With the United States tipping the scales in obesity, many people are making the effort to trim the fat. And while experts agree that a good diet and exercise is the best way to achieve weight loss, there is some disagreement about what numbers you should track as you trim those pounds.

Many exercise physiologists and fitness professionals rely on body fat measurements to gauge weight loss. They reason that the goal of weight loss is to lose fat and put on muscle. Others recommend using the body mass index (BMI) scale, which is a measurement that simply compares your height and weight. So which measurement should you monitor?

Body Fat vs. BMI

For years, doctors and researchers have used BMI to assess whether someone was at a healthy weight. BMI utilizes a simple formula that compares your height and weight. You can determine if your BMI is within a healthy range by looking up your height and weight on a BMI table available in many doctors’ offices or on the Web site of the National Heart, Lung and Blood Institute. A BMI between 18.5 and 24.9 is considered healthy, while a BMI between 25 and 29.9 is considered overweight. And if you have a BMI greater than 30, you are considered obese.

Advocates of measuring body fat point out that BMI cannot distinguish between body fat and lean muscle tissue.

"You can be heavy, but you could be heavy with muscle." says Richard Cotton, exercise physiologist certified by the American Council on Exercise and the American College of Sports Medicine. "Michael Jordan has a body mass index of 30, which is considered obese, but there’s no way he’s obese; he’s just heavily muscled."

Yet, experts who favor BMI say it helps the average Joe or Jane trying to trim down set a goal for a healthy weight. And it’s easier to measure than body fat.

In general, total body fat is reported as a percentage of your total weight. For example, someone who weighs 140 pounds with 10 percent body fat has 14 pounds of fat and 126 pounds of lean muscle, bone, organs and everything else that is contained within the body.

There are no standards when it comes to measuring body fat, but it’s estimated that a man should have between 15 and 20 percent body fat and healthy women should fall between 20 and 30 percent. If you are tracking body fat percentage over time, be sure to get measured by the same person every time as individual techniques and calculations may vary greatly.

The gold standards for body fat measurement are underwater weighing and dual-energy x-ray absorptiometry (DEXA). In underwater weighing, you compare your weight on land to your weight in a pool. Since fat is buoyant and muscle isn’t, the more you float, the more fat you have in your body—and the less you weigh underwater. While this is one of the most accurate ways to gauge body fat, few doctors’ offices feature a swimming pool. DEXA uses low-dose X-rays to measure bone and soft tissue mass. After the 20-minute scan, you will know your body fat with about a 2 percent margin of error, making it about as accurate as underwater weighing. Many doctors’ offices don’t offer DEXA either, but because it is also used to measure bone density, it is offered in some radiology centers.

Other methods to measure body fat exist, but with low cost and availability comes less accuracy. The caliper test, those scary pinchers that measure the folds of skin at specific parts of the body, can be accurate but only if used by a trained professional. Bioelectrical impedance, another means of measuring body fat percentage, is a feature on some at-home and gym scales. As you step on the metal plates, a tiny electrical signal is sent through your body. Because of its chemical properties, muscle creates no electrical resistance, but fat does. So the scale measures the total amount of electrical resistance—how fast it took for the signal to flow through your body—and reports your body fat percentage. While this technique is easy to use, the measurement may be greatly affected by your level of hydration, as water is highly conductive.

What It Means for Your Health

Weight loss is for many a means to a healthier life. While a lower body fat would seem to equate to lower chronic-disease risk, the evidence so far points to BMI as the best health indicator.

"BMI is much more correlated with chronic disease in medical studies, primarily because it’s so much easier to assess." says Ralph LaForge, managing director of the Duke Lipid Disorder Preceptorship Program at Duke University.

By contrast, body fat percentage alone has not been proven to be an indicator or risk factor for any disease. Body fat is sometimes more related to genetic and ethnic factors than diet or exercise regimes. So some people just have more body fat than others and have a more difficult time getting rid of it.

The only established role that body fat plays in health risk assessment is based on its location. Many studies have shown that the fat immediately under your skin, subcutaneous fat, does not affect your risk of heart disease, diabetes or other chronic diseases. However, high amounts of visceral fat, the fat located deep in your abdomen that surrounds your organs, can be a major indicator of vascular risk.

"If you had to hang your hat on any one variable that related to mortality, diabetes and coronary disease." LaForge says, "waist circumference is probably the single most important measure that a person could test from month to month or year to year."

For this reason, some doctors will measure your hip-to-waist ratio, another means of gauging your body fat. The larger your waist is in comparison to your hips, the more fat you have in your lower abdomen. This "apple" shape caused by high levels of abdominal body fat is correlated with high risk for cardiovascular disease as opposed to the "pear" shape, when more fat is located around the hips and thighs.

And that dimpled fat, cellulite, which tends to be more noticeable in the "pear" shapes, is no different than the rest of your body fat, just a bit tougher to remove because of its location and strong genetic basis.

What’s for You?

Most professionals agree that any form of diet and exercise will likely lower both your body fat and BMI and make you healthier. The key is finding a tool that will keep you motivated in your health endeavors.

"The most important thing is to get the exercise and the healthy eating habits rolling." Cotton says.