A few years after undergoing hip and knee replacement surgeries, Jane Fonda launched an exercise DVD for aging baby boomers.

The former queen of workout videos is now dedicated to helping herself and others thrive after surgery by making sure they stay active and don't pack on pounds.

That's smart. According to this year's Global Obesity Meeting, surgeries for everything from heart disease to kidney problems — although necessary and life-changing — can make it tough to maintain a healthy weight.

Weight gain often happens because of:

• Being relatively inactive after surgery, even though doctors want you to walk as much and as soon as possible after most surgeries

• Side effects of new medications

• Inflammation caused by trauma to tissues and/or organs

• Emotional stress triggering water retention as well as worry about how hardy you really are

• Cravings for comfort foods

Unfortunately, added weight interferes with recovery because it increases inflammation and saps a patient’s strength, making it harder to get appropriate physical activity.

So if you're headed for surgery or have recently undergone it, talk to your doctor about what physical activity you should do.

Cardio rehab and physical and occupational therapy after surgery are essential. They decrease unwanted blood clots and inflammation, and speed recovery.

When you complete programs, commit to a walking routine and strength-building exercises that you can design with your healthcare provider.

Also adjust your calorie intake to your activity level, so that you burn what you eat.