Some serious medical conditions such as hypothyroidism or Cushing's syndrome can cause your metabolism to slow. If you have an unusually slow metabolic rate, or if your metabolism has slowed significantly in a short period of time, you should consult a doctor.  Your doctor will have to take a blood test to measure your thyroid hormone levels to determine whether your thyroid is producing decreased levels of thyroid hormones that are causing your metabolism to slow. Keep in mind that hypothyroidism is fairly common – 1% to 2% of the population in iodine replete cultures. Cushing's is less common. Even if you don't have a medical condition that is causing your metabolism to slow, your doctor may be able to help you get back on track by providing suggestions or advice on how to boost your metabolism. Your metabolism often will slow when you diet because you're consuming too few calories. To combat this slow-down, make sure you're consuming at least as many calories per day as your body requires to function normally.  The difference between your BMR and the total calories you burn each day represents the range in which you can cut calories to lose weight. If you dip below your BMR, your metabolism will slow and your weight loss will plateau. If you're dieting, be prepared to do the calculations regularly as your BMR will change when you lose weight. People who are heavier tend to have a higher metabolism than people who are lighter. This change in BMR may be part of the reason you find it easy to lose weight in the beginning, but then plateau and find it difficult to lose weight the further along in your diet plan you get. While fiber may not necessarily boost your metabolism, if you eat more fiber, you'll gain less weight over time. Fiber also may increase how much fat your body burns. Cut out white bread and white rice, which lack the fiber your body needs.  Switching to whole wheat bread and pasta and brown rice can up your fiber intake. You'll also increase your fiber by eating more fruits and vegetables. Try to consume around 25 grams of fiber each day. You can get this fairly easily in three regular servings of fruits and vegetables. Just try to have a fruit or vegetable serving with each meal. You've probably heard that you should drink at least six glasses of water a day to keep your body optimally hydrated. Because the water you drink must be warmed to body temperature, drinking ice water burns more calories than drinking lukewarm water, boosting your metabolic rate.  According to one study, drinking six glasses (or 48 ounces) of cold water each day increased the participants' metabolic rates by an average of 50 calories. Even if you change nothing else, this trick alone could allow you to lose up to five pounds in a year. Protein is essential to build and strengthen your muscles. If you don't eat enough of it, you'll lose muscle tissue and strength, especially as you get older. Eating protein also increases the number of calories you burn after every meal.  Lean meats such as chicken and turkey are good sources of protein. If you're a vegetarian, consume soy products, nuts, and protein-rich vegetables such as spinach. Protein-rich foods such as lean meats and fish are also often rich in iron, which is an essential nutrient for raising your metabolism and burning fat. Maintaining an active lifestyle is one of the surest ways to boost your metabolism. If you consistently burn calories through exercise, your body's metabolic rate will adjust accordingly.  Plan on getting at least 20 to 30 minutes of exercise every day. This can be as light as a brisk morning walk, or as intense as doing interval training or taking a step-aerobics class. Make sure you include strength training as well as aerobic exercise. Stronger muscles burn more calories, which also will increase your metabolic rate. You don't need to strength train as often as you do cardio – 20 minutes two or three times a week is all you need. . Stress can have a negative effect on your metabolism by elevating your cortisol levels. Your body may also end up holding on to more of the calories that you take in when you are stressed. If your stress is chronic, then this can lead to weight gain, especially around your midsection. Make sure that you make time in your daily routine to practice stress reduction activities, such as deep breathing, meditation, and yoga.

Summary:
Rule out chronic medical conditions. Use your BMR to diet effectively. Eat more fiber. Drink cold water. Include protein with every meal. Make cardio and strength-training a regular part of your daily routine. Find ways to reduce stress