On its face, weight loss seems simple: As long as you burn more calories than you eat, you should shed pounds. But almost anyone who has tried to reclaim her waist can point to weeks or months when it doesn't seem to work that way. You exercise like a fiend and pass up the breadbasket only to find your jeans are mysteriously getting tighter. If it's not the dryer's fault - and trust us, it's not - you're likely in need of a mathematical reality check. Recent research shows that several popular methods of gauging your calorie needs may be inaccurate - and that costs you results. Here's the latest thinking on the five vital statistics for helping you meet your weight-loss goals.

 

Resting metabolic rate
There are a number of competing equations to calculate your resting metabolic rate (RMR) - the number of calories your body burns at rest in a single day. While these formulas offer a ballpark number of calories you can eat based on your age and weight, the most commonly used equations come from decades-old research. In fact, one study found the formulas to be off by as much as 15 percent, especially in obese individuals. All equations, even those based on body composition, can over- or underestimate the number of calories you should eat, says David Nieman, Dr.P.H., professor of health and exercise science at Appalachian State University in Boone, N.C. "Some people think they're cutting back quite a bit, but they aren't losing weight because they're still eating too much."

When scientists who study metabolism need to get it right, they rely on a "metabolic cart" - an elaborate tool that calculates RMR based on the amount of oxygen you breathe in and carbon dioxide you exhale. In the past, this kind of technology was expensive and inaccessible. But a Golden, Colo.- based company, HealtheTech, has recently used the same principle to create the BodyGem, a simple, hand-held breath test that is used for metabolic assessment at gyms and spas nationwide (log on to metabolicfingerprint .com for locations). For about $40- $100, you get results that rival the gold standard; studies found BodyGem was off by just 1 percent.

If you can't find a BodyGem test near you, turn to page 152 for the most accurate formula we've found to calculate your RMR.
 

Daily calorie count
Once you know your RMR, you will still need to account for physical activity to determine the total number of calories you expend each day. Here, an equation is the most practical method to gauge your calorie burn. Multiply your RMR by the appropriate activity factor:

If you are sedentary (little or no activity)                    RMR X 1.2
If you are slightly active                   RMR X 1.375
If you are moderately active (moderate exercise/sports 3-5 times a week)                    RMR X 1.55
If you are very active                   RMR X 1.725

The number you get represents the minimum number of calories you need to eat daily to maintain your current weight. Researchers believe that you have to burn roughly 3,500 calories to lose a pound of fat, so to lose 1 pound a week, a safe rate of weight loss, you'd need to diet or exercise your way to a 500-calorie deficit every day. But even if you are very carefully counting calories, you're probably significantly underestimating how much you actually eat. That's the finding of Wanda Howell, Ph.D., distinguished professor of nutritional sciences at the University of Arizona in Tucson, who instructed study participants to keep detailed food diaries for about two weeks. After being shown how to recognize portion sizes and account for extras like coffee creamer and salad dressing, even the most meticulous record-keepers missed about 30 percent of their true daily calories - a difference of up to 600 calories, Howell found.

The solution? Ask a friend or family member to help you get real. A recent study published in the British Journal of Nutrition found that calorie counts are much more accurate if someone else keeps track.
 

Maximum heart rate
Maximum heart rate is a measure of your body's ability to use oxygen, and it equals the number of times your heart would beat in a minute if you were running as fast as you possibly could. While the most precise tests are done in a lab, a more feasible approach in determining this number involves an equation recently created by researchers at the University of Colorado at Boulder.

The most popular way to calculate maximum heart rate is to simply subtract your age from 220. But when scientists took a closer look at that formula, they found it tends to overestimate the maximum heart rate in younger people and to underestimate it in the over-40 group. To get a better idea of your actual maximum heart rate, the researchers now recommend the following formula: 208 - 0.7 x age = heart rate max. For example, a 35- year-old woman would have a maximum heart rate of 183.5. See Target Heart Rate (below) for ways to use this figure to determine your ideal exercise intensity for weight loss.
 

Target heart rate
One persistent myth about exercising to lose weight is that low-intensity exercise - working at less than 55 percent of your maximum heart rate - is the best way to burn fat. While your body is burning a greater percentage of calories from fat when your heart rate is lower, the overall number of calories you expend during a workout is what counts. In fact, some scientists believe exercising harder burns more calories both on the treadmill and off. A study in the journal Metabolism-Clinical and Experimental suggests post-workout burn lasts three times longer (up to 101?2 hours!) for those who work out at 75 percent of their maximum heart rate than for those who coast at 50 percent.

So what's your magic number? For beginners, aim for between 50-70 percent of your maximum heart rate (just multiply your max heart rate by 0.5 and 0.7). A heart-rate monitor with a chest strap, costing between $80-$120, is the best way to tell if you're in your target zone (visit heart ratemonitorsusa.com to compare brands and prices). But the heart-rate grips on many fitness machines are a good substitute, says Jim Zahniser, spokesman for fitness-equipment maker Precor Inc. in Woodinville, Wash. They work best if your hands are slightly damp with sweat (water helps to conduct the electrical signals from your heart), your arms are relatively still and your grip is light, he says.

More advanced exercisers should shoot for at least 70 percent of their max heart rate, but don't go above 92 percent. At this point, most of us cross our aerobic threshold, according to a recent study by researchers at the University of Birmingham, England, meaning that almost all your calorie burn comes from stored carbohydrates. After about an hour at that pace (depending on how many carbs you're storing), your muscles will run out of fuel, causing you to experience what athletes call "hitting the wall." You'll feel weak and fuzzy-headed, and you can say sayonara to continuing your Spinning session - or your marathon.
 

Body fat percentage
Without exercise, once you hit your 25th birthday you'll begin to lose lean muscle mass and replace it with fat at the rate of up to 3 percent per year. By age 60, an inactive woman might weigh the same as she did at age 20, but have twice as much body fat. Excessive body fat, especially in areas such as the abdomen, is increasingly recognized as an important risk factor for killers like heart disease and diabetes.

That's why experts now suggest that women ditch body weight as a fitness benchmark and look to body composition as a better gauge of how healthy they are. The most practical and accurate way to measure body fat is a skin-fold caliper test. This can be up to 96 percent accurate if the average of three tests is used and it's done by an experienced tester. The test is offered at most gyms. However, results on people of color may be skewed by an additional 1-3 percent because the formulas most commonly used in health clubs are derived from research performed primarily on white subjects.

For optimum fitness, a recent study in The Physician and Sportsmedicine points to an ideal body-fat-percentage range between 16 and 25. Less than 12 percent can be dangerous to your health, while more than 32 percent puts you at higher risk for disease and a shorter life span.

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