plants. Or, as popularized by the motto emblazoned on the cover of In Defense of Food, the best-selling book by investigative food journalist Michael Pollan: "Eat food. Not too much. Mostly plants."

"We now have much stronger evidence that replacing red meat with plant sources of protein such a nuts or legumes improves blood cholesterol fractions and is associated with lower risks of heart disease, stroke, and diabetes," says Walter Willett, chair of the department of nutrition at Harvard School of Public Health.

Such data, like studies supporting the health benefits of aMediterranean-style diet, led to the emphasis on plant-based foods in the 2010 Dietary Guidelines for Americans, according to Linda Van Horn, professor of preventive medicine at Northwestern University Feinberg School of Medicine who chaired the dietary guidelines advisory committee. Those guidelines, which are released every five years by the U.S. Departments of Agriculture and Health and Human Services, showed that we're filling up on the wrong things—consuming, for example, nearly triple the recommended limit of solid fats and added sugars while getting only 15 percent of the whole grains and 59 percent of the vegetables we need.

"It's a double whammy," Van Horn says. "You're not only eating more of the things you shouldn't have, you're denying your body the very nutrients it should have."

Among the recommendations, the latest guidelines aim to "shift food intake patterns to a more plant-based diet that emphasizes vegetables, cooked dry beans and peas, fruits, whole grains, nuts, and seeds. In addition, increase the intake of seafood and fat-free and low-fat milk and milk products and consume only moderate amounts of lean meats, poultry, and eggs."

[See Best Diets for Healthy Eating .]

When it comes to what kind of plant-based diet one should follow, well, that's where things get complicated. There's not enough evidence to say that a vegan diet, for example, is healthier than eating a mostly plant-based one that includes some meat, experts say. Such distinctions are currently being investigated by Gary Fraser, associate dean of research at California's Loma Linda University, who is leading a study of some 96,000 Seventh-Day Adventists whose diets range from strictly vegan to non-vegetarian. This is the third iteration of the trial, which has linked vegetarian diets to reduced rates of heart attack and diabetes.

Of course, just eating plants doesn't necessarily confer health.

"A plant-based diet could include consuming large amounts of sugar, refined starch, hydrogenated oils, and soda, which would be about the worst diet possible," Willett says.

Or, take the example of palm oil, which comes from a plant but can cause inflammation in the body, which can lead to disease, says Michael Roizen, director of the Cleveland Clinic's Wellness Institute and co-author of the YOU series of books, which includeYOU: On A Diet. On the other hand, eating "seven baked sweet potatoes and broccoli every night" won't get you enough nutritional variety, he says.

Roizen agrees with Pollan's aforementioned dietary prescription, but he would add to it. "Don't eat those parts of plants that have saturated fat or that are refined or changed into simple sugars," Roizen says. Also: "You can have fun with salmon, trout, and skinless poultry," he says, noting health benefits associated with these foods.

For the purposes of this package, U.S. News defines a plant-based diet as an eating approach that emphasizes minimally processed foods from plants and is built around healthy protein like nuts, seeds, beans, and tofu. It allows for modest amounts of fish, lean meat, and low-fat dairy; you might eat these on a weekly basis, for example. Red meat can have a place in a plant-based diet but should only be eaten sparingly, such as once per month.

In keeping with this definition, we selected 11 diets that fit the profile of a plant-based diet without significant tweaking. A review by our esteemed panel of academics, physicians, and nutritionists weighed safety, healthfulness, and ease of compliance, among other issues, to score the diets.

Their top-pick? The Mediterranean diet, an approach to eating that emphasizes heaps of fresh produce along with seafood, olive oil, and red wine and whose followers historically boast longer life spans and lower rates of cancer and heart disease. Dawn Jackson Blatner's Flexitarian Diet took second place with its flexible approach to plant-based eating that minimizes meat and encourages more plants. The Ornish diet, a holistic approach to health that focuses on disease reversal and includes stress-management techniques, exercise, and social support, came in third place.

However, so many of the diets profiled in Best Diets 2013 may be easily adapted to focus more predominantly on plants. "It's hard to change ingrained habits, but I think you can often slowly move in a certain direction," says Fraser, suggesting that one might consider "several meatless meals each week or maybe each day."

According to Roizen, eating right is a matter of empowerment, starting with your food shopping. "The key change you have to make is when you go to the grocery store," he says. "You're the CEO of your body ... if you buy it, you will eat it."

[See In Pictures: 11 Health Habits That Will Help You Live to 100.]

With the release of Best Diets 2013, U.S. News hopes to empower you with the latest advice and insight about healthy eating to help you flourish in 2013 and beyond.