Basal metabolic rate (BMR) is the rate of energy expenditure that your body goes through at rest. Stated otherwise, it is the number of calories your body needs to survive if all you did was lie in bed.
BMR is important in the weight loss equation because it represents 60-75% of your total caloric expenditure each and every day. The BMR is proportional to your fat-free mass (ie. muscle), and after age 20 it decreases by about 2-3% per decade. In general, women tend to have a lower BMR at all ages due to lower fat-free mass.
At any body weight metabolic rate decreases about 0.01 calories/minute for each 1% increase in body fatness. Thus, simply by having more fat on your body decreases your basal metabolic rate. Conversely, doing things that will burn fat and promote muscle development will increase your metabolism.
While this 0.01 calorie/minute change may seem insignificant, this small difference can become much more meaningful if you progressively pack on the pounds. For example, a 5% difference in body fatness at the same body weight results in a difference of 0.05 calories/minute, which is equal to 72 calories/day. Since 3500 calories equals 1 lb of fat, in our example, it would take 48 days to add on one extra pound of fat (everything being equal). That might not seem like much but over the course of 1 year that would be an extra 7 lbs of fat just because of an initial 5% body fat difference. But it would actually be more than 7 lbs since your body fat would increase as you continued to pack on the weight.
Fat-free mass is not the only factor influencing BMR. As early as 1919, Dr. Benedict and his colleagues showed that prolonged dieting was associated with a 20% decrease in BMR. These findings were confirmed in the famous Minnesota Starvation Experiment where caloric intake was restricted by 25% over the course of 24-weeks. This study revealed that, per kg of body weight, BMR is reduced on a semi-starved diet. The researchers also showed that T3 (one of the thyroid hormones) and sympathetic nervous system activity decreased as well.
The latter two findings are rather significant considering tha thyroid is the master gland regulating your body's metabolism. A decrease in its function will inherently make it more difficult to lose weight. I have seen this in practice having worked with hundreds of men and women who suffer from an under active thyroid who at the same time, have had a very tough time losing weight.
The sympathetic nervous system plays a pivotal role in the body's ability to mobilize and burn fat. Since it stimulates the release of adrenaline from the adrenal glands, a hormone that breaks down fat, any interruption to its functioning will also impair fat burning.
What the data from these semi-starvation studies reveal is that during prolonged periods of low caloric intake, the energy production of the tissues decreases in an attempt to adapt to the lower caloric intake and reduce the rate of weight loss. It's simply a survival mechanism. Obviously, this is an appropriate adaptation in periods of starvation, but it is counterproductive if your goal is to lose weight fast and for good.
This would seem counter-intuitive. After all, if your BMR increases wouldn't it help you lose weight? In most cases, yes, but not if eating to a point of creating a dramatic positive energy balance.
In essence, during the dynamic phase of weight gain (going from a lower to higher weight), more calories are required per kg of body to maintain the weight gain than maintain normal body weight. This is obvious and it's also one of the reasons why you may feel hungrier when you are more active.
There is no doubt that your metabolic rate is elevated following exercise. The questions relate to how much and how long it is elevated, and to what extent it contributes to total daily energy expenditure.
Studies have shown that trained individuals have a higher metabolic rate than untrained individuals - but only when they did high intensity workouts and consumed sufficient calories to maintain energy balance. This suggests that the higher metabolic rate in trained individuals is not due to chronic adaptations associated with training, but more to do with the acute energy flux associated with the most recent workouts and dietary choices.
Therefore, it may be said, in the long run, daily exercise is needed to maintain an elevated metabolism. However, it should be remembered that what you do today will potentially have a greater acute impact on your metabolism than your combined workout efforts over the past few years. Just another reason why working out at least every other day is imperative for maintaining an elevated basal metabolic rate and helping you lose weight and stay lean!