Posted by: 4pack | June 8, 2008

Some Basic Concepts Regarding Body Fat

(From San Francisco Chronicle Online, Kelly Mills)

Here are three fitness concepts that deserve a closer look:

1. Body Mass Index isn’t as helpful as some might want you to believe. You’ve probably seen the formula: You enter your height and weight, and the ratio slaps you in a category: underweight, normal, overweight or obese. The idea is that BMI can help you judge the amount of fat you have on your bodyGo to fullsize image and presumably tell you if you are at risk for weight-related health problems.

BMI is a statistical measure and was not created to be used as a diagnostic tool for individuals. In other words, BMI may be used to group inactive people with an “average” body composition (according to the one used to develop the ratio) into one of the above categories. If you fall outside the average composition for any number of reasons, BMI will not provide an accurate diagnosis for you. But it has grown in popularity as a measure for individuals, probably because many doctors wanted an objective way to tell patients they should lose weight. You can feel the dilemma: It’s easier to say, “Your BMI puts you in the ‘obese’ category” than it is to say, “Hey, clearly you have some belly fat that might put you at greater risk for heart disease.”

Even BMI proponents acknowledge that BMI has limitations; it can overestimate body fat in dense, athletic folks with lots of heavy muscle, and it also miscategorizes some tall people. BMI can also underestimate body fat in people who have lost muscle mass, like many older folks. The measure also doesn’t say anything about fat distribution: Holding fat around the midsection, for example, can indicate a much greater risk of health problems than fat on the hips and thighs. Oh, and having a BMI under 23 can actually mean you have worse cardiovascular health, statistically speaking.

None of this would matter so much except BMI is being used in a number of questionable ways. Some insurance companies assess risk – and rates – based on BMI, meaning you could pay more if you are considered overweight or obese. Even the new Wii Fit game uses BMI as its measurement tool; the game company cites the fact that the CDC supports BMI. This could all be problematic if your BMI happens not to correlate with your actual percentages of body fat and lean muscle.

2. You can’t just lose fat from one part of your body. But, hey, maybe you aren’t so worried about your BMI, and you know you are carrying more fat than you’d like around your middle, so you pick up a fitness magazine that has a section on exercises to “flatten your tummy.” The lean, perky model inside is shown going through a range of exercises designed to target the abdominal region. While these exercises are often good for building core strength and muscle definition, nobody is going to see your perfect six-pack abs if you have a layer of fat on top of them. And the best way for most people to lose fat is through a combination of sustained cardio exercise (running, biking, swimming and so on) and some weight training (to build muscle, which can help you burn calories even while at rest.)

There is no one exercise that can selectively take fat off a specific part of your body. You can do crunches and sit-ups daily, and you are not going to see a significant reduction in waist size unless you burn more calories than you take in. Keep doing the ab exercises, but be sure to include cardio exercise as well. Most of us tend to store fat in certain places like belly or thighs, and this fat is the last to come off. I know. I think it stinks, too, but there you have it.

3. Don’t live in the fat-burning zone. So you have opted for a cardio program, and you bought a heart-rate monitor so that you can stay in your “fat-burning zone.” Much of the cardio equipment at the gym also has sensors so you can track your heart rate and ensure you hold true to that zone. The zone is that magical place where your heart rate stays at a certain level and your body is able to most efficiently burn calories from your fat stores. This level is between 55 and 70 percent of your maximum heart rate, and you can determine your zone using basic formulas or by having a professional administer a treadmill test.

One advantage of the fat-burning zone is that you can usually sustain exercise at this level for long periods of time, and burn lots of calories that way. But is it the most efficient way to exercise to lose weight? As someone who spent some time obsessively trying to stay in this zone in the past, I can tell you that while it does burn the greatest percentage of calories from fat, you actually burn more overall calories at higher-intensity heart rates. That means if weight loss is your goal, you are probably better off doing interval training, where you work below 70 percent of your maximum heart rate some of the time, and above 70 percent some of the time. You can do this in separate workouts or by combining things like slow jogging and fast running in the same workout. Either way, it’s also better for your overall fitness to train your heart at a variety of intensities. Just don’t bring your heart rate up to 90 percent and try and keep it there forever – even if your BMI puts you in a low risk cardiovascular health category and you’ve been doing all those ab crunches.

 Kelly Mills is a fitness instructor who lives in Oakland. Her Web site is www.fitnessfixation.com.

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