Want to keep your brain razor sharp for years to come? Just say no to middle-age spread.

When it comes to memory and concentration, it may pay to practice waist control. Packing on extra pounds can slow not only your time in the 50-yard dash but also your ability to remember things and stay focused. Need help shedding weight? Find a diet buddy.

Obesity and declines in both memory and attention span have long been linked in older people, but new research has found the same relationship in 30-somethings, too. And the slowdown only gets worse over time.

A large study tested 2,223 adults, ranging in age from 32 to 62, to see how their weight affected four different measures of cognitive function. Five years later, the researchers tested them again. Those who were seriously overweight -- meaning they had a body mass index (BMI) over 30 -- did poorly on the first round of testing. Worse, their mental abilities had declined even more at the 5-year mark.

Why? One theory faults leptin, a protein secreted by stored fat; high leptin levels may affect learning and memory. Another theory is that the brain is harmed by cardiovascular disease, which is commonly associated with high BMIs.

Although you don't need six-pack abs or a bikini-worthy waist to stay sharp and age healthfully, obesity is not a viable alternative. Join a health club, try the YOU: On a Diet plan, do whatever it takes. Do it not just for your body, but for your brain!