According to several recent studies, both waist size and the waist to hip ratio may be better indicators of heart disease risk than the traditional Body Mass Index (BMI).
The American Heart Association (AHA) recommends that people aim for a BMI of between 18.5 and 24.9 however most Americans have a BMI well above this range. Approximately 70% of American men and 60% of American women have a BMI greater than 25 and around 35% of Americans have a BMI greater than 30 (these individuals fall into the obese category). The proportion of Americans with a BMI over 25 has more than doubled over the last 30 years.
The major drawback of BMI is that it does not take into account a persons body composition. An athlete or body builder for example may have little body fat and yet have a BMI well over 25 due to the large amount of muscle they carry.
Waist size on the other hand gives a good indication of the amount of fat a person is carrying, particularly around the stomach area. Abdominal fat is considered an important risk factor for cardiovascular diseases such as coronary heart disease and stroke. Abdominal fat is also a risk factor for diabetes because it leads to a reduction in the body’s response to insulin which results in larger amounts of insulin needing to be secreted by the pancreas to control blood sugar levels. This eventually leads to insulin resistance and the onset of type-2 diabetes.
Most doctors consider the optimal waist size to be under 37 inches (84 cm) for men and 32 inches (81 cm) for women. As of 2008, the average waist size for men in America is around 40 inches, an increase of 5 inches compared to 40 years ago. The average waist size for women is 37 inches, an increase of 7 inches over the last 40 years.
One study, presented at the annual conference of the American College of Cardiology in Atlanta in 2006, found that a 14 cm increase in waist size correlated to around a 30% increase in heart disease risk. The data was collected from over 160,000 individuals in 63 countries and represents one of the largest studies on the effects of waist size on heart disease.
A study of post-menopausal women conducted at the Washington University School of Medicine also found that the correlation between waist size and three different heart disease risk factors: insulin sensitivity, HDL-cholesterol, and triglyceride levels was stronger than the correlation between BMI and the three risk factors. The authors of the study concluded that waist circumference was a better measure of heart disease risk than BMI in middle aged and older women.
Recently, it has been suggested that the waist to hip ratio is a better predictor of heart disease than either BMI or waist size alone. The waist to hip ratio is calculated by dividing the circumference of the waist at its smallest point by the circumference of the hips at their widest point. An optimal waist to hip ratio is less than 0.8 in women and 0.9 in men.
A study, published in the American Journal of Epidemiology in 1998, found the waist to hip ratio to be a far better predictor of heart disease than BMI. The results of that study are presented below.
As can be seen from the graph, the men in the top 25% for waist to hip ratio were twice as likely to develop coronary heart disease as men in the lowest quartile. BMI on the other hand appeared to have little effect on the probability of developing heart disease with those in the top group for BMI only around 15% more likely to develop heart disease than the men in the lowest group.
A reduction in waist size is best achieved through cutting body fat. Body fat levels can be lowered through cardiovascular activities such as brisk walking, running, swimming, and playing certain sports such as squash, tennis, and basketball. Some studies have also suggested that cardiovascular exercise in the morning on an empty stomach may be the most effective way to burn body fat as the body’s glycogen stores are lower at this time. Glycogen is the bodies preferred energy source and glycogen stores must be used up before the body begins using fat as its primary source of energy.
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