Why do some people eat lots and not put on weight?
Supposing humans are wired to store calories (as food was scarce for most of our evolution) and so many of us have a propensity to get fat, why do some of us stay skinny? Why do some of us eat lots and lots and never seem to gain weight? Shouldn’t natural selection have “deselected” those genes? That is, shouldn’t skinny genes have been out-competed long ago?—Shiloh
We can’t say for sure how much our ancestors ate, but most likely there haven’t been long periods of ‘too much’ food in our evolutionary history.
“We do know that frequent and extended famines in parts of the world have encouraged the evolution of the body’s ability to store fat quickly and be very efficient in its use of fat (cellular energy) to do the work required for muscle exertion and maintenance of bodily functions,” says Professor Michael Cowley, director of Monash University’s Obesity and Diabetes Institute.
It is thought between 60 to 70 per cent of our body weight is determined by our genes. Yet scientists have identified less than one-fifth of the genes involved, and still don’t know how many of these work, says Cowley.
“Sexual reproduction allows the genetic information of two parents to recombine and form a new individual. This produces considerable genetic variation through the shuffling of both beneficial and deleterious mutations down through the generations.
“It is why some people to this day carry genes that predispose them to skinniness even though historically food was generally in short supply, and carrying such genes was a disadvantage,” he says.
New world, old genes
The problem we now face is that due to food scarcity for most of our history, there has not been much evolutionary pressure to evolve genes that help us deal with our unprecedented access to plentiful, tasty and well-marketed high-kilojoule (energy) goodies rich in fats and carbohydrates.
“In other words, many of us still carry so-called ‘fat genes’ that predispose us to excessive weight gain because we haven’t had time to evolve a response to the smorgasbord on offer today,” says Cowley.
He says environmental factors such as how much we eat and the amount of physical activity we do determines around 30 to 40 per cent of our weight.
And while we now consume more high-kilojoule foods, the level of physical work most of us do has decreased dramatically.
“Consider clothes washing. It used to take two days of hard labour for a woman to do the family’s laundry; now all it takes is the push of a button. The result is that in our modern environment most of us stack on the kilograms.”
“Our weight is a product of our genes and how much we eat and exercise. In the push and pull between these two influences, a ‘settling point’ develops. For most of us, this is our ‘normal’ weight once we stop growing,” explains Cowley.
“So even though an overweight person might eat less and exercise more, losing weight can still be a challenge if, say, they’re genetically programmed to use energy more efficiently.
“Similarly, some slim people are lucky enough to possess a set of genes that ensure they stay svelte despite a sedentary lifestyle and so much gastronomic temptation,” he says.
“Such genes influence appetite, leading them to eat less and know when they are full, as well as allow for less energy (fat) storage and more energy use.
“It’s just the luck of the draw.
“But we should stop beating ourselves up about our weight and instead concentrate on eating healthily and doing regular exercise, and teaching these habits to our children.”
Professor Michael Cowley from Monash University was interviewed by Kathy Graham.