Basal Metabolic Rate & Exercise Metabolic Rate
Your basal metabolic rate (BMR) is the amount of energy your body needs per day while resting in a temperate environment when the digestive system is inactive. In such a state, energy is used only to maintain vital organs. For most people, BMR is about 70% or more of total energy burned each day. Physical activity makes up about a further 20% and about another 10% is used in the digestion of food (called thermogenesis).
An accurate BMR measurement requires that a person's sympathetic nervous system is inactive, which means the person must be completely rested.
Several formulas may be used to estimate BMR. The Mifflin-St.Jeor formula has proven to be fairly accurate for most people, based on their body weight, height, age and gender. Unlike the Mifflin-St Jeor formula, the Katch-McArdle formula calculates resting daily energy expenditure (RDEE) - which takes lean body mass into account. The Katch-McArdle formula can be more accurate for people who are leaner and know their body fat percentage.
The total daily caloric need is the BMR value multiplied by a factor between 1.2 and 1.9, depending on activity/exercise level. This is your exercise metabolic rate (EMR).
If, on average, the caloric value of the food you eat exceeds your EMR, you are said to be in a caloric surplus and will gain body weight over time. If less, you're in a caloric deficit and will lose body weight over time. People wanting to lose fat while retaining as much muscle mass as possible should weight train regularly and ensure their caloric deficit is only causing them to lose weight SLOWLY. For example, no more than about 0.5kg per week, which corresponds to consuming an average of about 550 kCal per day below EMR. Excessively overweight people can initially afford to lose weight at a faster rate.