A: One of the better answers I've heard to that question comes from writer Michael Pollan, who, in his book In Defense of Food, says: "Eat food. Not too much. Mostly plants." That's sound advice—placing the emphasis not on a particular nutrient but on a smart diet.
The trouble with focusing on, say, fat content, is that it doesn't tell you what you need to know. Avocados, almonds, and walnuts are high in fat but wonderfully nutritious. Olive oil is pure fat—and very healthy in the right amount. Bacon, cream, and fried chicken are another matter—high in saturated fat, and very hard on your heart. And you can run into the same issues with any nutrient you single out.
A better guide is to choose foods closer to their natural state. The challenge with Pollan's advice is that so many of our foods come in bags, boxes, jars, or cans, it can be quite hard to tell which foods are, in fact, closer to nature. One simple tip: Choose foods with shorter ingredient lists. Long lists usually indicate lots of additives and processing. The ingredient list for spinach is spinach. Make the one-word ingredient list true north on your nutritional compass and use it to guide your shopping cart; you'll wind up in a healthy neighborhood.