Event Review: Resisting temptation - the biology of appetite

Obesity is a growing problem. In England, around a quarter of adults are now classed as obese, and as many as 60 percent are overweight. With few exceptions, there is a global trend of expanding waistbands, putting ever-increasing strains on healthcare systems, with no signs of it slowing down.

So why are so many of us overeating despite the huge amount of information available on appropriate calorie intake? And what are the biological factors that control our insatiable appetites for food?

This was the topic of the Friday Evening Discourse at the Royal Institution, with Professor Sadaf Farooqi of the University of Cambridge. Professor Farooqi began by explaining that appetite is an incredibly complex behaviour. As well as the need to fuel our own body's energy consumption, we are also driven to eat by a broad range of social and cultural factors. We eat for pleasure, for comfort and as part of social bonding processes. However what Professor Farooqi was quick to establish is the growing body of evidence to suggest that appetite is heritable. What this implies is that the decisions that we make about what, when and how much to eat, are, at least in part, determined by our genetics.

Professor Farooqi then moved on to further demonstrate the role of genetics in obesity with an example of the geneticist's favourite experiment: the twin study. In the study, sets of twins were overfed by ten percent for a year and their exercise and other variables carefully controlled. As expected, all twins gained weight, but 90 percent of identical twins gained a similar amount of weight, compared to 45 percent of non-identical twins.

It turns out that our desire for food is controlled by a hormone called leptin. When our bodies need energy, leptin levels are high and appetite for food decreases. If our bodies are in need of food, leptin levels are low, causing a desire to eat.

When examining complex behavioural traits such as appetite, scientists often look to extremes to find underlying biological causes. Professor Farooqi went on to describe her own research that focused on morbidly obese children. She found that a small percentage of these children were completely lacking the ability to produce leptin. This meant that their bodies constantly felt that they were on the point of starving, causing them to compulsively eat anything they could get their hands on.

Fortunately Professor Farooqi was able to give these children synthetic leptin as part of a clinical trial. The truly remarkable results were displayed in a photo in the presentation. Two distressingly overweight children were transformed into happy smiling kids after a few months of treatment.

There was then some audience participation and live experimentation. Participants were given one of two milkshakes and asked to rate how much they enjoyed the drink. Although 'Milkshake B' was a clear favourite, both milkshakes in fact tasted exactly the same. The preference was caused by an increase of 20 percent in fat, despite this being imperceptible to the taste buds.

This preference for high-fat foods can be seen across all people, however a mutation in a gene known as MC4R can cause the preference to be significantly overinflated. Around one percent of obese people carry a MC4R mutation, making it the most common genetic contributor to obesity.

For me the most interesting parts of the lecture involved how people can learn to associate particular imagery and packaging with desirable food. For example children given carrot sticks in plain and McDonalds packaging exhibit a strong preference for the branded version. This information could well be used to nudge the population into making better dietary choices.

In the discussion the inevitable nature vs nuture debate arose, but was quickly answered by Professor Farooqi; complex behaviours such as appetite are clearly impacted by both genetics and environment. However at the extreme end, as in the case of leptin-deficiency biology has a much larger effect.

Professor Farooqi is an engaging speaker and the audience was excited and receptive thanks to live experiments. Unfortunately parts of the talk were let down by confusing and uninformative slides, some of which were far too complex. However, I left feeling inspired and with my appetite whetted to learn more about humanity's fascinating and changing eating habits.