Generally speaking, abdominal fat is either visceral (surrounding the abdominal organs) or subcutaneous (lying between the skin and the abdominal wall). Fat located behind the abdominal cavity, called retroperitoneal fat, is generally counted as visceral fat. Several studies indicate that visceral fat is most strongly correlated with risk factors such as insulin resistance, which sets the stage for type 2 diabetes. Some research suggests that the deeper layers of subcutaneous fat may also be involved in insulin resistance (in men but not in women).
Measuring up
Researchers have tried several ways of measuring the links between health risks and body weight or fat distribution:
Body mass index (BMI). A ratio of weight in kilograms to the square of height in meters, BMI helps identify people whose weight increases their risk for several conditions, including heart disease, stroke, and diabetes. People with BMIs of 25–29.9 are considered overweight, and those with BMIs of 30 or over, obese. However, some researchers think BMI isn’t always a valid indication of obesity, because it gives misleading results in people who are very muscular or very tall. To calculate your BMI, go to www.nhlbisupport.com/bmi or use this formula: Weight in pounds × 703 ÷ (height in inches)2.
Waist-to-hip ratio. To find your waist-to-hip ratio, divide your waist measurement at its narrowest point by your hip measurement at its widest point. As a marker of a person’s abdominal fat, this measure outperforms BMI. For women, the risk for heart disease and stroke begins to rise at a ratio of about 0.8.
Waist circumference. The simplest way to check for abdominal fat is to measure your waist. Run a tape measure around your torso at about the level of your navel. (Official guidelines determine the level at which waist circumference is measured by locating a bony landmark: the top of the right hipbone, or right iliac crest, where it intersects a line dropped vertically from the middle of the right armpit.) Breathe minimally, and make sure not to pull the tape measure so tight that it depresses the skin. In women with a BMI of 25–34.9, a waist circumference greater than 35 inches is considered high risk, although research suggests there is some extra health risk at any size greater than 33 inches. A study in the September 2006 American Journal of Clinical Nutrition found that in women, a large waist was correlated with diabetes risk — even when BMI was in the normal range (18.5–24.9). Since abdominal fat can be a problem despite a normal BMI, health assessments should include both BMI and waist circumference. The relationship between waist circumference and health risk varies by ethnic group. For example, in Asian women, a waist circumference above 31.5 inches is considered a health risk
source & more details:
http://www.health.harvard.edu/newsweek/Abdominal-fat-and-what-to-do-about-it.htm
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