But what causes the insulin resistance? The exact cause is unknown, but appears to be a combination of genetic and lifestyle factors. Type II diabetes is rare in developing countries and high in industrial countries where activity levels are low, and caloric intake and obesity rates are high. In individuals diagnosed with type II diabetes, a reduction in caloric intake, moderate weight loss (in people who are overweight) and aerobic exercise have all been shown to be helpful in improving blood sugar regulation. Why is abdominal fat risky? Type II diabetes often occurs in people with excess abdominal fat, but recent studies have found that not all abdominal fat stores are associated with increased risk of type II diabetes. Research using imaging techniques such as magnetic resonance imaging (MRI) or computed tomography (CT) has shown that adipose tissue located intra-abdominally (around the viscera) is much riskier than subcutaneous adipose tissue (fat stored under the skin).1 This observation helps to explain why some people with abdominal fat have type II diabetes while others do not. Scientists cannot yet say whether the excess visceral fat directly causes insulin resistance or not. It is possible that visceral fat and insulin resistance are both part of a larger metabolic imbalance that involves many variables, and these variables no doubt interact with each other. For example, a client with a strong family history of type II diabetes may experience onset of this disorder at a lower amount of visceral obesity than a client with less of a genetic predisposition.

Practical implications

What does all of this mean to the fitness professional? It is not likely that clients will be bringing in MRIs or CT scans of their visceral fat; nevertheless, plain old excess abdominal fat remains a good predictor of insulin resistance. Many fitness centers already include an assessment of extra abdominal fat by measuring waist and hip girths and expressing these as a waist-to-hip ratio. This gives some indication of a fat distribution pattern, and helps make clients aware of the risks associated with abdominal obesity.

One research group found that a waist circumference over 1 meter (40 inches) was associated with increased risk in men and women under 40 years old.1 A circumference of over 90 cm (36 inches) was associated with increased risk for men and women between 40 and 60 years old. The researchers found that these circumference predictions were not affected by a person's height or gender, and that the predictive power of the waist circumference alone was stronger than that of the waist-to-hip ratio. If these observations are confirmed by other studies, we may be taking waist circumferences as part of standard fitness tests in the future.

Exercise recommendations

Regular exercise should be the first line of treatment for clients diagnosed with type II diabetes. People diagnosed with the early stages of insulin resistance (sort of like borderline diabetes) are especially responsive to exercise. Some people are able to control their diabetes for many years with regular exercise and a nutritious, low-fat diet. Exercise acts directly at the source of the problem for people with insulin resistance: the cell receptors for insulin. An acute bout of exercise increases receptor sensitivity for about 48 to 72 hours. When the receptors become more sensitive to insulin, then insulin can do its job. Blood sugar can enter the cells to be stored or used for fuel instead of accumulating in the blood and causing metabolic havoc. After two or three days, this effect wears off, however, so regular, almost daily exercise is critical for diabetic control.

While regular exercise is strongly associated with improvement in blood sugar regulation, an increase in cardiovascular fitness, as measured by aerobic capacity, is not. And while aerobic fitness is more likely to increase with exercise of higher intensity, exercise volume (total energy expenditure) appears to be more important for improving insulin sensitivity. People who exercise for an hour or more daily at a moderate intensity (e.g., 50 to 70 percent VO2max) appear to have the greatest improvements in blood sugar regulation.1 This type of exercise not only improves insulin sensitivity but also helps address the other disorders commonly associated with type II diabetes, and, thus, may help correct other aspects of this metabolic disturbance. Regular exercise helps normalize blood pressure in people with hypertension, and improves lipid profiles, raising HDL levels and reducing levels of triglycerides. With an appropriate caloric intake, exercise also contributes to a reduction in the visceral fat stores that increases risk for type II diabetes.

REFERENCES


1. Despres, J-P. Visceral obesity, insulin resistance, and dyslipidemia: Contribution of endurance exercise training to the treatment of the plurimetabolic syndrome. Exercise and Sport Sciences Reviews 25: 271-300, 1997.

2. Fackelmann, K.A. Panel urges widespread testing for diabetes. Science News 152 (1): 4, July 5, 1997.