In an episode of "The Simpsons," Homer, who never completed high school, decides to take an exam to finally get his diploma. The problem is that his lifestyle choices have made his memory a little shabby.

"All right, brain," he pleads, "you don't like me and I don't like you, but let's just do this, and I can get back to killing you with beer."

At least Homer was smart enough to know his notoriously poor habits were hurting his academic performance.

A recent study published in the journal Neurology suggests that cardiovascular disease (Homer had a triple bypass in season four), diabetes, and smoking in middle age are reliable predictors of early decline in memory, executive function, and processing speed.

The 2,675 middle-aged adults in the study took thinking and memory tests at its start and then at a follow-up five years later. Overall, people with all three risk factors were nearly three times as likely to experience a faster cognitive decline in middle age as those without the risk factors.

Specifically, more than 10% of those with diabetes had an accelerated midlife decline in brain power, compared with 4.7% of those without diabetes. Nearly 8% of current smokers had faster cognitive decline, compared with 4.3% of those who never smoked.

The study underscores the importance of knowing your numbers — blood pressure, LDL cholesterol level, blood sugar, and body mass index (BMI) — from your 20s on, and using your cognitive powers to discover the joys of healthy nutrition, physical activity, and steering clear of all smoking.