Something I found interesting in the TIME Magazine article I reviewed a few days ago was the reference to an article in Obesity Research, which discussed methods of determining Resting Energy Expenditure, better known as Basal Metabolic Rate, or BMR.
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The most basic equation of weight loss is:
Weight gain/loss = energy in - energy out.
That is, measure the calories (energy) in the food you eat, and subtract the calories you "burn" through maintaining body functions, exercise, etc, and that number will tell you how much fat you can expect to gain or lose, at a conversion factor of about 3500 calories per pound of fat.
For example, if you eat just 96 calories per day more than you burn, over the course of a year you will gain ten pounds. Conversely, if you burn 96 calories more than you eat every day, you'll lose ten pounds in a year.
It's fairly easy to determine how many calories are in the food you eat. Easy, but tedious - you need to accurately measure everything, then convert to calories using a table or online calculator. There are smartphone apps that make this easier than ever before, but it's still a pain. However, it's still pretty easy. But how do you determine how much energy you burn?
The best-known BMR equations are the Harris-Benedict equations:
BMR (men) = (13.75 x body mass) + (500.3 x height) - (6.78 x age) + 66.5
BMR (women) = (9.56 x body mass) + (185 x height) - (4.68 x age) + 655.1
You can run these equations for yourself (body mass is measured in kilograms, height in meters, and age in years). But there are many online calculators that will do the math for you.
Knowing your Basal Metabolic Rate (that is, the energy your body needs to maintain normal body functions), you can determine your daily calorie burn by adding the additional calories you burn in exercise. Alternately, and more simply, you can multiply by a conversion factor based on how active you are and estimate your daily calorie that way. But how accurate is this estimate?
According to the article in Obesity Research,
These prediction equations were developed in 239 adults (136 men and 103 women) between the ages of 16 and 63 years for men and 15 and 73 years for women with very few overweight individuals.
The problem is, body fat requires MUCH less energy for maintenance than other tissues. Again according to Obesity Research,
Heart and kidneys have the highest resting metabolic rate (440 kcal/kg per day), whereas brain (240 kcal/kg per day) and liver (200 kcal/kg per day) also have high values. In contrast, resting metabolic rates of skeletal muscle (13 kcal/kg per day) and adipose tissue (4.5 kcal/kg per day) are low. Therefore, although skeletal muscle and adipose tissue are the two largest components, their contribution to REE is smaller than that of organs. The majority of the REE of the body (approx 60%) arises from organs such as liver, kidneys, heart, and brain, which account for only approx5% to 6% of BM.
Since the Harris-Benedict equations were formulated years ago, before the majority of the American population became overweight and obese, it's almost certain that these popular BMR calculations overstate calorie burn.
How much? Well, at my goal weight of 154 pounds, my BMR will be 1524 calories per day according to the first Harris-Benedict equation above.
My maximum weight was 229 pounds, when I was carrying an additional 75 pounds of fat. Taking the figure of 4.5 kcal/kg (2.04 kcal/pound) of energy required by those 75 pounds of fat (adipose tissue), I get a difference of 153 calories needed to maintain my extra fat mass at 229 pounds. In total, being 75 pounds overweight raised my BMR to 1677 calories.
But calculating my BMR at 229 pounds gives me a figure of 1991 calories, over 300 calories more! If I'd kept my food intake just under 1700 calories per day when I weighed 229 pounds, I would have expected to lose a pound every 12 days - but I would actually have been GAINING weight!
What to do?
Well, there are other BMR calculations. The next most popular one is the Katch-McArdle equation,
BMR = 370 + (21.6 x lean body mass in kg)
The drawback of the Katch-McArdle equation is it requires you to know your lean body mass. If you have a bathroom scale that ACCURATELY measures body fat percent or you've had your body fat percent measured by skinfold calipers, you can determine your LBM by multiplying that percent times your weight, then subtracting that result from your body weight to get your lean (i.e., non-fat) body weight. Divide by 2.205, multiply by 21.6, add 370, and you get your BMR. For me, my LBM works out to 134 mass pounds, and my BMR comes out to around 1683 calories per day.
Personally, I think it's easier to do what I did earlier - use the Harris-Benedict equation to figure BMR at your goal weight, then correct for the extra fat pounds that you're carrying by multiplying by 2.04 calories per pound. For example, my BMR at my goal weight of 154 pounds will be 1524 calories. At 173 pounds (as of this morning), I'm carrying 19 pounds of fat above my goal weight. Adding 39 calories, I get a BMR of 1563, instead of the overly-generous 1636 calories that the online calculator gives me.
Of course, what I actually did was buy a bodybugg, which uses a number of measurements to estimate daily calorie burn without needing to mess with calculations and corrections and estimates of energy burned in exercise. According to my bodybugg, I burned 2083 calories yesterday, when I slept in and took a sedentary rest day. Apparently, though, I was more active than I thought, because multiplying my calculated BMR of 1563 by 1.2 (for a sedentary lifestyle) gives a daily calorie burn of only 1875.
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