Obesity. Abstract artwork of the face of an obese woman trapped in a fat cell. Her brain is super- imposed, with the appetite-control centre - the hypothalamus - in colour. The different colours represent different regions in the hypothalamus. A person is obese when their weight is at least 20% more than the maximum desirable for their particular height. About a third of the UK adult population is considered overweight, but only 5% are obese. Why some people become obese but not others is poorly understood, although a genetic factor is known to be involved. One theory is that in obese people the perception of hunger and satiety is different, leading to over-eating.