Nutrition can be complicated. How do you know what foods are healthy and what aren't? How much should you eat? What's a healthy portion size? How many calories do you need? Do you weigh just the right amount—or are you underweight or overweight? Keeping track of how much you eat, weigh, and exercise can help you make the right choices. Discover how knowing the numbers—calories, BMI, weight, and more—is a big part of staying fit and healthy.