Which salad greens you choose matters less than what goes on top. (Scott Suchman/For The Washington Post)

Q: I’m trying to eat more salads because I’ve been told they’re filling and nutritious. Which salad greens are most nutritious?

A: Good job! Eating more salads is an easy way to achieve one nutrition goal that doesn’t stir the nutrition cauldron: eating more vegetables. Although dark green leafy lettuces are your best bet for a base, any greens tossed with raw vegetables and topped with a low-calorie dressing can be healthful. But it’s these toppings — rather than the right shade of greens — that matter most.

The 2010 U.S. Dietary Guidelines offer three bites of advice on vegetables:

1. Eat enough: The goal for most adults is 2½ cups per day, give or take, based on calorie needs and age.

2. Vary your vegetables to eat an array of nutrients.

3. Eat more dark green and red and orange vegetables. They’re more nutrient-dense and contain nutrients Americans don’t eat enough of (folate, magnesium, potassium, dietary fiber, and vitamins A, C and K).

Why eat vegetables? Moderate evidence shows that regularly eating 2½ cups of each per day can help reduce cardiovascular disease, including heart attacks and strokes. Sufficient evidence shows they protect against certain types of cancer. Plus they may assist with weight control. A recent study from the Harvard T.H. Chan School of Public Health’s large observational database, which tracked the eating and lifestyle habits of health-care professionals over 24 years, showed that eating one extra serving of vegetables a day led to a slower weight gain over that period.

Comparing salad greens

For many years iceberg, also called head lettuce, reigned supreme. But according to Elizabeth Pivonka, a dietitian and president and chief executive of the Produce for Better Health Foundation, Romaine and leaf lettuce (green leaf, red leaf, butter and bibb), combined, recently became the most consumed lettuces in the United States, followed by spinach. “Collard greens, escarole, kale, mustard greens and turnip greens are consumed at much lower levels but are increasing,” she told me in an e-mail.

Lettuce greens contain nearly no calories, around 5 to 10 per cup. Their virtue is their volume: lots to crunch and munch. Across the board they contain a gram or two of carbohydrates, barely a gram of protein and about a gram of fiber per cup. Their sodium content is next to nil.

The darker leafy greens, such as spinach, romaine and leaf lettuce, contain more of certain nutrients, including vitamin A and other carotenoids, vitamin C and potassium, than iceberg. “The cruciferous greens, like arugula, collard greens, kale, mustard greens and turnip greens, also contain these nutrients, along with other potentially cancer-preventing compounds, called isothiocyanates and indoles, unique to cruciferous vegetables,” Pivonka writes.

But, Pivonka adds: “The most nutritious greens are those that you actually EAT, so any of them, including iceberg, are fine!”

Lettuces also offer a palette for an array of other colorful nutrient-dense raw vegetables such as cucumbers, peppers, carrots, mushrooms, red cabbage and tomatoes. However, steer clear of calorie- and fat-dense salad toppers such as bacon, cheese, tuna salad or fried chicken. And especially beware dressing, which can be the downfall of a healthful salad. Most regular (not light, reduced-calorie or fat-free) salad dressings range between 50 and 80 calories per tablespoon. The sodium count of commercial dressings ranges from 100 to 500 milligrams of sodium per two tablespoons.

Get salad smarts

Salads can be healthful no matter where and when you eat them. Test out these tips.

On the homefront:

●Explore new greens, such as spinach, arugula, endive, escarole and watercress. Add color and nutrition with radicchio, radishes, beets, peppers and carrots.

●Cut up a few days’ worth of healthful salad toppers (cucumbers, peppers, red or green cabbage, carrots) and have them at the ready for easy salads in seconds.

●Enjoy entree salads as a quick, healthful dinner or a means to use up leftovers.

●Add texture, crunch and health with ingredients nuts, seeds, olives, raisins, dried cranberries, wheat germ or flax.

●Make your own salad dressing. Blend a healthful oil and your favorite vinegar, herbs, spices and garlic in a food processor.

At the salad bar:

●Build the base of your salad with a mound of greens. The more dark green ones, the better. Next pile on the raw vegetables.

●Take just a taste or two of higher-calorie items such as cheese, pasta or potato salad.

●Ladle only a tablespoon or two of your favorite dressing. Spread it with drizzles of vinegar or squeezes of lemon wedges.

In fast-food restaurants:

●Pour only a portion of the large packet of dressing.

●Take advantage of half orders when available.

●Steer clear of salads with high-fat items including fried chicken, Chinese noodles, candied nuts and cheese.

In sit-down restaurants:

●Start your meal with a salad to fill up.

●Split a side or entree salad. They’re often plenty for two.

●Request the kitchen go light on ingredients that are higher in fat and calories, such as avocado, cheese and bacon.

●Order salad dressing on the side to control the portion. Drizzle it gingerly or just dip a forkful of salad lightly in the side of dressing.

Warshaw, a registered dietitian and certified diabetes educator, is the author of numerous books published by American Diabetes Association, including “Eat Out Eat Well: The Guide to Eating Healthy in Any Restaurant,” and the blog EatHealthyLiveWell found on her Web site, www.hopewarshaw.com.

Have a nutrition question? Send an e-mail to localliving@washpost.com. Put “Nutrition Q&A” in the subject line and tell us where you live.