The metabolism is a set of chemical reactions, and there are several external factors that can affect it, such as medication or stress. Therefore, before you try speed up your metabolism, it's best to consult your physician to make sure all factors are considered and you are proceeding in a healthful manner. Besides, it's just good common sense to talk to your doctor before attempting to modify your diet or exercise regimen.

That said, there are several simple and natural ways to incorporate metabolism-boosting foods and activities into your lifestyle.

The quickest way to jump start your metabolism is to exercise. Any exercise will do, really, provided it's an activity that keeps you engaged and interested and coming back for more. However, there are two types of workouts you should incorporate into your routine to guarantee a high metabolic rate.

1. Lift Weights: It's well known that muscle burns more calories than fat, so the more lean muscle mass you build, the higher your resting metabolism will be. Plus, a sustained session of resistance training triggers muscle activity throughout the body, raising your metabolism for hours afterward.
2. Interval Training: Like weight lifting, alternating bursts of high-intensity cardiovascular exercise with periods of rest leads to a longer and higher resting metabolic rate after exercise. Also, intervals make your exercise sessions more efficient - you actually burn off more calories and take in more oxygen in a shorter time with intervals than with long, steady-state aerobic exercise.

Yes, you can eat to raise your metabolism. But it's important to know what to eat and how to eat to get your engine revving.

1. Eat breakfast. People who eat breakfast tend to have a lower body mass index than those who skip their morning meal, which makes sense. By skipping breakfast, we're asking our bodies to get moving after lying dormant for eight hours and fasting. This actually slows your metabolism. However, eating fiber-rich foods, such as fruit and whole grains, and low-fat calcium-rich foods, such as skim milk, has been shown to boost the metabolism.
2. Eat protein. Our bodies burn nearly twice as many calories digesting protein as either carbohydrates or fat, and they help our muscles repair after a particularly intense exercise session. Stick to lean meats, such as chicken, or fish, some of which are also rich in heart-healthy Omega-3 fatty acids. Finally, round out your diet with plant-based proteins, such as nuts and beans.
3. Eat spicy. A compound found in chili peppers known as capsaicin has been shown to rev up our metabolisms by as much as 25%. So unless you are sensitive to spicy foods, feel free to find ways to add spice to your diet.
4. Eat more often. Rather than having two or three large meals a day, nibble and snack - especially fiber- and water-rich fruits and vegetables - throughout the day. Eating large meals tends to cause a spike in our insulin levels, which slows the metabolism. Eating in small quantities throughout the day, however, keeps insulin levels steady and relatively low, thus raising the metabolism.
5. Eat organic. Yes, organic foods cost more, but in many cases they're worth it. The toxins found in non-organic meats and vegetables, including hormones, pesticides, and antibiotics, affect your thyroid function, thus hindering your metabolism.

No, not cocktails. In fact, stay away from alcohol. Rather, stay hydrated, because our bodies need fluids to process calories.

1. Drink water. Even mild dehydration will lower your metabolic rate. And what better way to stave off dehydration than with abundant, natural, and calorie-free water.
2. Drink water with ice. Consuming cold beverages triggers our bodies to burn as many as 10 more calories a day. That's 3,650 more calories each year we don't have to worry about.
3. Drink green tea. The caffeine and catechins in between two and four cups of green tea boost our metabolisms to the tune of 50 calories a day. Plus, unlike the caffeine in coffee, which stimulates a faster heart rate, the caffeine in green tea stimulates our nervous system and brain to fire faster. Also, some of the antioxidants in green tea have been shown to have anti-inflammatory properties, which might help post-workout.

1. Avoid simple carbohydrates. Alcohol, processed sugar, and products made with white flour all produce spikes in our insulin levels, which slows the metabolism.
2. Avoid late nights. A lack of sleep has been shown to affect how we process carbohydrates, which in turn raises insulin levels and slows the metabolism. Furthermore, the lower energy levels we experience as a result of sleep loss tends to also lower our activity levels while we're awake.
3. Avoid crash diets. Sustained large-scale calorie deprivation actually trains our bodies to lower our metabolisms to use fewer calories. Also, though diets that restrict you to under 1,000 calories per day result in weight loss, often that lost weight is in muscle. And the less muscle you have, the fewer calories you burn.