Many people don’t know that you can actually measure your metabolic rate in calories per day. Other people know that it can be measured because fitness facilities include a value for metabolic rate in their fitness evaluations. Even fewer people know the differences among the various methods and the wide variety in levels of accuracy.

So why bother to measure your metabolic rate? To answer that question, let’s first define some terms for clarification. Basal metabolic rate, the sum total of the energy required to perform the biochemical transformations that occur in the body, contributes 70% of the body burn rate, the daily total energy requirements. The 30% difference between burn rate and metabolic rate is based on the energy required to move muscles and digest food. Basal metabolism is an absolute term for metabolic rate that would require an absolute resting state. That is unable to be achieved, so the best we can do is measure resting metabolic rate (RMR) or it’s equivalent term, resting energy expenditure (REE). RMR is not theoretically the same as basal metabolic rate, but is the closest we can come. Once you measure RMR, you can determine your burn rate by adding 30% to the RMR, or some value ranging from 15-40% based on activity level. I prefer to use the standard of 30% and measure independently the calories burned during exercise to personalize the energy requirements.

Now that we know the terms, let’s go back to answer the question of “Why bother?”. Let me start by saying that I believe burn rate is a vital sign that should be monitored life long to individualize body weight regulation. Each person is unique in the amount of energy required to maintain any given weight and the energy needs change over time. As a child grows, their burn rate changes to alter the amount of energy that is used or stored. As an adult, burn rate changes in response to aging, loss of muscle mass, hormone changes, pregnancy ,and many other factors discussed in Chapter 2 of my book, “The Burn Rate Diet”. Keeping track helps you know when to adapt your daily calorie intake to regulate your body weight. It gives you the knowledge and the science to avoid fad diets, maintain a natural relationship to food, and keep your weight in control throughout your life. Most importantly, knowing your burn rate helps to build a healthy self-esteem. Weight control becomes based on science and biology, not on the lack of self-control that is the misconception of our entire culture. Knowledge of your burn rate provides protection from the cultural insults to people who are biologically designed to have larger shapes and sizes.

Since this vital sign is so significant to your health, it is important to measure it in the most accurate manner. Nobody would think to manage their diabetes with poor measurements of blood sugar. You want an accurate read of your blood pressure to know when to take medication, regulate your diet, or increase your exercise. The same holds true for burn rate. You want the most accurate method to guide your weight control efforts. Gas exchange is the most accurate by far. Gas exchange involves the measure of your oxygen and carbon dioxide content of your breath under conditions of fasting and resting. Your RMR is indicated by the breath by breath measurements taken over a period of 15-20 minutes.

I use a method in my book that is not nearly as precise as gas exchange. I ask people to eat a fixed intake of calories over 14 days and then measure the changes in body weight that occur over that period. For each pound that is changed, it reflects a 500 calorie difference per day in daily energy requirements above or below the level of the diet. While this method can provide a reasonable approximation, it relies on scale weight changes that are altered by changes in fluid as well as fat mass, and are only as reliable as the fluid loss can be controlled and the calories accurately recorded.

The most common method to measure metabolic rate is a formula, called the Harris-Benedict (HB) formula, that predicts metabolic rate based on height, weight, and age. For example, if a person’s percent body fat is measured by electrical impedance, the output will often contain the output of the HB formula reported as Resting Metabolic Rate. People erroneously believe that the output has something to do with the electrical impedance results. In fact, they have nothing to do with the measurement of fat mass and merely the outcome of the HB formula written into the output.

How reliable is the HB formula? I have found that most differences fall in the 100-200 calories per day range, with the highest difference being 540 calories per day. How important are these differences? Small differences in burn rate make large differences in what you will weigh and the amount of work it will take to manage that weight. In establishing individualized weight goals, each 100 calorie difference in burn rate corresponds to a 10 pound difference on average in weight goal. How much does a 20 or 30 pound difference in your weight make to you? Consider the impact on your eating and lifestyle. Is there much difference between a burn rate of 1500 or 1700 calories per day? Think about what happens if you eat 1400 extra calories eating and drinking at a party. If you have to cut back that amount to maintain your weight, you might reduce your intake to 1300 calories per day for several days to offset the increased intake. At a burn rate of 1500 calories, you would need to follow the 1300 calorie intake for 7 days to make up for the food at the party. At 1700 calories, you would need to restrict for 3.5 days, only half the time, giving you a better chance to manage your weight over the long term. Small differences in burn rate make large differences in what you will weigh and the amount of work it will take to manage that weight.

In summary, don’t use approximations on a vital sign. Get your burn rate tested by gas exchange on an annual basis and make it a part of your annual physical exam. It will take a load off your mind and your body.