If there's one food that no one — not your doctor, your nutritionist, or even your mother — will tell you to eat less of, it's leafy greens. Calorie for calorie, chard, collards, kale, and other leafy greens may just be the most nutritious food you can eat. They're packed with vitamins — A, B, K, and others — but also rich in essential minerals like calcium, iron, potassium, and magnesium, as well as antioxidants, which protect cells against damage. Leafy greens contain phytochemicals, natural compounds that can help prevent hardening of the arteries and lower inflammation linked to heart disease. The greens' synergistic combination of vitamins, minerals, antioxidants, and phytochemicals helps detox cells and expunge free radicals that damage DNA, both of which may inhibit cancer cells from forming and multiplying.

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Greens are also your single best source of natural nitrates, which get converted by the body into nitric oxide, a gas that lowers blood pressure, promotes blood flow, and can even improve erections. You produce less nitric oxide as you age — levels can dip by half after age 40 — which means you need to eat even more nitrates to keep everything working properly, says University of Texas biochemist Nathan Bryan. As if that weren't enough, greens have been shown to boost mental clarity, prevent depression, and reduce the risk of diseases like Alzheimer's. If you're looking to stay lean, high-fiber greens help speed digestion and make you feel full, and they're low in carbohydrates and calories, so you can practically eat as much of them as you want. At the very least, aim to consume three to five ounces of leafy greens a day, says Bryan. Here's how to get your fill.

1. Swiss Chard
Why You Need It: This green is a top source of two important, lesser-known antioxidants: syringic acid and kaempferol. The former can help stabilize blood sugar by inhibiting ­enzymes that turn carbs into simple sugars, while the latter protects cells against cancer-causing toxins, lowers inflammation, and may also reduce your risk for heart disease, diabetes, and other chronic diseases.

How to Eat It: Save calories while boosting your antioxidant intake by using Swiss chard instead of tortillas to make burritos and wraps. Cut leaves from stems, and steam leaves briefly. When cool, fill with your favorite healthy burrito staples: brown rice, quinoa, grilled shrimp or chicken, black beans, goat cheese, chopped tomatoes, sweet potatoes, or other vegetables; beans, grains, or grilled meats. Or sauté chard stems in garlic and olive oil for several minutes; add leaves, pine nuts, and currants; and cook two to three more minutes before serving.

2. Arugula
Why You Need It: Arugula has one of the highest nitrate levels of any leafy green, helping to ­increase blood flow and therefore enhance performance. It's also packed with flavonoids — antioxidants that fight heart disease and even some cancers. New research suggests it may also prevent ulcers.

How to Eat It: Arugula can be slightly bitter, so dress it in a salad with a fruity vinaigrette to counter the bite.

3. Collard Greens
Why You Need It: Of all leafy greens, collards are best at binding your stomach's bile acids, which can help lower your cholesterol levels and even protect you from some cancers. Collards also contain a special class of phytochemicals that nourish the body's natural detoxifying system.

How to Eat It: Boiled collards are a soul-food staple, but unless you eat the broth, you'll miss out on many nutrients. Steaming preserves more nutrients and increases bile-acid-binding activity. Jill Nussinow, a dietitian and chef, recommends kneading sturdy greens like collards or kale with olive oil for a few minutes before cooking to increase their flavor and make them easier to chew. Or massage with tahini and braise in garlic and lemon juice.

4. Bok Choy
Why You Need It: Bok choy is one of the best sources of potassium, which helps build muscle and keep blood pressure low. It's also packed with vitamin A, which strengthens the immune system by increasing white-blood-cell activity and the body's response to toxins.

How to Eat It: Chop up and braise the lower, white portion of the stems in chicken or vegetable broth and sesame oil. Add leaves after two minutes, and cook another one to two minutes.

5. Kale
Why You Need It:
Kale is a prime source of cancer-thwarting compounds called glucosinolates, as well as kaempferol, which researchers believe combats cancer and may also, incredibly, protect the heart, lower blood sugar, strengthen bones, and reduce inflammation in the body. Kale is rich in lutein and zeaxanthin, antioxidants that help prevent eye disease and vision loss as you age.

How To Eat It: Briefly sauté kale in olive oil with chopped onions, and then braise in white wine or vegetable stock for five to 10 minutes.

6. Watercress
Why You Need It: A single cup of watercress will bump you over your recommended daily value of vitamin K, which can help regulate blood clotting and reduce plaque on artery walls, and may lower inflammation linked to chronic diseases like arthritis. If you could stand to eat watercress daily for two months, you would cut DNA damage to your white blood cells, reducing your risk of cancer and lowering your triglycerides (unhealthy blood fats) by 10 percent, according to studies. At the very least, eat more.

How to Eat It: Watercress adds a peppery crunch to grilled cheese and other sandwiches, and also works well in salads. Or take a tip from Julia Child and simmer a pound of potatoes, three cups of leeks, and a little butter in two quarts of water for an hour; add a cup of watercress and simmer five more minutes before pureeing in a blender until smooth. Don't worry about losing nutrients: You'll retain them in the base of this flavorful soup.