Changes in the metabolic rate can have different causes. Here you'll find an overview with causes and tipps how to maintain a fast metabolic rate.

Exercise, Stress and Lack of Sleep can Affect your Metabolism

Metabolism refers to the rate at which your body utilizes calories to meet energy demands. The Basal Metabolic Rate (BMR) refers to the number of calories you burn while your body is at rest. Your BMR decreases as you age. This means that when you get older it is harder for your body to burn calories and harder for you to lose weight. A daily routine of cardiovascular exercise can improve your health and fitness and increase your BMR.

Stress has both positive and negative impact on the metabolism. Initially, it increases it to provide extra fuel for the body to react to any stressful situation. Prolonged stress, however, can actually lower the BMR by disrupting the digestive system. Lack of sleep makes you too tired to exercise or to work out intensely. It reduces your BMR. If you reduce the amount of stress in your life and get more sleep each night, your normal BMR will return. Not easy in the modern world, with its fast pace and demands on your time, but it can be done.

    Exercise
    That exercise increases the BMR is a well known weight loss fact. But did you know that some exercises will cause the body to continue burning calories at a higher rate after the exercise is finished? The amount depends on how hard and how long you exercise.

    For example:
A 154 pound person runs 8 mph and will burn 320 calories in 20 minutes. If that same person walks 3 mph for an hour, she will burn 235 calories.

Walking or jogging will restore the BMR within 60 minutes, meaning that, at best, you will lose 10-30 additional calories during your recovery period. Low-intensity exercise training does have a plus side. Studies have shown that fat oxidation increases by 40%. High-intensity exercise training, on the other hand, does not affect fat oxidation, but you will continue burning calories at the accelerated rate well into the next morning. So which form of exercise should you choose? Experts say that low-intensity exercise is preferable because the risk of musculoskeletal injuries is lower and you are more likely to stay with a low intensity exercise program.
    Stress
 During periods of stress, the hypothalamus instructs the adrenal glands to release epinephrine (adrenaline), norepinephrine and cortisol into the bloodstream to speed up heart rate, respiration, blood pressure, and metabolism. Epinephrine breaks down glycogen into glucose in the liver. Both hormones increase circulating free fatty acids. The extra glucose and fatty acids are used by the body as fuel in times of stress.

Prolonged stress disrupts the digestive system. The stomach produces excessive amounts of digestive acids. Irritable bowel syndrome develops when the smooth muscular contractions that move food along the large become spastic. When the abdomen becomes bloated, a person experiences cramping, constipation and diarrhea. Studies suggest that stress may make a person more susceptible to peptic ulcers or sustain existing ulcers. Stress has also been related to increased flare-ups of ulcerative colitis. Such conditions serve to lower the BMR. If the stress is removed, the body improves and the Basal Metabolism is raised.
    Lack of Sleep
The lack of sleep has been shown to change hormone levels in the body. As sleep decreases, the adrenal gland produces more of the hormone cortisol. It regulates appetite and when its levels increase people continue to feel hungry despite being full. Lack of sleep also causes levels of growth hormone to decline.

    This results in following consequences:
        Reduction of muscle mass and strength
        Increase of fat tissue
        Weakening of the immune system

Insulin is a hormone produced by the pancreas to carry sugar (glucose) from the blood to the muscles and other tissues within the body. Insulin levels increase when the body does not get enough sleep. Because insulin causes sugars to be stored as fats, increased insulin makes weight control difficult. Sleep deprived individuals often eat candy or cookies when they feel their energy level dropping. As their blood sugar rises, energy returns, but the unneeded calories are converted to fat.

    Tired people burn fewer calories because they lack the energy to exercise or work out intensely. They may exercise the same duration as a rested person, but they will burn fewer calories. Getting enough sleep reduces the effects of increased cortisol levels.
    Age
    When we get older, our metabolism decreases which makes it more difficult for us to maintain or lose weight. But there are a lot more factors that influence the metabolism. When we turn 30 our physical abilities will decline throughout our life. It depends on your fitness level and your lifestyle how fast the decrease will happen.

        Decrease in reaction an slower performance of tasks is about 15%
        Lung capacity will decrease approximately 40 %
        Reduction of muscles circa 40 to 50 

Because of less activity and a slower metabolism will the fat cells increase. When you train regularly you could slower the metabolism decrease.