Eat too much or exercise too little and you get fat: that's what most people believe.
But obesity is far more complicated, with multiple causes which differ from person to person, said Harvard Medical School's Associate Professor Lee Kaplan, an expert in the field. How each person responds to a specific type of treatment for weight loss also varies, making it important to look at tailored treatments, including a combination of surgery and medication, he believes.
Prof Kaplan was in Singapore recently to speak about his research into obesity and its treatments at the Obesity and Metabolic Surgery Society of Singapore's Annual Scientific Meeting, held at the National University of Singapore.
He blames changes in the modern environment for influencing the body to try to collect too much fat: factors include stress, sleep deprivation, and changes in the quality and the nature of the food we eat.
The 2010 National Health Survey revealed that 10.8 per cent of people in Singapore aged 18 to 69 are obese, putting them at risk of diseases such as heart disease and type 2 diabetes.
Prof Kaplan told The Straits Times that people with obesity have the "thermostat" for how much body fat is needed set wrong, which causes them to overeat. "So they are hungry at a body weight where they should be full," he explained.
Prof Kaplan, who is also director of the Obesity, Metabolism and Nutrition Institute at Massachusetts General Hospital in the United States, said scientists have over the years learnt more about the mechanisms that cause obesity and the way the environment acts to promote it.
More is also being understood about how there are different types of obesity and different types of patients. Research led by the British Sheffield University on more than 4,000 people with a body mass index of 30 or more was published in the Journal Of Public Health in April, in which it identified six different types of obese people: heavy drinking young men, young healthy women, unhappy and anxious middle-aged people, affluent and healthy older people, physically sick but happy older people, and those with the poorest health.
Studies have also found that surgery works through physiological mechanisms to decrease appetite and increase resting energy expenditure, and not by limiting food intake.
Research led by Prof Kaplan has identified a group of genes that predicts the degree of weight loss after a gastric bypass - a procedure in which a small stomach pouch is created and connected directly to the middle of the small intestine - in individual patients, which could point to whether this operation will be best for a patient.
Other available weight loss surgery includes lap band surgery, in which an inflatable ring is placed near the top of the stomach to restrict the amount of food ingested, and sleeve surgery to remove a large part of the stomach permanently.
The team found that not all pathways known to regulate body weight are activated in gastric bypass surgery, making such pathways potential targets for drugs to enhance the effects of surgery, said Prof Kaplan.
His centre in the US is one of the forerunners in combination therapy, using medication after surgery to enhance the effects of weight loss. With more research on the topic, more centres are also starting to adopt such methods. The National University Hospital (NUH) started using a combination of surgery and drugs for obese patients in 2013.
Said Dr Asim Shabbir, senior consultant at the NUH Centre for Obesity Management and Surgery: "For years, we believed the patient was doing something wrong and that is why he or she had not lost weight.
"But now after research we have understood that everybody behaves differently."
Dr Asim, who is president of the Obesity and Metabolic Surgery Society of Singapore, said complementing surgery with medical therapy has achieved good results.
Prof Kaplan said research in the next few years will make it easier to predict what will work best for each patient. "It is going to be slow progress... but eventually we would like to be able to say to the next person walking in the door, this person needs this kind of therapy," he said.