The law of bodyweight regulation states that "calories control bodyweight". To lose weight, you need to create a negative calorie balance (eat fewer calories than you burn). To gain weight, you need to create a positive calorie balance (eat more calories than you burn).
The first step you need to do is estimate what amount of calories you burn every day. All calculations start by estimating your Resting Metabolic Rate (that's the amount of calories you would burn per day if you don't move at all). Scientists have developed formulas for estimating your RMR (Resting Metabolic Rate). They do it by getting a bunch of people together, measuring their RMR using sophisticated methods and then trying to figure out a formula to match their RMRs using weight, height, age and other factors.
To calculate your RMR use this formula:
In pounds and inches:
Men: RMR = (4.54 x Weight in pounds) + (2.44 x Height in inches) - (4.92 x Age) + 5
Women: RMR = (4.54 x Weight in pounds) + (2.44 x Height in inches) - (4.92 x Age) - 161
In kilograms and centimeters:
Men: RMR = (9.99 x Weight in kg) + (6.25 x Height in cm) + (4.92 x Age) + 5
Women: RMR = (9.99 x Weight in kg) + (6.25 x Height in cm) + (4.92 x Age) - 161
Next, you will need to multiply the RMR by an Activity Coefficient (AC). Here are the average ACs for men and women.
Sedentary Men: AC = 1.55
Moderately Active Men: AC = 1.78
Very Active Men: AC = 2.10
Sedentary Women: AC = 1.56
Moderately Active Women: AC = 1.64
Very Active Women: AC = 1.82
Your total daily energy expenditure (the number of calories you burn per day) is: TDEE = RMR x AC
It is really simple. Calculate your Resting Metabolic Rate (RMR formula). Find your Activity Coefficient (AC) above and multiple the RMR by the AC.
If you want to gain weight, set a target calorie intake above your total daily expenditure. If you want to lose weight, set a target calorie intake below your total daily expenditure.
The question now is how much below/above your TDEE to set up your target calorie intake. It depends on your specific weight goals. Have in mind that gaining and losing weight is not the same as gaining and losing fat. Most of the time you are going to gain/lose both fat and lean body mass. The old premise that it takes 3500 calories per pound is wrong. It takes 3500 calories to gain/lose a pound of fat. Lean body mass requires about 500 calories per pound. In kilograms these values are 7700 calories per kilogram of fat and 1100 calories per kilogram of lean body mass.
For most people, a calorie plan of plus or minus 300 to 1000 calories per day will do the job. Every week weigh yourself and recalculate your total daily energy expenditure. If your progress is slowing down, add or subtract a bigger number of calories to your TDEE.
Finally, let me tell you that the above formulas are for "average people". In many situations, there are metabolic adaptations that will slow down/speed up your metabolism. All that means that you are going to cut more and more calories to keep the same rate of weight loss rate and you are going to overeat more and more in order to keep the same rate of weight gain.
If you are a female, never eat less than 1200 calories per day. If you are a male, never eat less than 2000 calories per day.
And remember, you can cut calories, but you can also burn more calories via exercise.
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