If I had to say there was ONE thing in nutrition that is the most important...

NATURAL (not supplemented) fiber, meaning vegetable intake, is in my opinion the single most important thing in the diet. I do not subscribe to eating only vegetables, or even half of one's calories as vegetables. What I have seen in the literature is a strong trend towards health with a broad and varied diet, where the countries eating the broadest spectrum of foods and tending to eat less saturated fat and sugar and more unsaturated fat and fiber have the healthiest populations. Walter Willet's Harvard Food Pyramid is designed to show the same thing, distinguishing between types of fats (saturated versus unsaturated) and carbohydrates (refined versus whole grain or unprocessed). This is AS OPPOSED TO eliminating all fats or all carbs or all of something else from the diet (fad dieting). But if we do eat some sugar and saturated fat (difficult to avoid considering the foods available to us), the negative effect is offset with vegetable intake. There are two important reasons for this: Vegetables, more than any other food, put bulk into the stomach with very few calories, dramtically reducing how many more calories a person will eat (unless the salad is just a tiny bowl of Romaine such as Caesar salad instead of a healthier, larger, bowl of darker greens and red and orange vegetables on top). Second, vegetables slow down the digestion process due the work load on the stomach breaking apart the courseness of the plants. Slower digestion means more of the calories that you eat go to active tissues (everything other than body fat) since those active tissues (such as muscle) have more time to consume calories when they enter into the bloodstream slower. Bottom line: Vegetables reduces hunger, increases muscle fueling and reduces fat fueling. Do you want more of your pizza to go to muscle? Eat a salad with it to slow its digestion. And, as if this were not enough, vegetables are the king of phytonutrients. You can not make up for a shortage of vegetables in your diet by eating whole grains and fruit, both of which contain only half as much fiber and half the phytonutrients but contain twice as many calories per gram as vegetables.