Keeping weight on is part of body's defence mechanism
Losing weight may be more than simply eating less and exercising more
Weight loss is often seen as a simple equation of burning off more calories than what is consumed.
But studies in recent years suggest a more complex process - that your body's biological signals play a part too.
One such study, published in journal Obesity last year, tracked the contestants of popular US reality show The Biggest Loserfrom 2009 for six years to find out what happened to them after they lost huge amounts of weight with intensive dieting and exercise.
The results were shocking.
Of the 14 contestants recruited for the study, 13 regained weight in the six years after the competition.
Four of them were heavier than before joining The Biggest Loser. Nearly all had slower metabolisms and burnt about 500 fewer calories than expected when at rest.
The slower metabolism in overweight people is a result of the body's regulatory system against weight loss, said Professor Tai E Shyong from the department of medicine at National University of Singapore Yong Loo Lin School of Medicine.
The body's primal instinct is to store fat as a defence against starvation.
In the past, when people had to hunt for or harvest food, only those who could store body fat survived the bad times.
The evolutionary adaptation continues today, even though food is now readily available.
Said Prof Tai: "When you lose weight, the body does not like it.
"This is because in an environment when food supply is not plentiful, the ability to conserve energy when there is no food is actually an advantage.
"So what happens is that energy expenditure typically goes down. As a consequence, we have to work harder and harder to continue to lose weight."
Today, our food intake is determined by social cues, not nutrition.
"Each time we eat a little more, the body packs a little more fat. The defence mechanism (activates)," said Prof Tai, who leads the metabolic disease summit research programme at the National University Health System.
A few extra calories here and there daily leads to visible weight gain over time.
One of Prof Tai's patients gained 40kg over three years, which amounted to an excess of about 300 calories a day.
The problem is that people underestimate the caloric content in the food they eat.
"They eat 'light' meals and think they are gaining weight from fresh air.
"Truthfully, something else is contributing to the calories. Very often, these are beverages," said Prof Tai, adding that the stomach does not sense the drinks as food.
Sometimes, people tell themselves to "just let go today", he added.
"The weight gain is so small, you do not notice it. People tell themselves, 'I got away with (eating more). Tomorrow I will let go again.'
"Soon, they find themselves gaining 3kg, 5kg or even 10kg."
One way to counter the metabolic slowdown is to exercise more - you build up lean mass by lifting weights or doing cardio, which in turn raises your body's energy expenditure when it is at rest, Prof Tai said.
But as with all ailments, prevention is better than cure.
When it comes to weight, it is easier to prevent weight gain than to lose weight.
"Small interventions such as walking a couple more thousand steps, exercising twice a week, or eating healthy are actually much more effective in preventing weight gain," he said.