Mehmet Oz, M.D., is host of “The Dr. Oz Show,” and Mike Roizen, M.D., is Chief Wellness Officer and Chair of Wellness Institute at Cleveland Clinic.

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QUESTION: There’s a guy in my office who is a vegetarian, but he’s overweight and doesn’t move around very well.

I thought I might try going vegetarian, but what’s up with that?

— Joannie D., Gainesville, Fla.

ANSWER: Chances are your coworker isn’t eating high-quality vegetarian food and is lacking essential nutrients. Being a vegetarian is no guarantee you’ll eat healthfully — after all, you could live on boxed mac ‘n’ cheese and fries and still claim to be a veg-head.

Interestingly, a study from Harvard’s T.H. Chang School of Public Health tracked more than 125,000 adults over a four-year period and found that eating unhealthy plant-based foods, like fries, refined grains or sugary juices, cancels out a plant-based diet’s benefits and puts you at an even higher risk of health problems, starting with significant weight gain.

That sounds like your coworker to us. But don’t let his poor choices put you off a plant-based diet that’s filled with colorful, unprocessed, low-saturated-fat foods.

A new Dutch study of more than 6,000 people found that those who ate more plant-based protein (plants contain great protein — especially when you eat a variety of colors and lots of whole grains, nuts and legumes) had a lower risk of developing coronary artery disease.

Another new Brazilian study of more than 4,500 people also found that a plant-based protein diet provided more arterial protection than an animal-based protein diet. The same study showed that not only were diets rich in high-quality, plant-based foods associated with less weight gain, but better plant choices also lowered a person’s risk of premature death by 30 percent.

There are heart-stopping, cancer-promoting, highly processed “vegetarian” foods out there, from canned soups with refined grains to microwavable dinners that are full of preservatives, salt, added sugars and trans fats. So stick with fresh, unprocessed foods and healthy oils like extra virgin olive oil. (If you sneak animal protein, stick with skinless poultry and fish like salmon and ocean trout.)