Obesity may soon overtake smoking as the leading preventable cause of death in the United States. Even now, overweight and obesity account for almost 216,000 deaths a year, and if present trends continue, the number is sure to rise.

As scientists have struggled to understand why obesity increases the risk of heart attack, stroke, hypertension, diabetes, cancer, arthritis, and other woes, they have refined the definition of obesity. Because the hazard is not body weight but body fat, simple height and weight charts have given way to a mathematic formula that uses these two measurements to calculate the body mass index, or BMI. A BMI between 25 and 30 signals overweight, and a score of 30 or above indicates obesity.

The poison apple

Although the BMI provides a reasonably accurate reflection of body fat, it does not tell how that fat is distributed. Research shows that not all fat is created equal. In fact, fat plastered around the body's internal organs (visceral fat) is much more dangerous than fat layered beneath the skin (subcutaneous fat); that's why liposuction and "tummy tuck" operations may improve your profile but won't help your metabolism or your health.

Scientists depend on fancy equipment to measure visceral fat directly, but you can get the practical information you need to evaluate your risk by using a humble tape measure. Many studies confirm that upper body fat (the "apple shape," or "beer belly") is much more dangerous than lower body fat (the "pear shape"). You can find out where you stand simply by measuring your waist circumference or by measuring your waist and hips and then dividing your waist measurement (in inches) by your hip measurement (in inches) to calculate your waist-to-hip ratio. The table below shows what your results mean.

A weighty body of evidence shows that upper body fat is more hazardous than lower body fat. Until recently, doctors assumed that even if lower body fat is less dangerous than upper body fat, it's no bargain on its own. But research may change that belief; without questioning the fact that upper body fat is a formidable foe, it raises the startling possibility that lower body fat may actually be a friend to health.

Body fat measurements that signal high risk Men Women Waist circumference 40 inches and above 35 inches and above Waist-to-hip ratio 0.95 and above 0.85 and above

Big thighs, better health?

To find out how thigh circumference affects health, Danish scientists evaluated 2,816 men and women ages 35 to 65 who were free of heart disease, stroke, and cancer when they joined the study in the late '80s. Each participant provided a detailed health history and each underwent comprehensive examinations that included measurements of height, weight, and thigh, hip, and waist circumferences, as well as body fat percentage, which was determined by the highly accurate impedance method.

Researchers tracked the volunteers for an average of 12.5 years. They found that people with big thighs had a lower risk of heart disease and premature death than those with thin thighs. In round numbers, a thigh circumference (measured where the thigh meets the butt) of about 62 cm (about 24.4 inches) was most protective; bigger thighs provided little if any extra benefit, but progressively thinner thighs were linked to progressively higher risks. The predictive value of thigh size held up even after the scientists accounted for other indicators of body composition, including waist circumference, BMI, height, and body fat percentage. And thigh size remained a strong independent predictor even after researchers adjusted for risk factors such as smoking, exercise, alcohol use, systolic blood pressure, cholesterol and triglyceride levels, and (for women) menopause.

It's only one study, but its results are impressive. Still, because the scientists measured thigh size but not thigh composition, they can't tell if the apparent protection of big thighs is due to more muscle, more fat, or both.

Muscle matters

It's tempting to wonder if national personality traits influence the design of scientific studies; for whatever reason, the 2009 Danish study focused on thighs, while a 2007 British study zeroed in on arms. In this case, scientists investigated 4,107 men between the ages of 60 and 79; men who were underweight (BMI below 18.5) were excluded from the study, as were those with heart failure. Researchers evaluated the body composition of each subject by measuring height, weight, waist and hip circumference, mid-arm muscle circumference, and overall fat mass and muscle mass. To evaluate other risk factors, each subject provided a medical history and underwent a physical exam and a fasting blood test.

During an average follow-up period of six years, two factors emerged as the strongest predictors of mortality. Large waist circumference, reflecting abdominal obesity, was linked to a high death rate, but large mid-arm muscle circumferences predicted a reduced death rate. The results held up even after researchers took other risk factors into account.

The British study agrees with other investigations that show the stronger live longer. Part of the reason may be that big, strong muscles reflect regular exercise and an active lifestyle. But muscle itself also contributes to health. Muscle burns calories faster than other tissues. Muscle cells are also more responsive to insulin, lowering blood sugar levels without requiring the high insulin levels that are associated with high cardiovascular risk. Perhaps that's why a 2010 Japanese study linked small thigh muscles to abnormally stiff arteries in men.

Big thigh muscles might help explain the apparent benefit of large thighs — but extra lower body fat might help, too.

The low-down on lower body fat

A 2010 British review makes the case that gluteofemoral fat, better known as the pear shape, has a protective role. Using thigh circumference, hip circumference, and direct measurement of leg fat mass as indicators of gluteofemoral fat, the scientists argue that lower body fat helps the metabolism by reducing LDL ("bad") cholesterol and triglyceride levels, raising HDL ("good") cholesterol levels, improving insulin sensitivity, and lowering blood sugar levels. And according to a 2011 study, this may help explain why women (who have more lower body fat) develop diabetes at a higher body mass index than men (who have more abdominal fat).

How might gluteofemoral fat do all this? For one thing, fat cells in the lower part of the body seem to vacuum up harmful fatty acids that are released into the blood when fat-laden foods are digested. Fat cells in the upper body also store up free fatty acids, but are quick to pour them back into the blood in response to stress-induced surges of adrenaline. And abdominal fat cells produce larger amounts of cytokines, chemicals that trigger harmful inflammation, while lower-body fat cells produce productive chemicals, including leptin and adiponectin.

Men and women, here and now

More research is needed to sort out the action of gluteofemoral and visceral fat. At present, the evidence implicating visceral fat is stronger than the data exonerating gluteofemoral fat. In addition, there is just no way you can selectively target either type of fat. The best advice is to keep your overall body fat in check by taking in fewer calories in your diet and burning up more calories with exercise. In particular, calorie-dense foods (fat, sugar) will tip the balance toward a big belly, while resistance exercise (strength training) can build up muscles in your thighs and arms.

In America and around the world, women tend to be pear-shaped, while men tend to be apple-shaped. It may be one reason that women live longer than men, both in America and around the world. And as scientists continue to investigate body shape and health, they are likely to pile up evidence that confirms what men already know: big bums trump big tums.

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