Understanding your metabolism is the first secret to improving your fitness.  How many calories do you need each day to maintain a healthy weight (or gain or lose weight)?  How can you increase your metabolism to burn more calories?  As it turns out this question is relatively easy to answer, we just need to do a few quick calculations – first, what does our body need to keep breathing each day (and regulate your temperature, keep your heart beating, and those sorts of things….).  This number is your Basal Metabolic Rate (BMR) and you can use the following formulas to calculate this number –

MALES – BMR = 66 + (6.23 x weight in pounds) + (12.7 x height in inches) – (6.8 x age in years)
FEMALES – BMR = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) – (4.7 x age in years)

Next you need to apply the Harris-Benedict Principle to account for your level of physical activity.  Basically you multiply your BMR by the appropriate factor from the table below to get your recommended daily caloric intake (to maintain your weight) –

Little or no Exercise     Daily Calories needed = BMR x 1.2
Light Exercise (1-3 days per week)   Daily Calories needed = BMR x 1.375
Moderate Exercise (3-5 days per week)   Daily Calories needed = BMR x 1.55
Heavy Exercise (6-7 days per week)   Daily Calories needed = BMR x 1.725
Very Heavy Exercise (twice a day/very heavy exercise) Daily Calories needed = BMR x 1.9

Once this is complete we know approximately how many calories you need each day to maintain your current weight, and with that information can start looking at your diet and exercise plan to begin working on improving your fitness level.

If you are like most Americans (me included), you probably need to lose a few pounds to get back to a healthy weight, which means either cutting back on how much we eat (i.e. eating less than recommended daily caloric intake that we just calculated) or increasing our activity level (i.e. increasing our daily caloric requirements while keeping what we eat the same (assuming this gets us at a level below our daily caloric requirements)).  One pound of fat is the equal to 3500 calories, so for each pound we want to lose we have to cut out 3500 calories.  So if you need to lose 10 pounds, you have to create a 35000 calorie deficit over time.  So if, for example, we reduce our intake by 250 calories a day, it should take us 140 days (~5 months) to lose 10 pounds.