By Olivia Judson
BEING fat is bad for your brain.
That, at least, is the gloomy conclusion of several recent studies. For example, one long-term study of more than 6,500 people in northern California has found that those who were fat around the middle at age 40 were more likely to succumb to dementia in their 70s. A long-term study in Sweden has found that, compared to thinner people, those who were overweight in their 40s experienced a more rapid, and more pronounced, decline in brain function over the next several decades.
Consistent with this, the brains of obese people often show signs of damage. One study of 60 healthy young adults (in their 20s and 30s) found that the fatter members of the group had significantly lower grey matter densities in several brain regions, including those involved in the perception of taste and the regulation of eating behaviour. A study of 114 middle-aged people (between 40 and 66 years old) has found that the obese tended to have smaller, more atrophied brains than thinner people; other studies have found similar results.
Brains usually atrophy with age, but being obese appears to accelerate the process. This is bad news: Pronounced brain atrophy is a feature of dementia.
Why fatness should affect the brain in this way is not clear, although a host of culprits have been suggested. A paper published earlier this week in the Proceedings of the National Academy of Sciences has identified a gene that seems to be involved. FTO, as the gene is known, appears to play a role in both body weight and brain function.
This gene comes in different versions: one version - let's call it 'troublesome' - appears to predispose people to obesity. Individuals with two copies of the troublesome version tend to be fatter than those with only one copy of it, who in turn tend to be fatter than those with two copies of the 'regular' version. Now, the troublesome form has been linked to atrophy in several regions of the brain, including the frontal lobes, though how and why it has this effect are still not known.
But genes are not the only guilty parties. Obesity exacerbates problems like sleep apnea, which can result in the brain being starved of oxygen; this can lead to brain damage. Obesity often goes along with high blood pressure, heart disease and diabetes, all of which are bad for the brain in their own right. Indeed, one study has shown that if, in middle age, you are obese and have high blood pressure, the two problems gang up on you, increasing the chances of your getting dementia in old age more than either one would do on its own.
Fat tissue itself may be a problem. Fat cells secrete hormones like leptin; leptin acts on the brain in a variety of ways, and is thought to play a role in the development of Alzheimer's. Obesity may thus disrupt the normal production of leptin, with dangerous results. Fat cells also secrete substances that cause inflammation; chronic inflammation of the brain, which is often found in the obese, impairs learning and memory and is also a feature of Alzheimer's.
Diet may play a role, too. Studies involving mice have shown that eating a very-high-fat diet increases brain inflammation and disrupts brain function. And the onset of brain decay may itself play a part. Since the regions of the brain most affected by obesity appear to be those involved in self-control and the regulation of appetite, erosion of these abilities may lead to greater obesity, which in turn may lead to more rapid brain erosion, in a downward spiral.
Whatever the causes, the implications are grave. In the United States today, around one-third of adults are obese. At the same time, dementia is already one of the most costly and devastating health problems of old age. The possibility that obesity today will lead to higher rates of dementia in the future is, therefore, deeply alarming.
The obvious question is: Can obesity-associated brain damage be reversed? No one knows the answer, but I am hopeful that it can. Those two old friends, a healthful diet and plenty of exercise, have repeatedly been shown to protect the brain. Foods like oily fishes and blueberries have been shown to stimulate the growth of new neurons, for example.
Moreover, one study found that dieting reversed some of the changes to brain structure found among the obese. Which suggests an interesting study. The most effective - and radical - treatment for obesity is bariatric surgery, whereby the stomach is made much smaller or bypassed altogether. Do people who have taken this option show a reversal, or at least a slowing, of brain atrophy?
But whether you are fat or thin, young or old, the best hope you have of guarding your brain is to eat well and exercise. Anyone seen my running shoes?
NEW YORK TIMES