Obesity is generally caused by consuming more calories – particularly those in fatty and sugary foods – than you burn off through physical activity. The excess energy is then stored by the body as fat.
Obesity is an increasingly common problem, because many modern lifestyles often promote eating excessive amounts of cheap, high-calorie food and spending a lot of time sitting at desks, on sofas or in cars.
There are also some underlying health conditions that can occasionally contribute to weight gain, such as an underactive thyroid gland (hypothyroidism), although conditions such as this don’t usually cause weight problems if they are effectively controlled with medication.