The Inefficiency of Humans

The human body converts the energy contained in food to mechanical output with an efficiency that may surprise you — it’s that low. What goes on at the gym reflects this. A person dedicated to keeping reasonably physically fit might use a rowing machine, an elliptical trainer, or a stationary bicycle. These machines usually provide readouts that show how much work the user is doing, and how many Calories she is burning. Checking the numbers now and then helps pass the time, and may provide a little reward at the end. But if you measure the work done, which is the power that went into driving the machine, it turns out to be only a fraction, between 18 and 26%, of the total energy burned during the session (the rest of this post will describe how to do this comparison). What’s going on?

Measuring the workout

Most of us going to the gym want to know how many Calories we are burning. It provides a faint hope that weight is being lost, even though for most of us, the number of Calories consumed during gym workouts is pretty small compared to total food consumption. (As in other posts on this site, I use the term “Calories” to measure energy. The Calorie is the same as the scientist’s kilocalories – the kcal – but is more familiar to most people. Food packages provide information in Calories.) The Calories burned by the average adult working out fairly strenuously for three hours a week is about 10% of the total Calorie content of their food. Regular exercise may provide context and discipline, but weight loss requires dietary control as well.

More serious trainees, for example competitive rowers or cyclists, look at a different measurement of their workout’s intensity: they are interested in their “work rate”, which measures how much power they are delivering to the exercise machine. They know that the ability to deliver a high level of power for a longer time is the key to improved performance in almost any sport, but particularly in events like bicycle racing or speed skating. (There’s an even more informative measure for sports like bike racing: the power delivered per kilogram of body weight. That’s what serious athletes are interested in.) Work rate, or power, is measured in watts, and good workout machines provide that number. A power output of 150 watts for one hour equals 150 watt-hours, or 0.150 kilowatt hours (kWh). This is the measure of how much energy was delivered to the machine. (The kWh is how the electrical utility measures the amount of energy consumption at your house.) Calories are also a measure of energy, and the units are interconvertible: 1 kWh is the same as 860 Calories.

A workout machine like a stationary bicycle provides both the rate of energy being delivered to it (in watts), and the total amount of energy expended by you (in Calories). But when you do the conversion, there’s a huge difference between the two. The number of Calories burned is way higher, at least four times higher, than the energy calculated from the power put into the machine over the period of time. What’s going on?

Looking at the numbers

Let’s say, as a reasonably fit grownup, one who has a life but finds time to work out two or three times a week, you are comfortable pedaling an exercise bike at a rate of 150 watts for 30 minutes. 150 watts for half an hour is 75 watt-hours, or 0.075 kWh. 150 watts is enough power to keep a few light bulbs going, but your hair dryer may consume ten times that amount. Professional athletes can do far more. In 1975, the Belgian bike racer Eddy Merckx, considered by many to be the greatest cyclist who ever lived, was put on a stationary bicycle. (Merckx used to annihilate his opponents; his nicknames included “The cannibal” and “One man forest fire”.) He produced 455 watts of power for one hour, about twice what a fit young amateur athlete can do. Even riders in the peloton are phenomenal engines; a typical rider can produce 800 watts for 15 seconds, and 600 for one minute. The strongest cyclists can put out 1200 watts for 15 seconds.

Eddy “The Cannibal” Merckx wins a stage on the 1974 Tour de France (which he also won). Photo AFP.

Back to your session at the gym. OK, you put out 150 watts for 30 minutes. Now you look at another part of the display and see that it reads 375 Calories burned. Feels good. But wait a minute. The work done, 150 watts for 30 minutes (0.075 kWh) is equal to only 64.5 Calories (.075 x 860). To repeat, what’s going on?

The difference between work done, and the total Calories burned, reflects the fact that our bodies are far less than 100% efficient at converting the energy released by the consumption of carbohydrate, fat and protein into mechanical work. No machine is 100% efficient, and we are no exception. Gasoline powered cars are 30% efficient at best. The efficiency of the human body is somewhere between 18 and 26% for most of us. The explanation has several parts. (There are many web sites dedicated to explaining the difference. I don’t recommend going there. They often get it wrong, and almost always make it even more confusing.)

Human energy metabolism

Energy metabolism is complex, and most people would rather work on their income tax return than have someone try to explain it to them. But in simple terms, it helps to think of energy metabolism, the conversion of the energy contained in sugar, fat, and protein into energy usable by the body, as a watercourse flowing downhill. There are little waterfalls along the way, some of which harness the energy of the water flowing over them and turn it into adenosine triphosphate, ATP. ATP is the chemical form of energy that the body can use to, for example, power its muscles or think its thoughts (the brain has a high level of energy metabolism; in a body at rest, about 20% of the total energy used).

After the water reaches the bottom, there are several products: carbon dioxide (yes, the greenhouse gas), water, heat, and ATP. The amount of ATP formed is a function of the design of the energy metabolism pathways. As constructed, in organisms ranging from humans to fruit flies to bacteria, it represents only about 40% of the energy made available by the path from foodstuff to CO2. If it was designed to extract more energy as ATP, it would be more efficient, but it would run slower, limiting the rate of ATP production. If less ATP was produced, less energy captured, the pathway would be faster, but the efficiency would be even less. What evolution has provided is a Goldilocks solution: just the right amount of ATP produced, about 40% of the energy of food oxidation captured, so that our normal energy needs are met, without having to pour a lot more food into the pathway. (If we were, say, only 10% efficient, we’d have to eat like pandas to provide the energy for our daily lives. Either that, or move like sloths.) 

The efficiency of muscular work, driven by ATP, is itself not 100%. Muscle fibers sliding over each other do experience friction, just like any moving machinery. And there’s a lot of other stuff going on. Even at rest, the human body consumes about 100 Calories an hour. This keeps the heart going, the blood flowing, and the lungs inhaling and exhaling. And then there’s the repair. Damaged molecules and cells are constantly being replaced. The cells of the gut, for example, only last on average 4 days, and by then new cell have to be available.

Nothing runs without energy, and during intense work, everything works harder. (Even repair. Dark colour in the urine after a marathon may be due to red blood cells being destroyed by the pounding of the feet on pavement.) At full tilt, the heart rate may be three times the resting rate, and the lungs puff faster and deeper. All of these things are machines that require ATP as a power source. So, not only is the production of ATP only about 40% efficient, the efficiency of its use is also less than 100%. Result? Only about 18-26% of the energy content of that chocolate bar shows up as work output of your arms and legs in powering the stationary bike or rowing machine. The rest is heat. (When Eddy Merckx performed his epic work rate on a stationary bike, he had to have fans blowing air over him to keep from overheating.)

Making sense of the numbers

Power output is a hard number; it is measured directly by the force applied to the pedals and the rate of pedal rotation. In other words, it measures the work you’ve actually done (assuming your exercise machine was correctly calibrated). The total Calories burned, on the other hand, is a calculated number, obtained by dividing the power output by your efficiency. This is necessary because measuring the Calories expended directly is a science project. The participant needs to be in a thermally-isolated enclosure in which the heat produced can be measured. A less demanding and more practical approach is to determine the Calories expended indirectly by measuring oxygen consumption and CO2 output. Still pretty technical, as it involves wearing a mask that tracks the oxygen and CO2 flows. The calculation of Calories expended by the computer in your stationary bike depends on what value it uses for the efficiency factor: how much of the total metabolic energy output that you produced actually drove the pedals? In the present example (150 watts for 30 minutes, equal to 64.5 Calories of actual work done), if we use an efficiency factor of 22%, a reasonable value for a healthy young adult, the total energy would be 64.5/.22 + 50 (50 is the basal rate of energy consumption, which keeps you alive), or 343 Calories. Your readout should be somewhere in that neighbourhood.

Heavier people are generally less efficient than lighter ones, and some machines ask for your weight and adjust the efficiency in calculating Calorie output. Some don’t. But there are no absolute numbers for a given age and weight, so the total energy output (Calories burned) is an estimate. There’s not much you can do about your energy efficiency, beyond losing some pounds if you’re overweight.  Even Lance Armstrong raised his efficiency by only a few percent over years of brutal training. And maybe that was the drugs, anyway.

According to online exercise geeks, some makers of gym workout machines inflate the numbers calculated by their machines. Not surprising.  Wouldn’t you rather work out on a machine that says you expended 375 Calories, rather than 343, for the level of effort reflected by heart rate and the feeling of exhaustion? When it comes time for you to suggest new equipment to your gym, or to buy a machine for home use, you might prefer such a machine. 


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    1. Thanks. As for where the information comes from, it’s what happens to old brains – they fill up with information, or at least, with a list of topics that can be looked up.

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