According to the Center for Disease Control, a whopping 40% of the adult US population is currently obese. That’s not all: an additional 30% of the adult US population is overweight, but not to the point of obesity. In addition to lowering people’s quality of life, obesity causes 21% of all health care costs in the United States, totaling over $190 billion per year.

Let’s take a look at the math behind weight gain.

Before we do, though, I must say that seldom has a simple math problem become so unnecessarily complicated. The weight loss industry was a 66-*billion*-dollar market last year, so there’s a lot of financial incentive for companies to keep people fat and confused. It doesn’t have to be that complex.

First of all, let’s examine what actually constitutes a healthy weight. First, predict: What do you think a healthy weight is for a 6-foot-tall man? How about a 5’5″ woman?

Here are two helpful calculators for figuring out ideal weight based on height and gender: the first is an ideal weight calculator, and the second is a BMI (Body Mass Index) calculator.

For a 6’0″ man, an ideal weight is approximately 165 pounds, and the healthy weight range is between 147-183 pounds. A weight of 184-220 would be considered overweight, and a weight of 221+ pounds would be considered obese. (Remember, 40% of the population is obese!)

For a 5’5″ woman, an ideal weight is approximately 130 pounds, and the healthy weight range is between 111-150 pounds. A weight of 151-179 would be considered overweight, and a weight of 180+ pounds would be considered obese.

A pound of fat contains approximately 3,500 calories. That means that a 5’5″ woman who weighs 180 pounds (right on the borderline of being obese) and wants to get down to 130 pounds has to get rid of an extra 175,000 calories worth of energy stored on her body.

According to Runner’s World, a 180-pound woman running a 10-minute mile would burn off 136 calories. So our hypothetical 180-pound woman would need to run a ridiculous 1,287 miles to burn off her excess fat and get down to her ideal weight of 130 pounds.

It gets worse. As our hypothetical woman begins to lose weight, her body would become more efficient, and she would begin to burn fewer calories per mile. One mathematician, after conducting experiments on a number of subjects, calculated that losing a pound of fat actually requires burning off not just an extra 3,500 calories, but closer to 7,000 calories.

At that rate, our woman is going to need to run from San Francisco to New York City to get back to her ideal weight.

So what’s the solution? Well, first of all, for our woman described above, a short-term diet isn’t going to work. What’s needed is for her daily caloric intake to match the daily caloric expenditure of a woman __at her ideal weight__. This calculator is perfect for figuring out what that would be.

In our example, we’ll say the woman in question is 30 years old, and she wants to weigh 130 pounds with a lifestyle of moderate exercise. According to the link above, her expected daily caloric expenditure would be 2,031 calories.

So our hypothetical woman should begin eating 2,000 calories or less each day, exercising moderately if possible. Her body will then slowly begin to find a new equilibrium weight where her caloric expenditure matches her caloric intake, and it should be right around 130 pounds.

Again, to summarize: __Each day, you need to eat no more than the number of calories that a person at your ideal weight would burn off in a day__.

Once you’ve got your target number of calories to eat daily, what’s the trick for staying at or beneath that number? Obviously a multitude of books have been written about this subject, but I think the simple answer is this: It will be easier to stick to your caloric limit if you don’t feel hungry, and the best way to feel full while eating fewer calories is to eat foods that are less dense in calories.

Caloric density, in my opinion, is the #1 key to losing weight. The chart to the left shows the number of calories found in one pound of some different common foods. (Credit tobere.com)

Your goal should be to eat the foods that have as few calories per pound as possible. Will power is a finite resource, and as such, you can’t plan to just use will power to fight off hunger pangs as a long-term weight loss strategy. Filling up on foods with fewer calories per pound will allow you to lose weight without feeling hungry.

That’s all there is to weight loss and staying fit: Eat like the person you want to become, and consistently eat foods with a low caloric density. I’ll dive deeper into what exactly constitutes a “good” caloric density and what your average caloric density should be in another post.