The science of exercise will be a series of articles meant for those of you who like to dig a little bit deeper, yet remain practical when it comes to exercise. Prior to my career in medicine I was an engineer, which has served me well when thinking about the science of exercise and the perfectly engineered but incredibly complex system that is the human body. Despite the complexity and the fact that we are all individuals with different body shapes and sizes, years of scientific study has allowed us to determine many of the basic laws which govern our physiology, although that knowledge is ever evolving. As a physician, I am forced to constantly confront obesity as a major cause of morbidity and mortality. This is often a difficult discussion with patients and I commonly hear comments such as “doctor I don’t eat much at all” or “my metabolism must be slow” or “I am just different than everyone else doc.” While there may be a grain of truth in each of these statements, with proper knowledge, habits, and patience, all of these can be overcome. We will address these issues in more detail in future science of exercise posts, but for now let’s discuss metabolic rate.
Science of Exercise: Basic Physiology
When discussing the science of exercise, metabolic rate, and weight loss, it helps to consider the basic laws of physics, including the laws of conservation of mass and conservation of energy. In their simplest forms, these laws state that total amount of energy and mass in a closed system must remain constant. The energy and mass can be converted from one form to another, but they must remain constant unless they are removed from or added to the system. So if you apply these basic laws to our body, we confirm our common sense notions about weight loss: in order to lose weight, we must add less calories of food to our body than the calories our body uses on a daily basis, such that we have a calorie deficit. There are 3 ways to achieve a calorie deficit: (1) eat less food so we take in less calories than our body uses, (2) increase our activity level so we require more calories than we take in, or (3) eat less food and exercise simultaneously. In fact, it is important to note that diet and exercise are synergistic, meaning that the combined effects of simultaneous diet and exercise are greater than simply adding their effects together if they had been done separately. However, in reality, diet and exercise as a means to lose weight are not always that simple for people because the human body likes to maintain stability. We will discuss these stabilizing mechanisms in a future science of exercise post.
Science of Exercise: What is metabolism, basal metabolic rate, and why do I care?
At its simplest, the metabolism of an organism refers to the sum of all ongoing chemical reactions that sustain the life of that organism. The metabolic rate refers to the speed at which these reactions occur and can be expressed in terms of energy use per unit of time. Thus, you can think of your own metabolic rate as the amount of energy required to keep your body alive and functioning on a daily basis. Practically, we can define the total energy expenditure as the total amount of energy used by our bodies in a day. For an average person, approximately 20% of the total energy expenditure is due to activity, 10% is due to digestion, and the remaining 70% is used in keeping our bodies alive. This 70% is what is known as our basal metabolic rate (BMR) and is defined as the amount of energy used while our bodies are completely at rest and fasting for 12 hours, and is typically expressed as Calories per day. One analogy for BMR that can be helpful is an automobile that is idling – gas is still being used to keep the car on even though it is not moving. We obviously cannot turn our bodies off – even at rest our cells are being repaired, our heart is pumping, our brain is busy worrying why we can’t lose weight, etc. The energy required for all of these baseline processes is our BMR. In reality, we do not generally lay around in a bed all day fasting, so the BMR is just a starting point to help us think about our baseline calorie requirement. Even on days when we are not exercising, we require a good bit more energy than our BMR simply because we are walking around, talking, problem solving, digesting food, etc. Therefore, in order to maintain a stable weight, we must take in more calories on a daily basis than our BMR.
Science of Exercise: How do you determine your metabolic rate?
It turns out that your rate of oxygen consumption is directly proportional to your metabolic rate. Therefore, metabolic rate can be determined at any given time by measuring oxygen consumption using a special mask and a technique called indirect calorimetry. This can either be done while performing an activity such as running on a treadmill or while resting quietly in a bed. If performed at rest and after 12 hours of fasting, this measurement will be your BMR. Obviously this method is not very practical because it requires special equipment to measure your oxygen consumption, so scientists have developed surrogate methods for calculating your BMR based on basic data about yourself that you can easily measure at home. This was done by collecting oxygen consumption data and body measurements from a large population of people and then deriving a formula which could be generalized. A few different research groups have done this, but the most commonly used equation is the Harris-Benedict Equation and its revision which was originally derived in 1919 (see below). You can see from the formula that you can estimate your BMR only by knowing your weight, height, age, and gender. This is not a perfect estimate, but it is accurate enough to be useful when planning for weight loss and exercise.
Men’s BMR = 66.5 + (13.75 x Wt in kg) + (5.003 x Ht in cm) – (Age * 6.775)
Women’s BMR = 655.1 + (9.563 x Wt in kg) + (1.850 x Ht in cm) – (Age * 4.676)
The underlying driver of these equations is the amount of muscle mass that you have, because muscle has a higher metabolic rate than fat. Thus, women typically have a lower metabolic rate than men due to their comparatively lower muscle mass percentage. The same can be said of aging – as we age, we move towards a lower percentage of muscle mass, and thus a lower metabolic rate. However, don’t make the mistake of thinking that an obese individual has a low metabolic rate. Fat is still metabolically active, especially when there is a lot of it.
Basal Metabolic Rate Variability
The variability of BMR among people with similar characteristics is generally small. While people can have “gland problems” (thyroid, adrenal) that affect BMR and “fluid problems” (heart failure, kidney failure) that falsely elevate true body weight, these generally do not affect the BMR that much. We will address these issues more directly in a future post. We have included a calculator below that you can play with to see how the different variables affect the BMR. One thing to note is that the formula is not as useful for people who are at extremes in the population, such as someone who is very muscular or very tall and in those cases, there are better ways to determine BMR. In the next post we will explore how you can use BMR and knowledge about food types in order to plan for and monitor your diet and exercise routines as we continue to explore the science of exercise. Check out this article on our site about What’s in My Food as a primer.