The Fuzzy Math of Exercise

Posted on November 26, 2014

There are many numbers that get tossed around pretty routinely in fitness. Calories are counted. Weight is measured. Repetitions are tracked. But, when you want to lose fat you need to think about the actual science behind the numbers.

Here’s one I hear all the time: it takes 3500 calories to burn a pound of fat.

That’s grand. But, is it correct?

I’m not going to dazzle you with math, but there are a few conversions that need to be made to get to the heart of this question.

Caloric information is reported in grams, so first let’s convert pounds to grams. There are 2.2 pounds = 1 kg and a 1000 grams = 1 kg…

So: Convert pounds to kg: 1kg/2.2lb = .455kg/lb

and then kg to grams: 1000g/1kg= 455 grams/lb rounded to the closest integer. This is the weight of one pound in grams.

There are 9 kcal/1gm of fat, so

455gm/lb x 9 kcal/1gm = One pound of fat contains approximately 4095 kcal.

So, a pound of fat is really almost 4100 kcals, not 3500. That’s a big difference!

Wang et al 2001 did a study where they measured the caloric expenditure of different types of resting tissue. Their results were reported in kgs. Christian Finn did a great article and summarized the results in pounds as:

Organ or tissue	Daily metabolic rate
Fat	2 calories per pound
Muscle	6 calories per pound
Liver	91 calories per pound
Brain	109 calories per pound
Heart	200 calories per pound
Kidneys	200 calories per pound

As you can see, the daily metabolic rate of a pound of resting muscle is approximately 6 calories, while fat is 2 calories. There may be a few studies that may show some variety, but in general estimates of muscle metabolism is 5-10 calories per pound per day and fat is 2-3 calories per pound per day.

Any important point that should be made is that there is a difference between resting muscle, recovering muscle, and active muscle. The metabolic rate of each depends on protein turnover. The more turnover, the higher the metabolic rate.

Another topic that gets a lot of buzz, and mis-quoting: EPOC, or excess post-exercise oxygen consumption. Many trainers discuss this recovery phase as crucial for fat burning, going as far as saying half (or even most) of the fat calories burned during/after a bout of exercise fall into this category.

What science says? EPOC only accounts for an additional 6-15% of the calories burned during a workout

To get to the heart of this misunderstanding we need to think about some basic physiology.

See, stored fat can only be mobilized into energy through a process that requires oxygen—called lipolysis. So, if you’re working at an aerobic level, well those calories can be accessed. If not, your body finds alternative fuels that don’t require oxygen. At rest your body can access fat stores easier, but once you hit a threshold of 65% of your VO2Max, your body looks to  first carbs and then protein for quick sources of energy.

HIIT exercise, because it operates near to the 65% VO2Max threshold for most of the workout, it will glean fat energy —and then again so will your normal daily activities—so what is to be gained in EPOC? Relatively little, but let’s look at an example.

More math….

If a person burns 500 calories in an aerobic workout, the majority of these calories will be from fat stores. If EPOC is roughly 10% of the total caloric expenditure, then you can figure 50 calories will be consumed in the post exercise phase. The calories consumed from fat during EPOC alone are not more than rest or even during the workout, assuming the workout maintained sub-anaerobic threshold levels for a considerable portion of the workout.