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Former President Bill Clinton had a legendary appetite: Hamburgers and steaks. Barbeque. Chicken enchiladas. But after having two stents inserted in 2010—on top of quadruple bypass surgery six years earlier—he radically changed his diet in the name of saving his health. Now a vegan, the strictest type of vegetarian, he has cut out meat, dairy, eggs, and most oils in favor of a super-low-fat diet that revolves around whole grains, fruits, vegetables, beans, and nuts. It appears to be working: He has said he's dropped more than 20 pounds and has never been healthier. In a televised interview with film producer Harvey Weinstein in June, Clinton explained that he'd decided he wanted to live to be a grandfather. "So I just went all the way. Getting rid of the dairy was great, getting rid of the meat was—I just don't miss it."

Vegan diets have lately been surging in popularity, thanks in part to the example of celebrities who are publicly forswearing all animal products (Michelle Pfeiffer, Carrie Underwood, Russell Brand, and Ozzy Osbourne, to name a few others). Ellen DeGeneres and Portia de Rossi have announced plans to open a vegan restaurant in Los Angeles. Vegan-centric books have been flying off the shelf, including Alicia Silverstone's The Kind Diet and The Engine 2 Diet by Texas firefighter and triathlete Rip Esselstyn, son of retired Cleveland Clinic physician Caldwell Esselstyn, whose research on the merits of plant-based eating inspired Bill Clinton. Vegan food trucks are making the rounds, schools are instituting meat-free days, and colleges are opening vegan dining halls.

While many vegans still take the stand because they believe in animal rights, a growing number are swayed by mounting research showing a profound impact on health. "It's dramatic," says Neal Barnard, a nutrition researcher and adjunct professor of medicine at the George Washington University School of Medicine in Washington, D.C., and president of the Physicians Committee for Responsible Medicine, a nonprofit group that promotes preventive medicine. "We've seen people whose chest pain has gone away within weeks, while their weight melts off, blood pressure goes down, and cholesterol plummets." Barnard's 2011 book 21-Day Weight Loss Kickstart is a three-week introduction to the case for and how-tos of the vegan life. The panel of 22 experts who analyzed 25 diets for U.S. News's ratings of the best eating plans overall—as well as the best for weight loss, heart health, and diabetes management and prevention—are not universally sold on absolute meatlessness. But without a doubt, the heavily plant-based plans tend to rise to the top of the U.S. News lists.

Exactly how you shape a vegan meal plan is up to you, but you'll typically aim for six servings of grains from bread and calcium-fortified cereal, for example; five servings of protein-rich foods such as legumes, nuts, peanut butter, chickpeas, tofu, potatoes, and soy milk; and four servings of veggies, two of fruit, and two of healthy fats like avocado, coconut oil, and olive oil. (Both of the Esselstyns advocate avoiding all oils, too.) There's no need to give up dessert, although you'll be baking without butter or eggs.

It should come as no surprise that becoming a serious vegan is apt to help you lose weight. By loading up on fruits, veggies, and whole grains, vegans tend to feel full on fewer calories, and indeed they tend to weigh less and have a lower body mass index than their meat-eating peers. In a 2006 study coauthored by Barnard, 99 people with type 2 diabetes followed either a vegan diet or a standard diet based on American Diabetes Association guidelines. After 22 weeks, the vegans lost an average of 13 pounds, compared to 9 in the ADA group. Both groups' control of their blood sugar levels also improved.

The cardiac case. A meatless diet's power against heart disease also is well documented. "It's an exceptionally healthy diet, especially when it comes to cardiac health," says Michael Davidson, director of preventive cardiology at the University of Chicago Medical Center. He notes that cutting way back on saturated fat and eliminating cholesterol is just part of the equation; also key is piling on "cardiac protective" fruits, vegetables, and grains, packed with antioxidants and other phytochemicals that protect cells from damage caused by free radicals. The soluble fiber found in plant protein also helps to lower cholesterol. In the 2006 Diabetes Care report, LDL cholesterol dropped 21.2 percent in the vegan group after 22 weeks, compared with 10.7 percent in the group following the meat-allowing guidelines. Triglycerides fell from 140.3 mg/dL to 118.2. In an earlier 12-year study that compared 6,000 vegetarians and vegans with 5,000 meat-eaters, researchers found that vegans had a 57 percent lower risk of ischemic heart disease—reduced heart pumping due to coronary artery disease, which often leads to heart failure—than the meat-eaters. Vegetarians had a 24 percent lower risk.

It was Dean Ornish, a professor of medicine at the University of California–San Francisco and founder of the Preventive Medicine Research Institute, who first showed that heart disease can actually be reversed, without drugs or surgery, by radically restricting fat intake, getting more exercise, and easing stress. The Ornish menu stops just short of veganism; nonfat dairy and egg whites are allowed. But it doesn't allow much wiggle room. Only 10 percent of calories can come from fat, very little of it saturated. Most foods with any cholesterol or refined carbohydrates, oils, excessive caffeine, and nearly all animal products are banned. Fiber and lots of complex carbohydrates are emphasized.

This regimen, combined with stress management techniques, exercise, social support, and smoking cessation, formed the basis of a landmark study of 48 heart disease patients published in 1990 in the Lancet. The program followers showed artery blockages receding after one year and continuing to do so at the five-year mark. The condition of control group members worsened at both points. In Caldwell Esselstyn's research on more than a dozen patients with advanced coronary disease, those who adopted his plant-based program also demonstrated a halting or reversal of the disease after five years.

Today, patients at hospitals and clinics nationwide can follow Ornish's Program for Reversing Heart Disease, which is covered by Medicare and private insurance companies. Among the nearly 4,000 people who had tried the program in Pennsylvania, Nebraska, and West Virginia by last fall, the average patient lost 13.3 pounds in the first 12 weeks and 15.9 after a year. There were significant drops in blood pressure, total cholesterol, triglycerides, and LDL cholesterol. Patients also reported less depression and hostility, the emotions most strongly linked with heart disease. And nearly 97 percent experienced improvements in the severity of their chest pain after a year.

Since those early days, the Ornish approach has evolved into a "spectrum" plan. He categorizes food into five groups from most (group 1) to least (group 5) healthful. It's the difference, for example, between whole-grain bread and biscuits, between soy hot dogs and pork or beef ones. The heart-healthy choices are at one end, and the choosing is up to you.

Despite all the advantages, vegan diets aren't a no-brainer move. There are downsides, from potential health risks to the challenge of sticking to such a restrictive eating plan. "Vegan diets are difficult to manage, and they're nutritionally incomplete," says Marion Nestle, a professor of nutrition at New York University and coauthor of Why Calories Count. "So you have to compensate for that in some way."

Since dairy is out, vegans often don't get enough calcium, for example. And in a report on the health effects of a vegan diet published in the American Journal of Clinical Nutrition in 2009, researchers warned that vitamin D, vitamin B12, and zinc are often lacking. One study showed, for instance, that more than half of vegans tested were deficient in vitamin B12, which is important for metabolism and is found only in animal products. These people were at risk for mental health problems, fatigue, trouble concentrating, decreased brain volume with aging, and irreversible nerve damage. But tofu, calcium-fortified juice and soy milk, bok choy, broccoli, kale, and okra all offer a plant-based source of calcium. Vitamin D- and vitamin B12-fortified products are available, too. And good sources of zinc include nuts, seeds, cooked beans, wheat germ, and cooked grains.

Vegans are often low as well in the omega-3 fatty acids EPA and DHA, important for brain, eye, and cardiovascular health and readily available in fish like salmon, tuna, shrimp, and sardines. "It's not true that you can get all the essential fatty acids you need from flax seeds," says registered dietitian Katherine Beals, an associate professor in the division of nutrition at the University of Utah, who worries about excluding salmon. "Vegetarian diets are demonstrably healthier than the standard American diet, but when you exclude all animal products, it gets questionable," Nestle says.

It's particularly important, nutritionists point out, for adolescents, the elderly, the poor, pregnant women, and those with psychiatric or chronic medical illness to be cautious. These folks are most at risk for inadequate nutrition. Still, the Academy of Nutrition and Dietetics says that a well-planned vegan diet is appropriate even for babies.

Diehard meat- and fish-lovers who aren't trying to clean out their arteries can aim instead for what David Katz, a clinical instructor of medicine at Yale University and founding director of the Yale-Griffin Prevention Research Center, calls "an optimal omnivorous diet." An eating plan on the Ornish spectrum is one good option. Another is the Mediterranean diet.

There actually isn't a single Mediterranean diet; the term describes a style of eating that emphasizes fruits, veggies, whole grains, beans, nuts, legumes, olive oil, and herbs and spices. Followers eat fish and seafood at least a couple of times a week; enjoy poultry, eggs, cheese, and yogurt in moderation; and save sweets and red meat for rare occasions. Healthier mono- and polyunsaturated fats are subbed—in moderation—for saturated fats, and meals are often topped off with a splash of red wine. The diet has been linked with a decreased risk for heart disease, and it's also been shown to reduce blood pressure and LDL cholesterol. "You can get similar benefits out of a Mediterranean-style diet as you can a vegan diet," says Lawrence Cheskin, director of the Johns Hopkins Weight Management Center in Baltimore. And while head-to-head studies are lacking, the Mediterranean approach removes concerns about getting sufficient omega-3 and omega-6 fatty acids, vitamin B12, and calcium.

The bottom line? Healthful diets fall on a continuum, and most nutritionists would agree that veganism is far superior to the typical American regimen. But the healthful choices that work best for you are the ones that actually work for you.

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