Yuri Elkaim, BPHE, CK, RHN
Your metabolism is important in the weight loss equation because it represents 60-75% of your total caloric expenditure each and every day. Metabolism 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 metabolism due to lower fat-free mass.
Your resting metabolism is the rate of energy expenditure that your body goes through at rest. It is the number of calories your body needs to survive if all you did was lie in bed.
For each 1% increase in body fatness, your metabolism decreases about 0.01 calories/minute. 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 change may seem like no big deal, it is! This small difference can become much more meaningful if you progressively pack on weight. For example, a 5% increase in body fat 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.
METABOLISM IS ALSO INFLUENCED BY…
Fat-free mass is not the only factor influencing your metabolism. Since the early 1900s researchers have shown that prolonged dieting is associated with a significant decrease in metabolic rate (as much as 20% in some cases). For example, the famous Minnesota Starvation Experiment where caloric intake was restricted by 25% over the course of 24-weeks revealed that, per kg of bodyweight, metabolic rate 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 that thyroid is the master gland regulating your body's metabolism. A decrease in its function makes it more difficult to lose weight. I have seen this in practice having worked with hundreds of men and women who suffer from an underactive 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 these “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.
IS YOUR METABOLISM DOOMED?
Your metabolism is also responsive to periods of over-feeding. In one dieting experiment, when the subjects were allowed a day of "free eating", their metabolism was elevated on the following day. Further, in long-term (14-20 days) overfeeding to cause obesity, increases in metabolic rate have been recorded.
This would seem counter-intuitive. After all, if your metabolic rate 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.
DOES EXERCISE IMPROVE METABOLISM?
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.
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.
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 “short term” 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 metabolism and helping you lose weight and stay lean!
Benedict, F. et al. (1919). Human Vitality and Efficiency Under Prolonged Restricted Diet. Washington, DC. The Carnegie Institute of Washington.
Keys, A. et al. (1950). The Biology of Human Starvation. Volume 1. Minneapolis: The University of Minnesota Press.
About the Author
Yuri Elkaim is a world-renowned fitness, nutrition, and weight loss expert. He is the creator Fitter U and Treadmill Trainer, author of Eating for Energy, and the Head Strength and Conditioning Coach for men's soccer program at the University of Toronto. His trademarked 12-week Fitter U iPod workout program has been helping thousands of people around the world get in shape and lose weight fast without the cost and inconvenience of hiring a trainer. Go to www.myFitterU.com now to get your FREE Fitter U workout and "How to Get Fit and Lose Weight Fast" report!