The energy the body takes from food is measured kilojoules or kilocalories.You need enough energy to meet the demands of your BMR and PAL.
BMR stands for Basic Metabolic Rate; the number of kilojoules you use to stay alive each day.
PAL stands for Physical Activity Level; the number of kilojoules you use to fuel all of your physical activity.
BMR + PAL = your daily energy requirement.
Big people need more energy for BMR. Athletes in endurance events and hard training need more energy for PAL.
If you consume [Consume: To eat and drink. ] more energy than you use you will put on weight. If you consume less energy than you use you will lose weight.
People who under-eat will not have enough energy to perform effectively.
Being overweight may not be a problem. It may be due to a person having a lot of muscle, rugby players for example, so it's not always harmful. However, people who are overfat [Overfat : Having excessive amounts of body fat. ] or obese [Obese : The official term for being very over-fat. Being obese is dangerous to health. ] will not be effective sportspeople.