Ask Jack

Jack,
My new bicycle computer displays a calorie count when I ride. How accurate is it?
Ms. Pattie Fattire

Pattie,
Not very. Calories, a measure of heat, are used to measure energy in food and the energy, from food, we burn to power exercise. Calories in a food are measured in a lab. A food is incinerated in a calorimeter and the heat given given off is measured and converted in to calories.
Getting caloric expenditure in humans is not that easy. Calories supply their energy to humans on a cellular level--not an easy place to stick a meter. However, accurate calorie counts can be made indirectly--by measuring the amount of oxygen the body uses to process the calories used to power exercise.
'Calories used' charts for hundreds of activities from typing with an electric typewriter to long distance running come from thousands of measurements of people doing those things while hooked up to an apparatus that measures, minute by minute, their oxygen consumption. The technology to do this and to do it accurately has been in laboratories for over 100 years.
Also, in a laboratory, variables which greatly tax the exercise effort such as headwinds and hills are eliminated.
In fact, calorie counters in treadmills can be quite accurate if variables that effect caloric expenditure, such as the weight of the exerciser and the degree of incline can be programmed in to the machine.
A one-ounce bicycle computer the size and thickness of two Wheat Thins strapped to the handlebars is not that sophisticated. The directions for my Cat Eye VELO8 state clearly how the calories are measured; "The calorie consumption data is only the accumulated value that is calculated from the speed data of every second. It differs from the actual consumed calorie."
What that means is that the computer does not know if your 14 miles per hour is coming from coasting (very little calorie burn) or from standing on the pedals breathlessly cranking up a steep hill (a serious caloric expenditure.) It only knows 14 miles per hour and that is what the count displayed on the screen is based on.
(Remember that even using exercise machines that can account for variables uch as weight and give fairly accurate calorie counts, the result includes calories you would have used if you had been sedentary during that time. Just because you are sitting still does not mean you are not burning calories. To get the contribution the exercise made to your daily caloric expenditure, you would have to subtract the calories you would have burned if you had spent the time not exercising but reading, watching TV or sleeping, from the total.)
I hope this helps you out. Keep up the good work.
Jack
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