When it comes to counting calories, remember this is not an exact science, and heart rate monitors vary by how they measure heart rates and calculate calories burned. Also, everyone’s metabolic rate is different.
According to Eileen Stellefson Myers, MPH, RD, LDN, FADA, “When one eats too little, the body can compensate and conserve calories to fuel the vital organs. Most people don’t realize that the Resting Metabolic Rate is the number of calories needed for the organs to function if one was simply sitting all day. When you don’t eat enough and exercise too much, the body needs to figure out how to supply the organs with fuel, and all organs, including the muscles, have less energy to burn.”
Muscle make up also has something to do with it. Some people have more efficient muscles that another, therefore they burn more calories than someone else. Other studies show that the temperature in a room could also affect calorie burn. According to the American Dietetic Association Complete Food and Nutrition Guide, both prolonged exposure to cold temperatures and perspiration to lower body temperature use extra energy. The effect of temperature on your caloric burn rate varies with your body mass and the extremity of the temperature.
Basal Metabolic Rate (BMR) changes as a function of temperature. BMR will change by seven percent for each temperature change of 0.9 degrees Fahrenheit. Thus, when external conditions affect your body temperature, your body will burn more calories as it tries to compensate for the anomaly and restore your body to its normal temperature, increasing your BMR. This effect occurs in conditions of both heat and cold.
Because each person’s metabolism reacts differently to heat and / or cold, calorie usage can vary widely.
Heat affects the body’s fuel preferences change in hot conditions as well. As this Spanish study from 2010 noted:
Exercise in the heat (40 C) increases muscle glycogen oxidation and reduces whole-body fat oxidation (Febbraio et al. 1994), in comparison to the same exercise intensity performed at 20 C.
This makes the effect even more pronounced: not only does exercising in the heat increase your intensity (which increases the carb:fat ratio), but it also increases the carb:fat ratio even at the same intensity.
So while many people live and die by their FitBits and other related technology, the bottom line, is not how many calories did I burn today, but how did I feel about that workout? Did I feel like I worked to the best of my ability? Could I have put more into the workout? Use this information to gauge the value of the workout, not to compare ourselves to others, but to compare ourself to ourself. Does the number go up over the course of a period of time? Or does it go down.
Improvements in fitness can only occur when the status quo is challenged.