Metabolism: The Starting Point for Planning Diets

How the Tests Work

Fitness in a Flash and Bally's use the same sort of technology to determine resting metabolic rate. The devices measure oxygen consumption to determine caloric needs.

Because oxygen is used in the metabolic process to create energy, you can determine a person's metabolic rate by measuring how much oxygen they consume when breathing.

Rob Wildman, director of nutrition nationwide for Bally's Total Fitness, explains how it works: "Think of a fire; you keep it going with oxygen. Oxygen fuels the body's fire. We breathe to burn."

All of our cells are constantly active. Even when you are sleeping or sitting at a desk, your cells are working, explains Ethel Frese, associate professor of physical therapy at St. Louis University and a board-certified cardiopulmonary specialist.

"There are active cells constantly at work in your body," Frese says. "They use oxygen ... And the more active tissue you have, the more oxygen you consume. Even when you are just lying there and not being active, you are consuming X amount of oxygen to fuel working cells in your body."

Once you know your resting metabolic rate, you can use that number to figure out how many calories you might burn in a day. For instance, if the device tells you your RMR is 1,400 calories, that's how much you burn at rest.

"That's your foundational metabolism," says Wildman, who has a doctoral degree in nutrition and used to teach at several universities. "That's how much your body's most vital processes (such as the beating of your heart) burn to stay alive and well. It usually makes up 65 to 75 percent of how much we burn.

"Once you add activity, that number starts going up. It's a measurement we use to build upon to get a better understanding of how many calories a body burns totally."

Here's a hypothetical example of how this device would help people know how much to eat to lose weight. Say your RMR reading -- the number of calories you burn sitting still -- is 1,500. To that, add the calories you burn from basic movements, such as walking to the restroom at work and typing on the computer -- say, 375 calories. Then add in the calories you burn from exercising an hour at the gym -- say, 400 calories -- and you are up to 2,275 calories a day to maintain your weight.

But if you want to lose weight, you need to burn 500 calories more a day through exercise, or eat 500 calories less each day, or a combination, to lose a pound a week. In this case, you need to eat 1,775 calories to lose weight.

"The bottom line is what you take in vs. what you burn off," Allen says. "There is no magic pill or magic equation. But it can be done."

Many people don't realize how many calories their body needs each day. For instance, if someone who has an RMR of 1,400 goes on a restrictive 1,200-calorie diet, that person's body isn't getting enough calories to process normal daily functions, Wildman says. He added that your calorie intake should never go below your RMR. If that happens, your metabolism can start going haywire, he says.

"Your body needs food for calories," Allen says. "If it doesn't get enough, it goes into survival mode; your body becomes more efficient at holding on to calories."

The Bottom Line

While Frese says knowing your RMR can help, she says there are some things you need to be aware of while testing at a gym.

"If you want to get an accurate measure of someone's resting metabolic rate, you should make sure they haven't eaten for three to four hours before taking the test. Also, they shouldn't do any physical activity, except maybe walking from the car, before the test ... Sometimes the test's accuracy depends on the measurer and how well the test is controlled."

But those who use the test still swear by its help.

"Your metabolism can be retrained," says House, of Fitness in a Flash. "Some things we can't change, like genetics, but things like diet and exercise, we can."

The Calorie Squeeze

Many teens eat constantly without gaining weight because their resting metabolic rate is higher than an older person's. As we age, our metabolism slows down.

Here is an example of how that might work, based on information from Natalie Allen, a dietitian at Barnes-Jewish Hospital, and from Anthony's Textbook of Anatomy and Physiology. This assumes that other factors that affect metabolism are equal.

At age 18: A typical person may be able to eat 2,400 calories a day and maintain current weight.

By age 30: That person can eat only 2,190 calories to maintain that same weight.

By age 40: 1,970.
By age 50: 1,775.
By age 60: 1,595.
By age 70: To maintain the same weight the person weighed at age 18, he or she could eat only 1,435 calories, almost 1,000 less than as an 18-year-old.

How to Speed Up Your Metabolism

  • Exercise: Physical activity, particularly weightlifting, can help rev up your metabolism. "You are not only burning calories as part of your workout, but because muscle burns more calories than fat, you are burning calories all day long," says Natalie Allen, a dietitian at Barnes-Jewish Hospital. That could be as much as a few hundred extra calories a day.
  • Eat small meals throughout the day: It's important to keep the fire burning by feeding it small meals every two to four hours. "Of course, we need to quantify small meals," Allen says. "We're talking half a sandwich or a piece of fruit or a container of yogurt."
  • Eat your breakfast: Getting food in your body early on can help boost your day's calorie-burning.
  • Drink adequate liquids: The recommendations keep changing on this, but essentially drink to quench thirst -- for most people that's about 64 ounces a day, more if you are sweating a lot.
  • Be sure to eat enough: Eat at least as many calories as your body needs to maintain basic bodily functions.

    Factors that affect metabolism

    Gender: Men have a faster metabolism than women.

    Age: The older you are, the more your metabolism slows.

    Amount of muscle: Muscles burn more calories than fat.

    Hormones: An imbalance in hormones, such as those that control your thyroid, can affect your metabolism.

    Genetics: Some people are just genetically predisposed to burn more calories.

    Previous diet history: If you've gone on a diet and didn't get enough calories, this could significantly slow down your metabolism.


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