Body weight is determined by energy intake from food and energy output, i.e. energy needed for basic body processes (such as keeping the heart beating) and for physical activity.
If a person regularly takes in even a little more energy from their diet than they need, they will start to gain weight and eventually become overweight.
The World Health Organisation (WHO) projected in 2005 that there were approximately 1.6 billion overweight adults (age +15) and 400 million obese adults.
WHO estimated at least 20 million children under the age of 5 years were overweight in 2005.