When it comes to your metabolism, age can slow you down, making it easier to gain weight once you hit 40.

But if you aim to turn that around, experts offer a few tips to boost your metabolism and shed a few extra pounds.

One way to do so is by altering your diet, exercise, and sleep habits, suggets Scott Isaacs, M.D., an endocrinologist in Atlanta and author of Hormonal Balance: How to Lose Weight By Understanding Your Hormones and Metabolism.

"Think of your body as an engine — your metabolism is the rate at which your engine runs," he explained to Healthmagazine. "By making adjustments to these three elements, you can actually make your engine rev higher."

On September 4, fitness blog FitSugar and Health magazine offer a few expert tips to get your metabolic engine burning — at any age. The promise? You can drop ten pounds, or 4.5 kilos, in 21 days.

1. Eat early
"Breakfast triggers a process called thermogenesis, where the body signals the brain to activate the metabolic process of turning food into energy," Mark Hyman, M.D., author of The Blood Sugar Solution, told Healthmagazine.

2. Watch your sugar
"As we get older, it's crucial to pay attention to how much sugar we're consuming," stated Diane Kress, R.D., author of The Metabolism Miracle. "Too much messes with your metabolism by causing your body to store extra calories as fat."

3. Eat often
To optimize your metabolic burn, Hyman recommends eating small meals every three or four hours.

4. Work out, with intensity
"Not only does it affect your metabolism while you're doing it, but research shows you can keep burning calories up to 24 hours after you finish because your metabolism stays elevated," Isaacs stated.

5. Sleep well
Research suggests that people who sleep two-thirds of their usual amount (such as five hours instead of eight) eat an average of 549 extra calories the following day without even noticing. Doing this regularly means a lot of extra weight.