The word “metabolism” has been thrown around and oversold so much by the fitness and weight loss industry that it’s true meaning has become a mystery to most people.  It has become something to be manipulated by supplements, eating patterns and special workouts.  This has led to the notion that the obese have been cursed with a slow metabolism and the lean are blessed with a fast metabolism.

A good definition of metabolism would be all of the chemical reactions in the body that keep us alive.  The energy used by our body’s metabolism is called our metabolic rate.  The two terms usually associated with metabolic rate are basal metabolic rate (BMR) and resting metabolic rate (RMR).  BMR and RMR are estimates of how many calories the body uses at rest and do not account for activity.

A basal metabolic rate is measured in a dark room, with the person reclined right after 8 hours of sleep and a 12 hour period of fasting.  The Harris Benedict Equation is often used to estimate this without the lab visit.

  • For men: (13.75 x w) + (5 x h) – (6.76 x a) + 66
  • For women: (9.56 x w) + (1.85 x h) – (4.68 x a) + 655

A resting metabolic rate requires less preparation and is often calculated with the Mifflin Equation.  I personally use this one and input the goal weight of my clients to determine how many calories they should eat per day.

  • For men: (10 x w) + (6.25 x h) – (5 x a) + 5
  • For women: (10 x w) + (6.25 x h) – (5 x a) – 161

For both of these equations w= weight in kg, h= height in cm and a=age.  You may wonder why height is used in these equations but height is a factor because the taller you are, the more lean body mass, i.e. bones, organ size, you have adding to your caloric needs.

Your resting metabolic rate, whether asleep or awake, is responsible for about 60-70% of your total calories burned.  Again, this does not include activity and is just the amount of calories needed to keep your tissues regenerating new cells.  Your heart, liver and brain account for a large amount of these calories in addition to your digestive system, skin and other bodily functions.  Muscle burns about 6 calories per pound per day and fat burns about 2.5 calories per pound per day which is why the only way to increase your “metabolism” or RMR is to gain weight.  So when you hear that building muscle will increase your metabolism, that is true but to what degree.  If you are fortunate enough to build 10 lbs of lean muscle in a year, which is possible for a person who is new to weight training, your metabolic rate will increase about 60 calories per day.  Most trained lifters are fortunate to add 2-3 lbs of lean muscle per year.  A lot of the “bulking up” you’ll see from overfeeding is water and fat.

Movement makes up about 20-40% of total calories burned, depending on how much you move of course,  A very elite athlete may have 40% of calories burned from movement but for most exercisers it would be closer to 20%.  If you’re a total couch potato on a random day, it would be much less. If you’re RMR is 1800 cal/day then 20% would be about 360 calories.  Remember to subtract your RMR from the number of calories your heart rate monitor or treadmill says you burned during exercise.  With an RMR of 1800, that would be 75 cal per hour that you would subtract from the total calories of an hour of exercise to know what you burned from the bout of exercise.  Those estimates are based on the formula of 1 cal burned for every kilogram moved 1 kilometer, presumably on foot.  So a very heavy, even obese, person will burn more calories walking a mile than a person of average weight going the same distance.  Most obese people have a higher lean body mass and a higher metabolism that a person of average weight of the same height, and as they lose weight, their metabolic rate will come down because they have less fat and muscle to support.

Approximately 5-15% of your energy expenditure, or metabolic rate, is caused by the digestion of food, called the Thermic Effect of Food (TEF).  The estimate for a normally balanced diet is a 10% average.  The TEF of protein is about 20-30%, carbohydrates is 5% and fat is 2-3%.  The thermic effect of alcohol is about 20% believe it or not.  So if you do the math, you can see why eating more or more often to lose weight really doesn’t make much sense.  If you eat an additional meal of 300 calories and only use 10%, or 30 calories, for digestion then you are left with 260 additional calories that you don’t need.  Surprisingly the New York Times actually ran an article about this myth in March 2010.  It is a brilliant marketing ploy to get people trying to lose weight or stay thin (pretty much everyone) to buy more food and meal replacement supplements.

Hopefully you can see why your metabolism is not something that can be dramatically manipulated by a supplement or a particular diet.  Your metabolic rate is pretty much set by your height and weight and can be moderately supplemented by exercise.  Don’t be fooled by the “boost your metabolism” marketers.  There is nothing wrong with your metabolism, you just need to know how it works.