The neurobiology of obesity
When most of us think about obesity, we think about eating, and therefore, we think about the stomach and other digestive organs. However, Dr. William Colmers, a neuroscientist at the University of Alberta, does not; he thinks about the brain. "Eating is a behaviour, and behaviour in higher animals is controlled by the brain. If we want to understand obesity, we have to look at the brain."
He studies the interplay between two key neurologic systems: the homeostatic system, which regulates the intake of calories in relation to the output of energy, and the hedonic system, which is focused on perceived rewards. If you eat because you are hungry, the homeostatic system is at work. If you eat because you crave chocolate, for example, your hedonic system is at work. Basically, the homeostatic system is overridden by the fact that eating is pleasurable.
"There is good reason for highly caloric foods to be pleasurable," notes Dr. Colmers. "These systems evolved to cope with the occasional mastodon that keeled over in front of you. You ate as much as you could and stored the energy as fat, which didn't spoil and you could easily carry around with you. Sadly, now it's as if that mastodon dies every time you go into a restaurant. We haven't evolved to cope with this situation. As a consequence, we've got an archaic survival trait, which was once very useful, getting in the way of our health."
How exactly does the hedonic system affect the homeostatic system? It seems that a high-calorie diet can raise the body weight set point, similar to raising the thermostat in your house to a higher temperature. However, in the case of the homeostatic system, it appears that the change is permanent, which makes it very difficult to lose weight.
Dr. Colmers' research illuminates the complicated machinery that underlies energy balance. He works on neuropeptide Y (NPY), a chemical messenger in the brain, and its interaction with the ventromedial nucleus of the hypothalamus, an area of the brain that is very sensitive to energy balance signals. NPY injection into this area dramatically increases food intake.
"The work on NPY and other neurotransmitters is beginning to bear fruit in that we are starting to understand the important factors that regulate the body weight set point," says Dr. Colmers, "but we still can't explain how these things work together to change the set point in a permanent fashion. That is the major question I would like to answer."
Dr. Colmers leads a Canadian Institutes of Health Research (CIHR) Team on the Neurobiology of Obesity, which includes Dr. Keith Sharkey from the University of Calgary. Dr. Sharkey is known for his research on the bidirectional communication between the gut (digestive tract) and brain, especially the roles of the nerves in gastrointestinal inflammation, such as Crohn's disease and ulcerative colitis. Interestingly, the role of the gut in obesity has not been well explored until recently. The work of Dr. Sharkey and others has shown that hormones released by the gut affect not only the digestive tract but also special regions at the edge of the brain called the circumventricular organs.
Dr. Sharkey says that the importance of the gut in the maintenance of energy balance is best illustrated by the fact that "a relatively simple change in plumbing can allow you to lose weight. Gastric bypass surgery has a dramatic impact on energy expenditure and the patient's perception of food. Type 2 diabetes is reversed in many patients, well before they lose weight. There's much more going on than a simple reduction in the gastric volume. We believe gut hormones are playing a big role."
Given that the gut is the largest endocrine organ in the body, it is very likely that gut hormones are key players in altering the body weight set point. Now the challenge is to understand how these hormones change systems in the brain.
"This is a very exciting time to be in obesity research," says Dr. Sharkey. "New technologies are allowing us to do ever more sophisticated work. Collaborations such as our CIHR team are bringing together people with different expertise to work on a common problem. Converging lines of evidence are starting to push the fog away and show us which systems are really important. We're getting a clearer picture of how obesity changes the brain."
Measuring BMI
Take your weight in kilograms (wearing light clothing and no shoes), and your height in metres. Weight should be recorded to the nearest 0.2 kg; height should be recorded to the nearest 0.5 cm. Then use this equation to calculate BMI:
BMI = weight (kg)
height (m) x height (m)
Example: For someone 170 cm tall who weighs 80 kg:
BMI= | 80 1.70 x 1.70 | =27.7 |
The following categories of BMI are used to identify levels of health risk:
| <18.5 | Underweight | Increased risk |
| 18.5 - 24.9 | Normal weight | Least risk |
| 25.0 - 29.9 | Overweight | Increased risk |
| 30 and over | Obese | High risk - Extremely high risk |