A new study finds obese people have 8 percent less brain tissue than normal-weight individuals. Their brains look 16 years older than the brains of lean individuals, researchers said today.

Those classified as overweight have 4 percent less brain tissue and their brains appear to have aged prematurely by 8 years.

The results, based on brain scans of 94 people in their 70s, represent "severe brain degeneration ," said Paul Thompson, senior author of the study and a UCLA professor of neurology.

"That's a big loss of tissue and it depletes your cognitive reserves, putting you at much greater risk of Alzheimer's and other diseases that attack the brain," said Thompson. "But you can greatly reduce your risk for Alzheimer's, if you can eat healthily and keep your weight under control."

The findings are detailed in the online edition of the journal Human Brain Mapping.

Obesity packs many negative health effects, including increased risk of heart disease, Type 2 diabetes, hypertension and some cancers. It's also been shown to reduce sexual activity.

More than 300 million worldwide are now classified as obese, according to the World Health Organization. Another billion are overweight. The main cause, experts say: bad diet, including an increased reliance on highly processed foods.

Obese people had lost brain tissue in the frontal and temporal lobes, areas of the brain critical for planning and memory, and in the anterior cingulate gyrus (attention and executive functions), hippocampus (long-term memory) and basal ganglia (movement), the researchers said in a statement today. Overweight people showed brain loss in the basal ganglia, the corona radiata, white matter comprised of axons, and the parietal lobe (sensory lobe).

"The brains of obese people looked 16 years older than the brains of those who were lean, and in overweight people looked 8 years older," Thompson said.

Obesity is measured by body mass index (BMI), defined as the weight in kilograms divided by the square of the height in meters. A BMI over 25 is defined as overweight, and a BMI of over 30 as obese.

The research was funded by the National Institute on Aging, National Institute of Biomedical Imaging and Bioengineering, National Center for Research Resources, and the American Heart Association.