It's no mystery that your metabolism will change with age. You won't be able to eat three hamburgers for a mid-afternoon meal and easily work off the calories later. Being aware of those changes and making adjustments to your lifestyle to complement each stage of life can help you maintain a svelte, healthy body.

Your metabolism's efficiency will start to decrease in your 40s. You won't need as many calories to fuel strenuous activities, but it will be harder to maintain and / or lose weight. So what are the keys to keeping your metabolism working? Specific food, different types of exercise and plenty of sleep.

"Think of your body as an engine; your metabolism is the rate at which your engine runs," explains Dr. Scott Isaacs, an endocrinologist in Atlanta and author of Hormonal Balance: How to Lose Weight by Understanding Your Hormones and Metabolism. "By making adjustments to these three elements, you can actually make your engine rev higher."

1. Focus on eating heart-healthy fats, such as coconut oil, to help lower cholesterol, and foods that reduce stress and inflammation. Fill up on green tea, Brazil or walnuts, fish packed with omega-3 fatty acids, and fibrous fruits and vegetables. Shrink your portion sizes, and limit snacking to eliminate unnecessary calories.

2. Start using resistance training, walk wherever possible, and explore aquatic fitness so you can stay lean and limber without straining yourself.

3. Get enough quality sleep. An article from reported that just one night of interrupted sleep can cause your body to burn up to 20% fewer calories the next day. Make an effort to get at least seven to eight hours of quality slumber per night.