The real reason why you are fat (or thin)

In the UK, the University of Cambridge is deep into its Science Festival, and one particular speaker's topic caught our eye. Metabolic researcher Stephen O'Rahilly has determined that the genetic reasons that some people become fat or thin are not quite what we had thought. He's giving a talk on his findings this week at Cambridge, but we got a little preview of what he's going to say.

According to a release from the University:

At the core of the problem lies one immutable fact: this is the abundance of cheap food now available to almost everyone in the developed world, an "unnatural" situation that has evolved over the past 50 years, a period that is the merest blip in terms of human evolutionary history.

"Human beings are not genetically geared up to limitless high-calorie food being available to them 24/7. We evolved as hunter gatherers. If we all reverted to the lifestyle of our ancestors, the problem of obesity would disappear," said Professor O'Rahilly. "But we live in the real world where you can purchase a takeaway meal, containing enough calories for a day, for less than £2 – so we need to look at the whole picture."

As well as presenting the big picture, he will outline the latest genetic research, which looks not just at why so many people are fat but also at why many others remain lean, even when food is equally available.

The most exciting recent hypothesis adjusts the notion of "thrifty genes", genetic variants that promote the acquisition and storage of calories, to that of "drifty genes", genetic variants that have not been selected for or against during most of human evolution and which are somewhat randomly distributed throughout human populations, but which now, in the face of abundant food availability, strongly influence why some people are predisposed to becoming fat while others effortlessly remain lean.

"Everyone has views about obesity and views are often rather fixed and based on personal experience or anecdote. Obese people are significantly discriminated against. Research tracking the careers of large numbers of US teenagers has shown that those who are fat go on to earn substantially less as adults than those who are not fat, and that is even when factors such as education, parental income and scholastic aptitude are accounted for," he said.

"To be blunt, in today's society it's definitely not ok to be racist or sexist and it's increasingly not ok to be ageist which is all to the good – but it still seems to be quite ok to be fattist even, and perhaps especially, in highly educated circles. This isn't helping us with the serious science that will help provide some answers."

Heading a team of scientists looking at the underlying reasons for obesity and its adverse consequences, O'Rahilly describes himself as being on the "nerdy end" of metabolic research. While other colleagues in the Institute of Metabolic Science institutes are investigating environmental contributions to obesity and diabetes, his own particular expertise is in exploring the biological systems that control human body weight and metabolism and how genetic variation influences how well, or badly, these systems work in one person vs. another

"We know for sure that a propensity for obesity - or its opposite, a propensity for leanness - is rooted in the genes. Some 70 per cent of the variation between people in terms of their amount of body fat is explained by inherited differences that are built into our genetic make-up and passed from generation to generation," he said.

"What our work has shown is that, rather unexpectedly, a lot of these genes primarily influence appetite and satiety, i.e. how hungry we get and/or how satisfied by a particular amount of food we are, rather than how fast or slow we burn off calories."


If you're near the Cambridge area, you can see the lecture on the evening of March 18 - get your tickets here and find out more about the festival here.


Klebert L. Hall


I'm fat because I eat too much, and don't get enough exercise.

I know this, because when I don't eat too much, and do get enough exercise, I get thinner.