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Metabolism by numbers

November 4, 2008

In my post about calorie counts on menus I opined that one of the big problems with emphasizing calories is that so many people have absolutely no idea what their actual energy needs are.

So that begs the question, how do I determine my individual metabolic needs?

Before we get into the different methods, let’s define some terms:

Metabolism: This is the sum total of all chemical reactions in the body. This is important to remember when talking about metabolism. Most people tend to think of metabolism as if it were just an organ or some central furnace in the body. But really, metabolism is just all those things that the body needs energy to do: breaking down food, keeping organs functioning, building proteins, storage, etc… in addition to the energy we need to use in order to move around.

Basal Metabolic Rate (BMR): This is the amount of energy your body needs at rest. Most metabolic measurements are giving you this number so it’s important to remember that, unless you’re comatose (which seems a tad unlikely), this number does NOT represent the amount of energy you actually need on a daily basis.

There are several possibilities for measuring your metabolism, all of which have their drawbacks.

1) If you are maintaining your current weight, count how many calories you eat over several days, and find the average.

      Problems: Time consuming. Sanity consuming for some people. You really need a ‘typical’ set of days when the way a lot of people eat is fairly uneven. People tend to underestimate their intake.

 
2) Pay to get your metabolism measured. Rachel mentioned this in her comment to my other post. Lots of gyms, hospitals and dietitians offer this service.

  Problems: Depending on where you live, it might be hard to find a place to get this done. Cost can range from $30 or more.

3) Use one of the formulas you can find all over the internet for determining your BMR.

  Problems: Formulas have differing levels of accuracy, especially where heavier people are concerned.

Ideally, option number 2 is preferable because it’ll be the most accurate. However, in my case, there is only one place near me that offers indirect calorimetry and you must pay $100 for a meeting with a nutritionist in order to get the test. 

So if you don’t want to count calories and indirect calorimetry isn’t available, you’re left with using available formulas. Lucky for us, the internet has lots of neat little BMR calculators.

However, most online BMR calculators use the Harris-Bennedict equation. This is problematic as some information suggests this particular equation will, for many women, overestimate their BMR. Another, newer equation is the MIfflin St Jeor equation which some studies suggest is a bit more accurate for overweight individuals.

If you want to use the Mifflin equation, you first must convert your weight into kg and height into cm. To convert pounds to kg, divide by 2.2. To convert inches to cm, multiply by 2.54. Or, you can use these two calculators to do you conversions.

Then, depending on your sex, use one of the two equations:
The Mufflin equation for RMR:
w = weight in kg
h = height in cm
a= age

For men: (10 x w) + (6.25 x h) – (5 x a) + 5
For women: (10 x w) + (6.25 x h) – (5 x a) – 161

So, for me, a 27 year old woman who weighs 247lbs and is 5’5″, the formula looks like this:

(10×112.3) + (6.25x 165.1) – (5×27) – 161 = 1859

Once you have this number, you need to multiply it by an activity factor. Remember, BMR is your metabolism completely at rest, so even if you’re basically sedentary, your energy needs are higher than you BMR. The most common activity multipliers are

Activity Factor Category Definition
1.2 Sedentary Little or no exercise and desk job
1.375 Lightly Active Light exercise or sports 1-3 days a week
1.55 Moderately Active Moderate exercise or sports 3-5 days a week
1.725 Very Active Hard exercise or sports 6-7 days a week4
1.9 Extremely Active Hard daily exercise or sports and physical job

In my case, I do both light and moderate exercise most days so I use both 1.375 and 1.55 as multipliers to figure out a general range for my metabolism. This gives me 2556 – 2881 as the range in which I will maintain my weight.

Another equation is the Katch-McArdle. This equation requires you to know the weight of your lean mass. You can estimate your lean mass if you know your body fat %, but remember that a lot of commercial body fat analyzers are not very accurate so any estimates of lean mass will probably not be that accurate. The Katch-McArdle is fairly simple

BMR (men and women) = 370 + (21.6 X lean mass in kg)

This one is gender-neutral, because it relies on lean mass which, if you recall, is a huge determinant in terms of metabolic rate.

So why do you want to know any of this information?

You may not. However, knowing this information isn’t just for weight loss. For a lot of women, I think it is a revelation to see what their energy needs are. It can also help you know when something is wrong with your metabolism. A lot of people, if they start to gain weight, just assume they must be eating ‘too much’, but if you know that you generally need 2,200 calories a day and on the same amount, you start gaining weight, it can be a signal that something is not quite right.

There are several conditions that effect metabolism. Remember, these equations are based on averages so there are some genuine outliers, but if the numbers you’re getting are more than 15% away from what your observations tell you about your energy needs, it might be worth it to get checked out by a doctor.

But if weight loss is a goal, knowing these numbers is very helpful especially if you’re counting calories. Remember, most experts only recommend cutting calories by 500 calories/day.

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3 Comments leave one →
  1. gnomeprincess permalink
    November 4, 2008 9:52 pm

    “To convert pounds to kg, multiply by 2.2”

    I know you knew how to do this, but you accidentally said multiply when it is divide 🙂 Some people might not know.

  2. November 4, 2008 10:13 pm

    Ack! LoL, I’ve got too much election-brain, I think.

    Thanks, I’ll edit that.

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