Cheeseburgers per day?

Last post we talked about the units used for describing the energy content of food.  For the purposes of an energy balance on the body, we also have to consider the efficiency of converting food energy into activity.
The first obstacle would be estimating how much of the energy in the food would be locked into undigestible forms and so would not be available for any human activity.  Luckily, this is already taken care of for us.  Recognizing that a certain percentage of food energy is not available to the body, food labelers have adopted systems to account for that effect before the energy content of food is reported.  So, the reported energy should be a pretty close approximation to what is available for your body to use.
 
A second issue concerns how much of the food energy that we ingest can be converted into mechanical activity—things like lifting a weight or running up stairs.  This varies depending on the kinds of food (relative amounts of fats, carbohydrates, protein, etc.) and the kinds of activities (aerobic or anaerobic muscle activities, organ or tissue involved, etc.).  Some estimates for conversion of energy in skeletal muscle are in the range of 15-25%. 
 
Yet another issue involves the rate at which food energy is used by our bodies just to exist. This is called the Basal Metabolic Rate (BMR).  One popular equation for estimating this number is called the Mifflin St Jeor Equation and uses mass, height, age and gender for the estimate, as follows:
 

Where “G” is +5 kcal/day for males and -161 kcal/day for females.  Using this formula, the vast majority of adults fall between about 900-2200 kcal/day.  Changing units, this corresponds to 44 to 117 watts.  So, based on Calories alone, a person might be able to survive indefinitely on 3-5 small cheeseburgers per day.

This figure shows, for the typical USA distribution of height and weight, a range of estimated BMR as a function of age.  Next time we’ll consider activities and the rates that they use food energy.