Under the Microscope:In the poorest parts of the world, people worry about getting enough food to avoid starvation, whereas in the better-off world many people eat so much it injures their health, writes Prof William Reville.
Overeating is now a major health problem in the developed world and this, together with a general preoccupation with body-image, has spawned a large dieting industry. The reasons so many people overeat are complex. Recent research, described by Oliver Grimm in Scientific American Mind,April/May, 2007, has uncovered similarities in the body's biological underpinnings to its craving of food and its craving of drugs.
Overeating leads to excess weight and this can be bad for your health. An index called the body mass index (BMI) is used to assess overall body weight status. Technically BMI is your weight in kilograms divided by the square of your height in metres. If this ratio is less than 18.5, you are classed as underweight, 18.5-24.9 is normal, 25-29.9 is overweight and a BMI of 35 or more is defined as obese. I think the overweight criterion is a bit strict - my BMI is 25 but I am a physically magnificent specimen! (You can easily calculate your BMI at www.nhlbisupport.com/bmi).
Being overweight is the most significant risk factor for cardiovascular disease and diabetes. Using the BMI index, one third of American adults are overweight and almost one third are obese, and one study has estimated that almost 10 per cent of America's health expenditure goes on overweight/obese patients. Overweight and obesity is now a significant problem in Ireland.
The body has feedback control mechanisms that both alert us to take in food and drink and to signal to us when we have enough. Hunger and thirst trigger intake of food and water and feelings of satiety trigger cessation of intake. Problems arise when the thresholds of signals that trigger intake are lowered and/or we become desensitised to the signals that trigger cessation. Erosion of the efficiency of these feedback mechanisms can occur by gradually allowing bad habits to build up. In some cases there may also be a congenital predisposition to overeat.
It was discovered in 1994 that fat tissue operates a feedback mechanism that inhibits further eating. Fat cells secrete a protein that passes into the blood and travels to the brain where it suppresses hunger feelings. This substance was named leptin, from the Greek leptos, meaning "thin". Genetically engineered mice in whom leptin does not operate rapidly become obese. Leptin also plays a role in addictive behaviour - heroin-addicted animals suffer more from withdrawal symptoms if they are kept hungry.
GRIMM ASKS THE QUESTION - is food a drug? Experiments show that the brain reacts similarly in many ways to an intake of palatable food as it does to an intake of cocaine and amphetamines. Nerve fibres that run from the mid-brain to a structure called the nucleus accumbens secrete large amounts of dopamine, a neurotransmitter chemical, whenever we experience something pleasant. Dopamine causes a rush of pleasure. Dopamine levels in the nucleus accumbens rise when we see something nice to eat or when we take cocaine. Extremely overweight people seem to have a shortage of dopamine which they try to increase by overeating.
Dopamine also affects another region of the brain called the hypothalamus which, in addition to other things, regulates eating behaviour. Genetically modified mice that produce no dopamine show no interest in eating. However, when dopamine is administered to these rats, their eating behaviour returns to normal.
The amygdala is a part of the brain involved in arousal and emotion responses. The amygdala becomes active once a hungry person sees food, but goes quiet once the food is eaten. The amygdala behaves in the same way in cocaine addicts, becoming active immediately they see the white powder.
However, it would be dangerous and wrong to conclude from this that obesity is an addiction in the same league as drug addiction. For example, we all know that when we go on a diet, we do not suffer the awful symptoms of a drug addict in withdrawal. Some dieters suffer more than others from residual cravings, but, for most people, dietary withdrawal symptoms would only be an extremely pale shadow of drug withdrawal symptoms. To seriously compare the two would be mockery.
If you are seriously overweight, reducing to a healthy weight means breaking a well-established food habit. Breaking any habit requires effort, but only moderate effort is involved in dieting. A little perseverance and willpower will do the trick for most people.
One useful tip to cut back on food intake that I read somewhere is, when you are eating, always chew and swallow the food in your mouth before putting more food into your mouth. This slows down your eating, allows you to enjoy the taste more and, most importantly, it also allows feelings of satiety to develop to persuade you that you have eaten enough earlier than would happen if you are bolting down your food.
I will finish with a little story:
An elderly couple were killed in an accident and found themselves being given a tour of heaven by St Peter. "Here is your ocean-side apartment, over there are the tennis courts, swimming pool, and two golf courses. If you need any refreshments, just stop by any of the many bars located throughout the area."
"Heck, Gloria," the old man hissed when Saint Peter walked off, "we could have been here 10 years ago if you hadn't heard about all that stupid oat bran, wheat germ, and low-fat diets!"
William Reville is Associate Professor of Biochemistry and Public Awareness of Science Officer at UCC - understandingscience.ucc.ie