Energy and Exercise

Introduction

The topics of health and exercise are never far from many of our minds. It is definitely late enough in the year now that new year’s resolutions about exercise probably haven’t lasted this long. We are now well past the January spike in interest in gyms and gym memberships (as pointed out in this interesting article about the economics of gyms). But it is still a good time to explore the topic of the energy of exercise.

Hopefully most of us realize that food, exercise, health, and weight are all intertwined. Of course, those interactions and discussions get very complex and I can’t begin to scratch the surface of all that that entails. Thus, my purpose in this post is not to attempt to comment on all of the implications of energy and exercise. Rather, I will simply consider a few seemingly simple questions just for the joy of thinking about energy. The questions:

• How much energy is used during exercise?
• How is energy measured during exercise?

How Much Energy is Used during Exercise?

Often, this question is asked in the context of attempting to burn as many calories as possible so a person can lose weight. To answer this question, various tables have been put together with a quantification of the amount of energy “burned” by humans performing various activities including different forms of exercise. And not only exercise, but different types of activities in general. Of course, a person still burns some amount of energy even when sleeping or watching TV.

The following table shows how many food calories (about 4,184 joules per food calorie) a person weighing 155 pounds “typically” burns in half an hour of performing the listed activities. These calorie values are based on studies performed of people doing these exercises while metabolic rates are monitored. I found two sources for the information in the table, which mostly agreed, so may be from the same original source, but I included both just to show the comparison. The two sources were Harvard Medical School and website VeryWellFit.com.

Of course, I just selected a few of the many, many activities listed on these web sites, just to give a flavor. Obviously, there is a great deal of variation in the energy requirements of various exercise undertaken by humans. It is important to note, however, that the thing about a “typical” person performing “typical” exercise is that there is actually no such thing as “typical”. The actual energy burned during exercise varies significantly based on a variety of factors, some of the most prominent of which are the following:

• Weight
• Relative amount of different types of body mass
• Actual activity rate

The first factor is weight. The more a person weighs, the more energy they burn off during exercise. In fact, for a typical person, energy burned is directly proportional to body weight. So a person who weighs 200 pounds would just need to multiply the numbers in the table by 1.3 (200 divided by 155) in order to get the number of calories they would burn performing that same activity. So, for example, a 200-pound person would use about 408 calories in half an hour bicycling compared to the 316 calories a 155-pound person would burn. This makes sense. It takes more energy to move more mass around, and it takes more energy to support more body tissue.

However, the second factor accounts for different types of body mass. There is a perception out there (which I maintained as well before doing some research) that a person with more muscle mass burns a lot more calories at rest than a person with more fat. It turns out that this isn’t as big of a factor as one might be led to believe. In fact, for a person at rest, typically at least 80% of energy consumption occurs in the major organs of the body (heart, lungs, kidneys, brain, and liver). See a good summary presenting this information in this article. Another good reference can be found here. Thus, the proportion of the body that is muscle tissue versus fat tissue is a relatively insignificant factor for total energy consumed at rest compared to the impressive metabolic rate of organ tissue.

The third factor is the actual activity rate maintained for a particular exercise. While the ratio of muscle mass to fat mass does not significantly effect resting metabolic rate, it can significantly affect daily activity rate. That is, a person with higher muscle mass will be more likely to have more active periods during the day and have higher metabolic rates during those active periods. It is pretty simple: the more muscle a person has, the more easy it is to be more active.

So, while a “typical” person weighing 155 pounds will burn 316 calories in a half hour of playing basketball, not all basketball is equal, and those casual playing will burn less energy than those involved in an intense game. Thus, the actual calories burned will vary significantly from the listed amounts in the table.

Of course, elite athletes will of course burn more energy than casual exercisers. This can be shown in an example from a book I read recently. While the table indicates that “vigorous” rowing still burns significantly less calories than running at a seven minutes per mile pace (316 versus 539 calories per half hour), rowing by an elite athlete might just be the most energy-demanding activity there is, as described in a quote in the book The Boys in the Boat:

The result of all this muscular effort [of rowing]… is that [a rower] burns calories and consumes oxygen at a rate that is unmatched in almost any other human endeavor. Physiologists, in fact, have calculated that rowing a two-thousand-meter race – the Olympic standard – takes the same physiological toll as playing two basketball games back-to-back. And it exacts that toll in about six minutes.
A well-conditioned oarsman or oarswoman competing at the highest levels must be able to take in and consume as much as eight liters of oxygen per minute; an average male is capable of taking in roughly four to five liters at most.

The Boys in the Boat, Daniel James Brown, Penguin Books, New York, 2013, pages 39-40.

While we all may not be able to become Olympic rowers, we all can control to some extent our metabolic activity rate through each day. This takes us to our next question, how is energy consumption during exercise (or any other activity for that matter) measured?

Measuring Metabolism

As one can imagine, the topic of measuring human body metabolism is vast and covers a lot of ground. We don’t have time to explore all of it in this blog post, but one useful topic for our consideration is the concept of the metabolic equivalent of task (MET). In order to understand the concept of MET, it is helpful to know that the average energy used by a person at rest is about one food calorie per kilogram of body weight each hour. Thus, a person who weighs 155 pounds (70 kilograms) uses 70 calories each hour. For 24 hours, that equals about 1700 calories.

Every nutrition facts label contains the statement “Percent Daily Values are based on a 2,000 calorie diet.” It is easy to see that a 2,000 calorie diet is derived from a typical person at rest for a day (1,700 calories) plus a few hundred more calories for being active beyond resting.

So while a MET can be defined in various ways, the most useful definition for our purposes is “the ratio of exercise metabolic rate to the resting metabolic rate.” So if a person is a couch potato all day, he would have a MET of 1.0 all through the day (and a MET of about 0.9 while sleeping, as activity level then is below the “resting rate”). So let’s look at our chart from earlier, this time in terms of METs instead of calories burned.

It is pretty astounding, at least to me anyway, that the human body can increase its rate of energy use by at least a factor of 15. Human bodies can be amazing energy-consuming machines when needed (or if that is your idea of fun). Of course, not everyone needs to be an elite athlete or maintain such high metabolic rates. The key to beneficial exercise is increasing METs in whatever ways are available.

The graph below shows a possible scenario for a person going throughout the day with different MET levels. Getting some exercise in first thing in the morning is a great idea. One could run for 10 minutes, or bike or row for 20 minutes, or do weightlifting for 30 minutes, or walk for 45 minutes. Any of those scenarios (assuming the remainder of the hour is spent “resting”) would result in an hourly average for the 6:00 to 7:00 a.m. hour of about 3 MET. Then maybe this person has a few opportunities in the day to take a walk or climb some stairs. Then when the person gets home from work, he does some chores or plays with the kids to get some increased metabolic activity.

The end result of the day shown in the chart is a total of 27.5 MET-hours, or about 3.5 MET-hours above the baseline level of 24. Increasing daily METs is a powerful method for overall better health and life satisfaction.

What about Fitness Trackers?

Maybe some of you have been thinking this question all along. Many these days wear little devices that can simply tell them exactly how much calories they have burned in a day. We can simply use a fitness tracker to tell us how much energy we are using in real time all the time, right?

Maybe. As with any data, we need to be cautious how we use it. As noted in this spot on National Public Radio, fitness trackers can have significant error in calculations of calories burned. Another study (summarized in another article on NPR) noted that people wearing fitness trackers actually lost less weight than those that didn’t, presumably due to the “look how many calories I burned, now I can eat a donut!” effect.

Conclusion

The human body has an amazing capacity for converting food into energy to be used at rest (mostly by our internal organs) and during all the various activities that people enjoy. I hope some of you will have a line graph of hourly average METs pop in your heads sometimes to remind you to do something enjoyable to get those METs up!