Most individuals will not have their energy expenditure measured using a metabolic measurement system. If energy expenditure is not measured directly or indirectly, it must be estimated. In this section, you will learn how to estimate your energy expenditure using a prediction equation. There are three major categories of energy expenditure: basal or resting metabolic rate, energy burned during physical activity, and energy burned to digest food. If we have an idea of an individual’s daily habits, we can estimate how much energy is required for all three categories of energy expenditure and a personalized total energy expenditure can be calculated.

Figure 3.5 Components of Total Energy Expenditure

Basal Metabolic Rate/Resting Metabolic Rate

Basal metabolic rate and resting metabolic rate are often used interchangeably. However, there are slight differences between the two.

Basal Metabolic Rate (BMR) is the bare minimum amount of energy the body requires to perform basal (basic) life sustaining functions over a 24 hour period. Basal metabolic processes include breathing, maintaining organ function (including brain function – yes, using your brain uses energy), maintaining a heart beat, circulation, core temperature regulation, and other life sustaining functions. BMR does not include energy for voluntary muscle contraction or digestion, absorption, and transportation of nutrients. Because of this, BMR is difficult to measure and subjects must not have eaten or exercised recently. BMR is best measured right as a subject wakes up naturally from a deep sleep and is often measured under very controlled laboratory conditions that require the subject to spend the night in a sleep lab.

Resting Metabolic Rate (RMR) is very similar to BMR in that it measures the minimum amount of energy the body requires over a 24 hour period, except that in RMR metabolism is measured after a 12 hour fast when the subject is alert and at rest. Therefore, RMR is easier to measure and approximately 10% higher than BMR. RMR still does not include energy required for voluntary muscle contraction or digestion, absorption, and transportation of nutrients. Typically, RMR is measured over a short period of time (30 minutes) and then extrapolated to reflect calorie burning over a 24 hour period. For practical purposes we will be using the term RMR, since most assessment of energy expenditure is conducted on individuals in an alert state. For the average individual RMR is the largest contributor to calorie burning each day and represents approximately 60-70% of all the energy the body uses.

There are many prediction equations to estimate RMR. For this class, we will be using the simplified equation.

In the example below, you will see how to use the simplified equation to predict the RMR for a 180 pound man and 150 pound woman. Remember, this is just estimating the number of kcals that the individual burns at rest, it does not include any physical activity that they may do.

In addition to the simplified equation, there are other prediction equations based on research. You can think of these equations as algorithms. If you use MyFitnessPal or other weight loss apps/websites and plug in your information, the program will give you an estimated number of kcals that you need based on the equation the app developed and programmed in. This is why your estimated kcal needs may vary if you use different equations, apps, or websites. Most prediction equations estimate RMR because it is easier to measure, but some do estimate BMR. This is important to differentiate because technically RMR and BMR are two different things. Another limitation of prediction equations is that they may not be as accurate for populations historically underrepresented in research. A 2013 study found that common prediction equations overestimated RMR in young Hispanic women (2). Just keep in mind that these equations are guesstimates – not an exact measure of the number of calories that you burn.

Because RMR is the largest component of energy expenditure, it’s important to understand factors that can increase or decrease your resting metabolism. Some factors can be controlled but others cannot. If, at any point in your life, you want to gain or lose weight or change your body composition, the first step is to understand factors that influence your resting metabolism.

Non-Modifiable Factors that Influence RMR

• Age – RMR declines as we grow older. Much of the decline is due to aging related changes in body composition such as the loss of lean mass. However, some of the decline in RMR cannot be explained by loss of muscle mass and may be related to other body processes slowing down.
• Biological sex – Men tend to have a higher RMR than women. This is primarily because men have higher amounts of testosterone which leads to more lean muscle mass.
• Height  -When comparing two individuals of different heights but of the same weight, the taller person will generally have a higher RMR than the shorter person. This is because the taller person has a larger body surface area which allows for a greater loss of heat. The body then has to use energy to generate more heat to replace what was lost.
• Genetics – As with many physiological processes, there are large variations between individuals. Some people have higher metabolic rates and some have lower metabolic rates. Some people struggle to put on weight and others gain weight very easily. However, metabolic rates tend to be similar among members of the same family. This suggests that there is a genetic component to metabolism.
• Thyroid hormones – The thyroid is the organ that produces many hormones that regulate metabolism. There is a wide range of “normal” thyroid hormone levels and thyroid hormone levels in the high end will cause RMR to be higher while lower levels of thyroid hormone will cause RMR to be lower. There may be a genetic component to fluctuations in thyroid hormone levels. However, secretion of thyroid hormones outside the normal range is a medical condition that needs treatment.

Modifiable Factors that Influence RMR

• Fat free mass – The term fat free mass or lean mass refers to any tissue in the body that is not fat (muscle, bone, organs etc.). Fat is not very metabolically active. In other words, it does not take a lot of energy to maintain fat tissue. Other tissues in the body require more energy to maintain at rest. Therefore, individuals with higher amounts of fat free mass burn more calories at rest. Because muscle composes a large portion of fat free mass, increasing muscle mass through strength training will increase calorie burning so fat free mass is considered a modifiable factor.
• Low calorie intake – Very low calorie diets that people use to lose weight induce a state of “starvation” in the body. The body adapts to starvation with conservation. In other words, starvation causes the body to conserve energy by making body processes more “energy efficient” which leads to a decrease in RMR.  Low calorie diets have been shown to reduce RMR by 20% or more. In addition, the effects of starvation can persist even after the individual returns to “normal” eating patterns. For this class, we are defining a starvation diet as a diet that does not meet your RMR (the minimum amount of energy required to keep your body alive).
• Growth – Anyone going through a period of growth requires more energy to build more tissues. This includes children going through growth spurts and athletes or active individuals in a state of exercise induced muscle growth or recovery.
• Physiological stress – Injuries, illness, or surgery are all things that put temporary stress on the body. When recovering from stressors the body requires more energy. The amount of energy required is proportional to the stress on the body. Minor illness or injury leads to a small increase in RMR but serious infections or injuries can lead to a significant increase in RMR.
• Environmental factors – Factors such as ambient temperature and altitude can temporarily increase RMR and energy needs. This occurs with variations in temperature such as seasonal temperature changes or when people travel to warmer or colder climates. This change in metabolism is temporary and calorie burning returns to normal within 3 weeks.
• Pregnancy and lactation – Pregnancy requires an extra 300 calories per day beginning in the second trimester and lactation requires an extra 500 calories per day. Both of these are temporary changes in RMR.
• Stimulant Drugs – Caffeine, nicotine, adderall, and other drugs such as methamphetamine or cocaine all temporarily speed up RMR. Prescription amphetamines are sometimes used as a weight loss tool under physician supervision but they should not be used without medical supervision. As discussed in chapter 1, many supplements advertised for fat loss contain hidden ingredients such as controlled substances or prescription drugs that may speed up your metabolism but can be quite harmful for your health if not used appropriately.

Even though there are many factors that affect RMR, most of them have a very small and temporary effect. The two factors that have the greatest, and potentially long lasting, effect are starvation and loss of lean mass. To maintain healthy body composition it is important to avoid very low calorie dieting and maintain optimal levels of fat free mass. It is especially important to maintain muscle mass as we age since some of the decline in RMR with age is due to loss of muscle. This is another reason why strength training is so important. Some people are hesitant to lift weights because they are afraid of getting big bulky muscles. In fact, it is very difficult for most people to gain big bulky muscles and requires an individual to eat in a calorie surplus and follow a progressive overload strength training program that focuses on hypertrophy. However, people do not need to be concerned that they will automatically bulk up from participating in strength training. Strength training is the single best way to increase your resting metabolism which increases calorie burning 24/7.

Thermic Effect of Exercise (Physical Activity)

The thermic effect of exercise is the energy required for physical activities. We previously defined physical activity as bodily movement caused by muscle contraction. Energy is required for all muscle contractions, therefore, physical activity increases energy expenditure above basal or resting needs. Certain factors can increase calorie burning during physical activity, for example, heavier people burn more calories while exercising because they are carrying more weight. Increasing the intensity, frequency or duration of the activity will also increase calorie burning. As you become more efficient at an activity, calorie burning decreases. This means you need to increase the intensity, frequency, or duration if you want to burn the same number of calories. If you are an athlete nearing the end of your competitive career and don’t want to gain weight when your training volume decreases, pick up a new sport. For example, if you’re a runner, when your competitive career ends, take up swimming or cycling. You are not as efficient as these new sports and will burn calories faster which can help prevent weight gain as you adjust to a new routine. When it comes to total energy expenditure, the best way to manipulate your energy output to increase, decrease, or maintain weight is by changing your physical activity. An easy way to estimate calorie burning through physical activity is to use activity factors. This table shows activity factors for various levels of activity. Once an activity factor is chosen, it can be used to estimate total energy expenditure by multiplying the activity factor by the estimated resting metabolic rate. See Table 3.1 for activity factors for various levels of activity. We will go over some examples of how this is put together in the total energy expenditure section.

Table 3.1 Activity Factors for Various Levels of Structured and Unstructured Activity

Thermic Effect of Food (TEF)

The energy required for all the enzymatic reactions that take place during food digestion and the absorption and transportation of nutrients is called the thermic effect of food (TEF) and accounts for about 10 percent of total energy expended per day. For example, if you eat 2000 kcals per day, approximately 200 kcals will be required to digest, absorb, and transport the nutrients in that meal. TEF is the smallest component of energy expenditure so when estimating energy expenditure, emphasis is placed on resting metabolism and physical activity.

Total Energy Expenditure (TEE)

As the name implies, total energy expenditure (TEE) is how much energy your body burns over a 24 hour period. TEE includes both RMR and energy burned for physical activity. When estimating TEE, we simply multiply RMR by an appropriate activity factor. Because there is a margin of error when estimating RMR and physical activity, we do not factor the thermic effect of food into the TEE calculation. To calculate TEE, simply multiply the RMR that you calculated by the activity factor you chose based on the individual’s average level of activity.

Here is an example of how you can put this all together.