Aerobics works the most important muscle, your heart, and keeps your good cholesterol up and your blood pressure normal. But muscle mass can only be built through weight-resistance training. The more lean muscle we have, the more we burn calories, and from the age of 30 on, we lose half a percentage point of muscle each year. The reason why is that he hasn't done any kind of resistance training. He's doing the aerobics classes, riding his bike, he's not pigging out but he's losing his furnace.
Muscle burns twice as many calories as fat.
As we get older, it's that much more imperative that we work out by pumping iron. I'm not saying you have to be a Peter Nielsen or Arnold Schwarzenegger. I'm saying get into at least 2-3 days of resistance training a week. Weight-resistance training is the best medicine for women as they go through hormonal changes. Weight training increases bone density, increases your metabolism so even at rest you're burning fat and it allows the body to better utilize the calcium and magnesium that strengthens bones.