When it comes to eating a healthy diet and staying fit, what's your biggest motivator? Is it feeling good? Staying active? Fitting into your jeans? Maybe it's all of the above. Those are all good reasons for staying fit, but here's maybe the biggest one of all: Being obese could cut your life by 10 years. I don't know about you, but I'm suddenly feeling very motivated.

This finding comes from an analysis of 57 different studies that included almost 900,000 people. The short story is this:

• A normal BMI is 18.5 to 24.9.
• Death rates were lowest among people with a BMI of 23 to 24.
• A BMI of 30 to 35 could potentially take two to four years off life.
• Being morbidly obese, or having a BMI of 40 to 45, could shorten life by eight to 10 years.
• Being underweight, with a BMI of less than 22.5, was also associated with a shorter lifespan.

Researchers attribute early deaths to cardiovascular, kidney and liver disease, and to a lesser extent, cancer. "Obesity is the new dark horse for public health officials," Dr. Arne Astrup told MSNBC. "People need to be aware of the risks they're taking when they gain weight." In fact, lifelong obesity might just be as bad as a lifelong smoking habit.

Let's put this in practical day-to-day terms: Spending 20 to 30 minutes on that workout today could translate to 10 (or two or four) extra years down the road. Ditto on nixing that nightly dessert. It's a small sacrifice that could pay off big for you down the road. It's like an insurance policy, but one that pays dividends during your life, not after it's over.

Here are three things you can do today, this minute, to get yourself on the road to fitness:

• Know your numbers. BMI isn't a perfect measurement, but it's a start. Use this calculator from AOL Health to figure out your BMI.
• Make one small change. Walk 10 minutes in one direction, then turn around and come back. Cut soda. Start packing your lunch. Visit Small Step for more ideas.
• Set a goal. In fact, set two ... one for the short-term, or the next six weeks, and one for farther down the road. Maybe you want to lose five pounds, exercise at least five days a week or eventually run a 5K. Goals keep us motivated.

Does knowing that serious weight gain could shorten you life span spur you to make healthy changes? If not, share with us what does motivate you.