The good news is that more and more restaurants are catering to meatless customers, which now number approximately 8 million adults in the United States, with many more trying to eat less meat in general for health reasons.

"I am seeing more options out there for plant-based eaters who want to grab fast food," said Sharon Palmer, a vegetarian, nutritionist and author of "Plant-Powered for Life."

Vegetarian dining requires some careful planning, however. Here are some tips and strategies for eating well at fast food restaurants if you are a vegetarian or vegan:

Check out menus in advance. Since some chains have more vegetarian options than others, you should go online or try calling a restaurant to see whether there's something you will eat. If the menu seems limited, it may be worth traveling a bit farther if it means you'll get healthier, more appealing meatless options.

Seek out newer fast food chains. Traditional chains offering burgers and fried chicken are sometimes less vegetarian-friendly than newer chains. "Look for plant-based fast food restaurants, such as Veggie Grill, which are on the rise," Palmer said. "Also, look for ethnic fast casual restaurants -- Indian, Vietnamese, Chinese, Ethiopian -- as they have many traditional vegetarian and vegan dishes on the menu that offer delicious options just as quickly as a drive-through."

Veggie up. "I like to look for plenty of veggie-rich options -- salads, sandwiches with a side salad, a wrap filled with veggies -- so I am gaining all of those health benefits and the satiety value and lower calorie load of eating more vegetables at a meal," Palmer said.

Ask about "off-menu" items. Don't assume that if a vegetarian option is out of sight, it's not available. "You'd be surprised how many fast casual restaurants I've been to that offer a veggie burger, but it's not listed on the menu," Palmer said. Additionally, you can try custom-ordering your meal. For example, vegans can ask for burritos, pizza or tacos without cheese and sour cream.

Breakfast, for lunch. Some chains serve breakfast all day long, which means an egg and cheese sandwich from the breakfast menu can be a healthful option, especially if it's paired with a side salad for some veggies.

Pack foods that can balance out the meal. "If you're traveling, you can use fast food restaurants to round out your own picnic fare. I always take whole-grain crackers, peanut butter and mixed nuts on the road with me and then grab a side salad at a fast food restaurant when nothing else is available," Messina said.

Palmer agrees. "Sometimes, your options are limited to a piece of whole fruit and a green salad, leaving you hungry and improperly nourished. Look for ways to make a balanced meal no matter where you go; if you really are stuck, you can combine that salad and piece of fruit with a bag of peanuts and some whole-grain crackers for a more balanced option."

Check ingredients. "Bread used for burgers and sandwiches is sometimes not vegan," Messina said. The breads might have milk products or honey added to them, for example. Veggie patties may also contain milk or egg ingredients. "People may often assume that veggie burgers are vegan; they often are not. So it's important to ask or to check ingredient lists online."

Beans in general are protein-rich. "Beans are getting more common in fast food restaurants, making it an excellent go-to option," Palmer said.

Go for grain salads. They can provide a tasty, protein- and fiber-rich option for vegetarians. Palmer recommends Panera Bread's Greek salad with quinoa (take out the feta to make it vegan), Starbucks' hearty veggie and brown rice salad bowl, and Veggie Grill's quinoa power salad.

Combine side dishes. Sometimes, this can be your best strategy for maximizing the nutritional value of a vegetarian meal. Plus, it adds variety. Sides such as beans, veggies, hummus and rice can be combined to offer more nutrition and flavor than any one by itself.

Consider a smoothie. "This is a great option when you can't find anything else," Palmer said. Options can be vegan or vegetarian and have a mixture of proteins, nuts, vegetables (such as kale or spinach), fruits (such as frozen raspberries or peaches) and other additives, such as spices or ginger.

Lisa Drayer is a nutritionist, author and health journalist.

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How to make fast food healthier for vegetarians and vegans - CNN