At the food court in the mall there are three people in line waiting to buy pizza. The first person in line is a man who is 6 feet tall and weighs 250 pounds. He is very muscular and has no discernable body fat. The second is a woman who is 5 feet 2 inches tall and weighs 131 pounds, but has a lot of belly fat. The third person is a woman, 5 feet 6 inches tall and 149 pounds, with heavy legs. What is the risk for heart disease for these three people?
Obesity is a well-known risk factor for heart disease. There are a variety of ways to measure obesity. The body mass index (BMI, derived from a formula using weight in pounds and height in inches) is the standard measure used to define whether patients are normal weight (BMI 18.5-24.9), overweight (BMI 25-29.9) or obese (BMI > 30). There is a strong relationship between BMI and heart disease. An elevated BMI (obesity) significantly increases the risk for developing heart artery disease. However, BMI is only a crude measure. It doesn’t distinguish the total fat content of the body, the body shape or the distribution of the fat. Waist circumference can detect abdominal fat and the presence of central obesity (a “fat belly” or “love handles”). Obesity is defined by a waist circumference >40 inches in men and > 35 inches in women. An elevated waist circumference is associated with heart artery disease and increased risk for cardiac death. Other measures to assess the body’s fat content and distribution include CT scan, MRI and nuclear imaging. While very accurate, these tests are expensive and not used routinely. So BMI, may not be the best measure to assess cardiac risk. For example, take our muscular mall man. His BMI is elevated at 34, but he has no body fat and the high BMI may be due to increased muscle mass. His risk for heart disease may be the same as someone with a normal BMI. What about the two ladies? They both have a normal BMI (24), but is one more at risk than the other?
It turns out that where the fat is located is just as important as how much fat a person may have. The Framingham Heart Study showed that patents with increased abdominal fat, fat around the midsection or central obesity, had higher risk for heart disease than those with fat elsewhere. This was true for both men and women and independent of the BMI. In other words, even if the BMI is normal, an increased amount of stomach fat confers a higher risk for heart disease. The Women’s Health Initiative adds to the data. In postmenopausal women with normal BMIs, the presence of excess abdominal fat was associated with a higher risk for cardiac death compared with women without central obesity. In addition, they found that postmenopausal women with normal BMIs and high abdominal fat were at risk for heart artery disease, while those with elevated leg fat were at low risk for heart artery disease. The combination of low leg fat and high abdominal fat conferred the highest risk for heart disease. Why does the distribution of fat matter? The biologic function of fat depends on where it is located. Abdominal fat interferes with blood sugar regulation and lipid storage, leading to diabetes, elevated triglycerides, high blood pressure and subsequent heart disease. Leg fat is associated with less metabolic disturbance and thus lower risk for diabetes, cholesterol problems and heart disease.
Body shape is a stronger predictor than BMI for heart disease. The regional distribution of fat is more important than the total amount of fat. An increased waistline and excess abdominal fat can lead to heart disease in both men and women, regardless of BMI. Older women with normal BMIs and with fat around the midsection (“apple-shaped”) are at higher risk for heart disease than women with fat around the thighs (“pear-shaped”).