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At some point in your high school curriculum you will come across the "laws of thermodynamics" which are all about energy. They say that any amount of energy may change its form many times (from wave to electricity to heat to light) but essentially stays the same amount. That's also true for the energy that goes into your body (see the chapter "Understanding and Enjoying Foods"). Your body constantly requires energy to grow and stay healthy. But when you take in energy, you must spend it, too.
Do you know how much energy you need? That depends on your age, gender, height, weight, metabolism, physical activity, geography, and genetic structure (which you inherited from your parents). Each person has different energy needs. However, there are guidelines for the "average" person: teenage girls need between 1900 and 2,300 calories each day, teenage boys require between 2,200 and 3,000 calories per day (based on the national guidelines of several countries in North America and Europe).
The most common measures used to calculate whether you eat enough, too little or too much is the Body Mass Index (BMI) and your waist line/weight ratio. The BMI relates your height to your weight through a certain formula. Follow this link to calculate your BMI. A BMI between 18 and 25 is considered healthy, and you want to keep it that way. If your BMI is over 30, you are obese and with a BMI of over 40 you are considered morbidly obese. If you're BMI is under 18, you are underweight and at BMI 14.9 you are close to starvation. The size of your waist is another indicator of how much energy in your body retains. A waist too large for your height and weight is considered especially harmful. For girls and women, health risks start at a waist size of 31.5 inches and rise substantially at 35 inches. Boys and men should be concerned about waistlines of 35 inches and 40 inches, respectively. On the other extreme, waist sizes below 20 for an adult person may be too low (depending on height and body built).
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