When Sade (pronounced “sha-day”) sings “Smooth Operator” your heartbeat slows, your shoulders relax, and you feel calmer. That's good for heart and soul.

Smoothies can do the same for you if you make them with spinach or other dark green leafy vegetables.

Researchers from Sweden's Linkoping University have studied the nutrient lutein — a fat-soluble pigment that is abundant in dark green vegetables — and found that it tamps down inflammation in immune system cells. Your body also stores it to use if needed to help cool heart-damaging inflammation.

That made the researchers wonder: What's the best way to prepare lutein-rich foods to make sure you get the most of that nutrient in every tasty mouthful?

So for a new study, published in the journal Food Chemistry, they fried, boiled, and steamed baby spinach, and then measured the surviving lutein content.

They then compared that to raw spinach and found that the highest lutein levels were maintained when uncooked spinach was chopped into tiny pieces — just like when it's added to a smoothie.

Here are some recipes you can try:

• Green Drink: For three to four servings, blend 2 cups of spinach, 2 cups of cucumber, 1 head of celery, 1/2 inch or teaspoon of ginger root, 1 bunch of parsley, 4 teaspoons of psyllium fiber (sugar-free Metamucil), 2 apples, the juice of 1 lime, and the juice of 1/2 lemon.

• Spinach Shake: In a blender, put 3 cups spinach, 2 cups ice, 1/2 banana, 2 tablespoons peanut butter, 1 scoop vanilla protein powder, and 3/4 cup unsweetened almond milk.

You too can be a “Smooth(ie) Operator.”