Many of us look for motivation and information from fitness machines as we try to reach and maintain a goal weight. I participated in an indoor triathlon this morning and, without me having entered any information about myself into the computers associated with the stationary bike and the treadmill, I got readouts reporting that I had “burned” about 350 calories in 30 minutes on the bike and about 250 calories in 20 minutes on the treadmill. Are these numbers accurate and, if not, how far off are they? Should I smile and have a cheeseburger?
When I began to look for information on the question of accuracy of measurement of calories, I expected to find a few scientific papers and reviews on the subject with easy-to-understand numbers. For example, I was hoping to find statements such as: “treadmills are off by 20 percent plus or minus, elliptical trainers are off by another percentage plus or minus,” and so on. As it turns out, it is very difficult to get good information on this topic. This is in part due to the large degree of variation between individual people. For example, would a 70 kilogram professional marathoner expend the same amount of energy to run 8 miles in an hour as a 70 kilogram novice runner? There are so many variables to consider, including percentage of body fat, resting metabolic rate, efficiency of movement, VO2 max (this is a measure of the efficiency with which an individual uses oxygen to perform strenuous activities and is defined as: the maximal oxygen uptake or volume of oxygen that can be utilized in one minute during maximal exercise), altitude, hydration, stride length, underlying medical conditions, and medications (prescription or not). Furthermore, different models of exercise equipment may not all follow and record the same types of data, such as distance covered, heart rate, resistance (such as an incline on a treadmill), weight of athlete, and age of athlete. Accuracy can be affected, of course, if the equipment is not well-maintained and calibrated. Entering inaccurate personal information and holding on side rails (or, in the case of elliptical machines, deciding whether or not to use the hand levers) may affect results. The manufacturers of exercise equipment may use a variety of algorithms (which are usually not publicly available), to use the data that has been recorded to estimate calories expended. Good algorithms would need to be tested against known controls with large numbers of individuals.
Instead of relying on the readouts from exercise machines, it may be more helpful to take another approach. The Compendium of Physical Activities, last updated in 2011, is a research-based approach to estimating the energy costs of various activities (they even have rodeo sports, juggling, and curling). The authors emphasize the limitations of these estimates:
‘When using the Compendium to estimate the energy cost of activities, investigators should remind participants to recall only the time spent in movement. The Compendium was not developed to determine the precise energy cost of physical activity within individuals, but rather to provide a classification system that standardized the MET [metabolic equivalent] intensities of physical activities used in survey research. The values in the Compendium do not estimate the energy cost of physical activity in individuals in ways that account for differences in body mass, adiposity, age, sex, efficiency of movement, geographic and environmental conditions in which the activities are performed. Thus, individual differences in energy expenditure for the same activity can be large and the true energy cost for an individual may or may not be close to the stated mean MET level as presented in the Compendium.”
MET is derived from estimates of resting metabolic rates (RMR). RMR, in turn, is higher in men than women and increased with height, weight, and lean mass. It decreases with age.
It is very difficult to find online calorie calculators that use the, above, 2011 updated figures. The sites I have found that name their references, even the sites of highly-respected academic centers, use pre-2011 formulas. However, there is another way to get this information. The equation to determine RMR is the Harris Benedict equation:
Male: 66.4730 + (5.0033 * height in cm) + (13.7516 * weight in kg) – (6.7550 * age in years)
Female: 655.0955 + (1.8496 * height in cm) + (9.5634 * weight in kg) – (4.6756 * age in years)
Fortunately, you don’t have to do the math. This website http://www.bmi-calculator.net/bmr-calculator/ uses this equation (I checked) in calculating RMR (called BMR on the website).
RMR is set at one MET unit. You can refer to the tables on the Compendium website to get MET units for various activities. Divide your RMR by 12, then multiply this figure by the MET units for the selected activity to get an estimate. Using these tables, I was disappointed by the accuracy of these figures for myself. For example, 60 minutes of bike racing would cost me about 1040 kcal while 60 minutes of running at 8 minutes per mile would cost me 780 kcal. Based on my perceived need for calories during and after cycling and running, I would predict that the caloric costs of these activities would be flipped (i.e. higher calories for running than cycling). This discrepancy may be explained by the fact that I am a much more efficient cyclist than runner. This is where individual variation starts to significantly affect the value of equations such as these.
So, considering the limitations of readouts on fitness machines and of population-based estimates of caloric expenditures, what can you do? Well, you can go to an exercise lab and have measurements taken with indirect calorimetry. This is a technique that provides accurate estimates of energy expenditure from measures of carbon dioxide production and oxygen consumption during rest and exercise. Since this is not practical for most people, for any number of reasons, please allow me to make some suggestions. Use information from the Compendium, above, to get ballpark estimates. Then try to select exercise equipment that have been well maintained and that ask you for as much information about yourself as possible. Equipment that uses more personal information in its algorithms is more likely to provide better data. Then, compare your estimates from the Compendium with the readout to get a sense if the readout is realistic However, please remember, unless you are in a fitness lab using sophisticated equipment to measure your caloric expenditures, all you can really hope for is a rough estimate. Skip the cheeseburger.
Ainsworth, BE, Haskell, WL, Herrmann, SD, et al. 2011 Compendium of Physical Activities: a second update of codes and MET values. Medicine and Science in Sports and Exercise, 2011;43(8):1575-1581
Harris, JA and Benedict, FG. A biometric study of human basal metabolism. Proc Natl Acad Sci USA, 1918;4(12):370-373