Good nutrition and regular exercise go hand in hand: a healthy diet gives you the energy you need to maintain an active lifestyle. As a general rule, athletes are advised to obtain 60 percent of calories from carbohydrates, 20 percent from protein, and 20 percent from fat, though your particular needs may be different. Often, the recommendations vary by sport: those taking part in long-distance activities like marathons may want to consume large amounts of carbohydrates, which their bodies can store as fuel. When losing weight and increasing muscle mass is your goal, you'll probably want to reduce fat intake, and eat a little more protein. It's important to consume some amount of protein every day, because unlike fat, protein can't be stored. Another important step is getting enough liquids. Most people don't drink the recommended eight glasses of water per day, and when you exercise, these requirements increase. Water is vital for your muscles and circulatory system, and also helps keep you from overheating. As a final note, there are various supplements on the market that promise to boost endurance or create muscle mass. Not all of these products are beneficial. When in doubt, it's best to consult a doctor, before adding these types of substances to your daily routine.