Carbohydrates contain four calories per gram. According to MayoClinic.com, your body likes to use carbs as its primary source of energy. A 3-ounce portion of lettuce has over 10 calories in the form of carbs. Carbohydrates are higher in starchier veggies. Like other plants, potatoes contain carbohydrates. Your body uses these nutrients to build muscles and burn fat. Meat contains about 9 calories per gram of protein. Protein helps repair muscles and is used by the body for many other functions including building antibodies against bacteria and viruses. Lettuce provides more than just fuel for your body. It's also good for your heart! Lettuce is a low-fat food that can help reduce your risk of heart disease and stroke. A salad with chicken, tomatoes, cheese and pasta sauce is an easy way to get your daily intake of vegetables. This meal has less than 100 calories and 5 grams of fat. It's also high in fiber and will fill you up without filling you out.