As the evidence against abdominal fat mounts, researchers and clinicians are trying to measure it, correlate it with health risks, and monitor changes that occur with age and overall weight gain or loss. The most accurate measurement techniques, magnetic resonance imaging and computed tomography, are expensive and not available for routine use. However, research using these imaging methods has shown that waist circumference reflects abdominal fat. It has largely superseded waist-to-hip ratio (waist size divided by hip size) as an indicator of fat distribution, because it is easier to measure and about as accurate. There’s also evidence that waist circumference is a better predictor of health problems than body mass index (BMI), which indicates only total body fat (see “Measuring up”).
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.
The good news is that visceral fat yields fairly easily to exercise and diet, with benefits ranging from lower blood pressure to more favorable cholesterol levels. Subcutaneous fat located at the waist — the pinchable stuff — can be frustratingly difficult to budge, but in normal-weight people, it’s generally not considered as much of a health threat as visceral fat is. In fact, a study published in the New England Journal of Medicine in 2004 found that liposuction removal of subcutaneous fat (up to 23 pounds of it) in 15 obese women had no effect after three months on their measures of blood pressure, blood sugar, cholesterol, or response to insulin. Weight loss through diet and exercise, on the other hand, triggers many changes that have positive health effects.
What’s wrong with abdominal fat? Body fat, or adipose tissue, was once regarded as little more than a storage depot for fat blobs waiting passively to be used for energy. But research suggests that fat cells — particularly abdominal fat cells — are biologically active. It’s more accurate to think of fat as an endocrine organ or gland, producing hormones and other substances that can profoundly affect our health. One such hormone is leptin, which is normally released after a meal and dampens appetite. Fat cells also produce the hormone adiponectin, which is thought to influence the response of cells to insulin. Although scientists are still deciphering the roles of individual hormones, it’s becoming clear that excess body fat, especially abdominal fat, disrupts the normal balance and functioning of these hormones.
Scientists are also learning that visceral fat pumps out immune system chemicals called cytokines — for example, tumor necrosis factor and interleukin-6 — that can increase the risk of cardiovascular disease by promoting insulin resistance and low-level chronic inflammation. These and other biochemicals, some not yet identified, are thought to have deleterious effects on cells’ sensitivity to insulin, blood pressure, and blood clotting.
One reason excess visceral fat is so harmful could be its location near the portal vein, which carries blood from the intestinal area to the liver. Substances released by visceral fat, including free fatty acids, enter the portal vein and travel to the liver, where they can influence the production of blood lipids. Visceral fat is directly linked with higher total cholesterol and LDL (bad) cholesterol, lower HDL (good) cholesterol, and insulin resistance.
Insulin resistance means that your body’s muscle and liver cells don’t respond adequately to normal levels of insulin, the pancreatic hormone that carries glucose into the body’s cells. Glucose levels in the blood rise, heightening the risk for diabetes. Together, insulin resistance, high blood glucose, excess abdominal fat, unfavorable cholesterol levels (including high triglycerides), and high blood pressure constitute the metabolic syndrome, a major risk factor for heart disease and stroke.
Excess fat at the waist has been linked to several other disorders as well. A European study of nearly 500,000 women and men found that, for women, a waist-to-hip ratio above 0.85 was associated with a 52% increase in colorectal cancer risk. A long-running community study on atherosclerosis conducted by researchers at Wake Forest University found that even among normal-weight people, those with higher waist-to-hip ratios had just as much difficulty as those with higher BMIs in carrying out various activities of daily living, such as getting in and out of bed and performing household chores.
A larger waist measurement also predicts the development of high blood pressure, regardless of total body fat, according to a 10-year study of Chinese adults published in the August 2006 American Journal of Hypertension. Finally, a study presented at the 2005 annual meeting of the Society for Neuroscience found that older people with bigger bellies had worse memory and less verbal fluency, even after taking diabetes into account.
Now for the good news So what can we do about tubby tummies? A lot, it turns out. The starting point for bringing weight under control, in general, and combating abdominal fat, in particular, is regular moderate-intensity physical activity — at least 30 minutes per day (and perhaps up to 60 minutes per day) to control weight. In a study comparing sedentary adults with those exercising at different intensities, researchers at Duke University Medical Center found that the non-exercisers experienced a nearly 9% gain in visceral fat after six months. Subjects who exercised the equivalent of walking or jogging 12 miles per week put on no visceral fat, and those who exercised the equivalent of jogging 20 miles per week lost both visceral and subcutaneous fat.
Strength training (exercising with weights) may also help fight abdominal fat. A University of Pennsylvania study followed overweight or obese women, ages 24–44, for two years. Compared to participants who received only advice about exercise, those given an hour of weight training twice a week reduced their proportion of body fat by nearly 4% — and were more successful in keeping off visceral fat.
Spot exercising, such as doing sit-ups, can tighten abdominal muscles, but it won’t get at visceral fat.
Diet is also important. Pay attention to portion size, and emphasize complex carbohydrates (fruits, vegetables, and whole grains) and lean protein over simple carbohydrates such as white bread, refined-grain pasta, and sugary drinks. Replacing saturated fats and trans fats with polyunsaturated fats can also help. But drastically cutting calories is not a good diet strategy, because it can force the body into starvation mode, slowing metabolism and paradoxically causing it to store fat more efficiently later on.
Scientists hope to develop drug treatments that target abdominal fat. For example, studies of the weight-loss medication sibutramine (Meridia), which was approved in 1997, have shown that the drug’s greatest effects are on visceral fat. Rimonabant (Acomplia) — not yet FDA-approved — is the first of a new class of drugs that block a receptor in the brain that increases appetite. Acomplia has been shown to modestly reduce the accumulation of fat at the waist.
Because levels of the hormone dehydroepiandrosterone, better known as DHEA, decline with age, many people believe that DHEA supplementation can reverse age-related changes, including increased abdominal fat. DHEA is converted in the body to testosterone and estrogen and regulates various functions. Some studies have linked DHEA to longevity in animals and people, and others have linked it to modest health benefits. But the results of a two-year randomized trial published in the Oct. 19, 2006, New England Journal of Medicine showed that DHEA had no effect on aging markers, including body-composition measurements, in women and men ages 60 and over.
For now, experts stress that lifestyle, especially exercise, is the very best way to fight visceral fat.