Math for Grownups (26 page)

Maybe he’s better off asking for another week of vacation, a cappuccino machine in the break room, and a VIP parking space. Kyle gets to work on his counteroffer.

In the Gym: How Many Miles on the Treadmill to Burn Off One Doughnut?

You’re feeling a little thick around the middle, and your couch is beginning to show signs of permanent stress where you’ve planted your rear end for the last several months.

It’s time to clean out the fridge and hit the gym.

If you’ve done this before, you know that maintaining weight is a balancing act: Calories in = calories out. To lose weight, you’ll need to shake things up:

Fewer calories in
+
more calories out
→ weight loss

Yep, there’s math involved in losing weight and staying fit. Whether you’re at the gym or the kitchen table, a few computations can keep you on the right track.

There are hundreds of different criteria for tracking changes in your weight—how you look in your favorite pair of jeans, whether you can still play touch football with the guys on the weekends, or whether your snoring is why the next-door neighbor’s dog is howling all night long.

But for nutritionists, physicians, and personal trainers, that number on the bathroom scale matters. The pros also count on another important number: BMI, or
body mass index
.

BMI is used to evaluate a person’s health on the basis of their body weight and height. Here’s the formula:

Let’s say June weighs 155 pounds and is 5 feet 2 inches tall. What is her BMI?

Before you start substituting the numbers into the formula, take a look at what you have. To use the BMI formula, you need to know June’s weight (in pounds) and her height (in inches):

w
= 155 pounds

h
= 5 feet 2 inches

But June’s height is listed in feet
and
inches. That number needs to be converted to inches only. There are 12 inches in a foot, so multiply 5 feet by 12 and add the leftover 2 inches, like this:

(5 • 12)
+
2
=
62 inches

Now
you can use the BMI formula:

June’s BMI is 28.35. So what?

We need the experts to tell us what this number means. And they’ve helpfully provided a chart. (This one comes from the World Health Organization, a group that is usually more concerned with starving children in Africa than with whether you can zip up your pants—but it’s still helpful.)

Where does our friend June fit on the BMI table? She’s considered overweight and pre-obese. Her doctor should suggest that she lose a few pounds to get into the normal range.

Everybody knows someone who can eat anything and stay slim. After devouring her second slice of chocolate-mocha cake with full-fat vanilla ice cream and sprinkles, she leans back in her chair, pats her flat tummy, and says, “I think I may have gained 5 pounds at dinner tonight!”

It’s tempting to throw the rest of the cake at her.

It may not be fair, but each of us burns calories differently. Some folks have good genes—they run through calories like water, which means they can eat what they want and forgo long sessions with Claus at the gym. Others seem to gain a pound by simply looking at the leftover French fries on their kid’s McDonald’s tray.

Fair or not, gender, age, weight, and height all play a role in how efficiently your body handles energy or calories. Did you notice? Many of these variables are numbers. Your age: a number. Your weight: a number. Your height: another number.

And where there are numbers, math is bound to be right around the corner.

Your
basal metabolic rate
(BMR) is another important number. It describes how many calories you would need to stay alive if you were to spend all day in bed asleep. In other words, BMR is the minimum calorie intake for a resting individual.

Of course, BMR varies from person to person. If you know your BMR, you can calculate the number of calories you need to consume in a day. But what is BMR based on?

A person’s size is important: The more a person weighs, the more energy it takes to do daily tasks. This additional energy translates into a higher BMR. And as we age, we tend to burn calories less efficiently. That’s because muscle mass decreases with age—particularly when achy joints and busy schedules keep us from exercising as frequently. It also means that, in order to maintain your weight as you age, you may need to take in fewer calories—or, better yet, burn more calories.

Finally, because of their muscle mass, men need to consume more calories than women, so gender plays a role. So, there are two formulas for BMR, one for women and one for men.

For Women:

655
+
(4.3 • weight in pounds)
+
(4.7 • height in inches)
–
(4.7 • age in years)

For Men:

66
+
(6.3 • weight in pounds)
+
(12.9 • height in inches)
–
(6.8 • age in years)

Want to get really geeky? Here are the formulas using variables:

BMR
_women
= 655 + 4.3
w
+ 4.7
h
– 4.7
a

BMR
_men
= 66 + 6.3
w
+ 12.9
h
– 6.8
a

w
is weight in pounds

h
is height in inches

a
is age in years

These formulas show that BMR depends on gender, weight, height, and age. In other words, your gender, size, and age play a role in how efficiently you burn calories.

Let’s look at an example. Say your best friend, Susan, weighs 185 pounds, is 5 feet 7 inches (67 inches) tall, and is 25 years old. What is her BMR?

To find out, you’ll use the formula for women and substitute the information that you have for Susan:

655
+
(4.3 • weight in pounds)
+
(4.7 • height in inches)
–
(4.7 • age in years)

655
+
(4.3 • 185)
+
(4.7 • 67)
–
(4.7 • 25)

655
+
795.5
+
314.9
–
117.5

1,647.9 calories

But remember, BMR tells you how many calories your body will burn if you were asleep all day. And that doesn’t require much energy at all. Even climbing out of bed, pouring a cup of coffee, and brushing your teeth burns calories. So to find the number of calories you can burn in a day without gaining weight, you’ll need to do one more calculation, and this one is based on your activity level.

Total calories
=
BMR
+
(BMR • level of activity)

Your level of activity is represented as a percent:

• Sedentary→20%

• Lightly active→30%

• Moderately active (exercise most days a week)→40%

• Very active (exercise intensely and daily or for prolonged periods)→50%

• Extra active (hard labor or athletic training)→60%

If you’re sedentary, your recommended daily calorie intake would be your BMR plus 20% of your BMR. As an equation, that is

Total calories
=
BMR
+
(BMR • 0.20)

(Did you catch that? You have to turn the percent into a decimal before you can multiply. All you need to do is move the decimal point two places to the left and then drop the percent sign: 20% = 0.20).

Let’s take a look at Susan again. Her BMR is 1,647.9. She walks a couple of miles two or three days a week, so she’s lightly active. So this is how she’ll find the total number of calories she should consume each day to maintain her weight:

BMR
+
(BMR • 0.30)

1,647.9
+
(1,647.9 • 0.30)

1,647.9
+
494.37

2,142.27 calories

(Do you see why she used 0.30, rather than the 0.20 that a sedentary person would use?) In order to maintain her weight, Susan must take in 2,142.27 calories each day.

A pound of body fat equals just about 3,500 calories. This means that in order to lose 1 pound of fat, you need to consume 3,500 fewer calories (or burn 3,500
more
calories).

But if you look at those weight-loss advertisements, they make promises like “Lose at least 7 pounds a week on MeltAway, a revolutionary new diet pill that melts fat!” Is that reasonable?