You know those super-model-skinny gals everyone loves to hate? The ones who chow down on whatever they want and never seem to gain an ounce, never mind that they don’t exercise? Well they may not be as lucky as you think. When it comes to your overall health, the numbers on the scale might not be as good a predictor as previously thought. What you see isn’t necessarily what you get, especially when it comes to weight.

Over the last couple decades a number of studies have cast doubt on conventional wisdom that skinny people are generally healthy, and fat people are generally not.

American research published in the Archives of Internal Medicine in August shattered the stereotypes even further: researchers looked at 5440 adults over the age of 20 who had participated in national health and nutrition surveys between 1999 and 2004.

They used the long-held standard measure of healthy weight—body mass index—to determine who was normal weight, overweight and obese. They then looked at the major risk factors health professionals usually associate with being overweight—things like high blood pressure, cholesterol, blood fats and blood sugar. What they found was that more than 51 percent of overweight adults—and nearly 32 percent of obese adults—fell into the normal range. But 23.5 per cent of people in their target weight range had dangerously high levels of at least two of the risk factors. (Arch Intern Med 2008 Aug 11;168(15):1617-24)

But when the researchers dug a little deeper to see what set the healthy folk in the study had in common, they found a few things. People who were inactive, older or had thick waistlines were all more likely to have the risk factors for heart disease.

Overweight and obese people in the study who were metabolically healthy had smaller waistlines. And in the normal weight group, the ones who were most likely to have heart disease risk factors had extra weight around their tummies.

Surprising? Hardly, says Associate Professor Katherine Samaras, an obesity guru who heads up the Australian Centre for Metabolic Health at St Vincents Hospital, as well as the Diabetes and Obesity clinical group at the Garvan Institute in Sydney.

“The no brainer here is that it’s not about your weight, it’s about where you keep your fat,” Samaras says.

Abdominal fat has been linked to increased risk of diabetes, hypertension, high cholesterol, cardiovascular disease and death.

“Women with twiggy little legs who are apple shaped are the ones that have these health problems, whereas women who are pear shape with a little waist even though they are overweight, do not,” Samaras says. “There are intrinsic differences between the type of fat cells you find on the hips, thighs and buttocks and those found in the abdomen.”

In women, the fat cells on the hips, thighs and bum are mostly just there for storage purposes.

“So if there was a famine or a crop failure or a war, pregnant women could still lactate. It is a guaranteed food supply for infants,” she says.

The fat cells around the abdomen, on the other hand, don’t just store fat. They constantly leak fat out into the circulatory system. At the same time they also release inflammatory molecules (called adipokines) that can lead to all sorts of nasties—such as hardening of the arteries, arthritis and even some cancers.

Samaras says abdominal fat is “malign” but it does serve some evolutionary purposes too.

“There’s a ready supply of energy if there’s an immediate threat. If you were being chased and needed to run fast now, the abdominal fat would provide you quick energy.”

Which may be why you find elite endurance athletes, such as brilliant swimmers, tend to put weight on around their tummy in older years.

“Biologically there are some advantages to central fat, but its very much genetically determined,” Samaras says.

In other words, there’s not a lot you can do about where your body stores fat—it’s mostly luck of the draw, but some things can accentuate it. For example certain nutrients are associated with abdominal fat—probably the most common one is fructose, which is often added to prepared foods under various nomers: corn starch, maize starch, all natural fruit juice, and apple juice among them. And it’s important to note that despite its ties to central obesity, fructose often appears in fat reduced products including yoghurts.

Stress has also been linked to gaining weight around your middle. Abdominal fat has more receptors for the stress hormone cortisol than other types of fat, so the more stressed you are the more cortisol is produced, and the more fat ends up around your waist. Some people are more vulnerable to stress than others, and again there’s not much you can do about the way you’re wired—but you can learn to deal with stress more efficiently. Getting enough sleep, learning relaxation techniques, making time to exercise, prioritising your life and cutting things from your schedule to free up time, and resisting the temptation to gorge out on comfort foods are a few examples.

So how do you know if you’re on the right track health-wise?

Once upon a time health professionals almost uniformly recommended checking your body mass index, which is calculated by dividing your weight in kilos by your height in meters squared. If the number fell between 19 and 25 you were given the green light for healthy. But while BMI estimates total body fat, it doesn’t take lean muscle mass into account, which is why elite athletes sometimes score in the obese category. Likewise, BMI doesn’t take into account how your fat is distributed on your body, so you’re not really getting an accurate picture of your health risks. Which is why experts now say measuring your waist circumference is a better indicator of how you’re going.

To get an idea of your overall health, grab a tape measure and measure your waist circumference at the narrowest place between the bottom of your ribs and the top of your hips. For women experts say your ideal circumference should be under 80 cm, for men it should be under 94 cm. Wider than that means you are at increased risk of health concerns.

How do you know if the apple-shape is in your genes? One way to gauge your risk is to have a look at your parents, if either of them is apple-shaped you are at risk as well.

"If that’s the case it’s ideal to not ever become overweight,” Samaras says. “ Keep an eye on what you are eating: no junk, minimise bought or processed foods and sweet drinks—even fruit juice—and stay active. Preventing central obesity is the best cure."

Are you in the danger zone?

Waist circumference

For women:

80cm or more – increased risk
86cm or more – substantially increased risk

For men:

94cm or more – increased risk
102cm or more – substantially increased risk.

for Madison, 2008