The real reason we gain weight as we get older...

As we get older, we tend to gain weight.

If you thought it was purely because our metabolism is slowing down. You're right.

But what exactly makes our metabolism slow down?

There's actually a variety of reasons why the pounds become more difficult to keep off.

Dr. Caroline Apovian, the Director of the Nutrition and Weight Management Center at the Boston Medical Center, has revealed a change in hormones, a more sedentary lifestyle, an increase in stress and decrease in sleep due to added responsibilities are all factors.

"But a major reason for middle aged weight gain is the natural muscle loss we all experience," Dr. Apovian says.

"The amount of lean muscle mass we have is the primary determinant of metabolic rate. In other words, the more muscle mass we have, the more calories we will burn. Our muscle mass naturally begins to decline around age 30, and that process, called sarcopenia, accelerates around age 40. Unless something is done to actively protect and build up that lean muscle mass, our bodies will require fewer calories, our metabolisms will slow, and the lost muscle will be replaced by fat."

So, what can you do to prevent sarcopenia?

Dr. Apovian suggests "exercise with weights at least twice per week, building up in both weight and intensity as you progress. Eat a diet rich in lean protein sources, including protein smoothies. And get plenty of sleep. Amongst other health benefits, this gives the body time to repair and rebuild the muscles."