Maintaining a healthy weight requires maintaining a state of approximate equilibrium between the fuel you consume in the form of food and the fuel you burn in the form of activity.
Judging from the sad reality that 2/3 of Americans are overweight or obese, I'd say we could all use a little refresher on how to figure out just how much fuel (how many calories) we need each day.
First, let's define a couple of key terms: Basal Metabolic Rate (BMR) and Total Daily Energy Expenditure (TDEE)
Basal Metabolic Rate is the amount of fuel you need to maintain basic bodily functions such as breathing, heartbeat, temperature regulation and tissue repair. You might think of BMR as the number of calories you would need to consume if you were lying in bed all day. Factors that increase BMR include gaining lean muscle tissue, running a fever, or having an overactive thyroid. On the flip side, your BMR will be lower if you lose muscle tissue, you eat too few calories, or you have an underactive thyroid.
Total Daily Energy Expenditure is the combination of BMR plus the calories you burn through activity. Daily activity includes both your regular exercise and all of your normal movements throughout the day, whether it's standing at your job, folding laundry, walking to your car or preparing meals.
Although your BMR makes up some 60-75% of your total calorie needs, you can see that there's not much you can do to influence your BMR besides gaining lean muscle tissue. You can have the biggest impact on your daily calorie needs by increasing your activity.
To estimate how many calories you need, we use one of several calculations that estimate BMR, then we multiply by an activity factor to estimate TDEE. Remember that your metabolism is uniquely your own, and calculated estimates can never be more than approximations. That said, if you're asking "how many calories do I need," one of these two options should get you in the ballpark.
The Mifflin-St. Joer Equation. This equation offers a good, general estimate of your daily calorie needs.
For men: BMR = 9.99 x wt (kg) + 6.25 x ht (cm) - 4.92 x age (yrs) + 5
For women: BMR = 9.99 x wt (kg) + 6.25 x ht (cm) - 4.92 x age (yrs) - 161
[To convert pounds to kgs divide by 2.2. To convert inches to centimeters multiply by 2.54.]
So let's calculate the BMR for Isabel, a 55 year old woman who is 5'4" and weighs 145 lbs.
9.99 x 66 + 6.25 x 162.5 - 4.92 x 55 - 161 = 1243. Isabel's BMR is 1243. Remember that this is the minimum number of calories she needs to maintain the function of her organs. Next we'll estimate her TDEE by multiplying the BMR by an activity factor:
Sedentary: 1.20 (little or no activity and sedentary work)
Lightly active: 1.375 (light exercise one to three days per week)
Moderately active: 1.55 (moderate exercise six to seven days per week)
Very active: 1.725 (vigorous exercise six to seven days per week)
Extra active: 1.90 (very hard exercise daily and a physical job)
Isabel is like most of us; she works at a desk job and only exercises sporadically. So we'll multiply her BMR by 1.2 to get a daily calorie need estimate of 1492. That's the number of calories per day that Isabel needs to maintain her current weight. If she wants to lose weight, she'll need to create a calorie deficit by eating fewer calories or by increasing her activity.
The greatest shortcoming of this formula is that it doesn't account for lean body mass. Since lean muscle tissue requires significantly more fuel than fat, a 145 lb. person with 40% body fat will need far fewer calories than a 145 lb. person who is only 20% fat.
The Katch-McArdle Equation. The Katch-McArdle calculation attempts to account for differences in body composition in estimating daily calorie needs.
370 + (21.6 x lean body mass kg)
If Isabel is 40% body fat, her BMR using the Katch-McArdle formula would be 1224 (very close to the calculation we got using the Mifflin-St. Jeor equation).
Isabel's friend Brianne is also 145 lbs, but she's a muscular, athletic 20% body fat. Her BMR is 1509. So before she even factors in her daily activity, the leaner friend gets to eat an extra 285 calories each day.
I know, it doesn't seem fair, but it explains why your lean friend can eat so much and not gain weight. Her muscles simply burn more calories than your fat!
(Image credit: bradleygee)