Even with no physical activity, your body burns calories to fuel normal functions such as breathing and pumping blood. The number of calories you burn in a day of total rest is called your resting metabolic rate, or RMR, and you can estimate it using a mathematical formula. However, your RMR represents the number of calories needed to maintain your weight, not to survive. Individuals have reportedly lived as long as 40 days with no food, according to "Scientific American."
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The Harris-Benedict equation yields a rough estimate of your RMR. For men, add 66.47 to 6.23 times your weight in pounds, add 12.67 times your height in inches to that total and then subtract 6.76 times your age. For women, add 655.1 to 4.34 times weight in pounds, add 4.69 times height in inches and subtract 4.68 times age in years.
Using the Harris-Benedict formula, a woman who is 140 pounds, 5 feet 6 inches tall and 35 years old would add 655.1 to 607.6 to get 1,262.7; add that to 309.54 to get 1,572.24; then subtract 163.8 from that total to find an RMR of 1,408.44 calories per day.