While many of us worry unnecessarily about extra inches or a few extra pounds, we also know the real signs of being overweight or obese. Or do we? It turns out that numbers on a scale can lie when it comes to carrying too much fat, and that you can be of normal weight and fat at the same time. A recent report from the Mayo Clinic, reported by Ron Winslow in the Wall Street Journal (The Scales Can Lie), calls this phenomenon "normal weight obesity" - and it brings into question one of our most commonly assumed signs of health.

Keep reading for all of the details - and to figure out if you're fat or fit.

The Mayo researchers found that individuals of normal weight who had a high percentage of body fat had bodies that behaved like they were obese - and that means that they were at significantly higher risk for heart disease and other ailments linked to excess weight. The researchers estimate that as many as 30 million Americans could fall into the normal weight obesity category - many of whom are unaware that they're at higher risk for disease and metabolic syndrome, which includes elevated blood sugar and blood pressure and is often a precursor to diabetes and heart disease.

So how can you determine whether you're both normal weight and obese? You can buy a specialized scale that measures body fat, or you can get an assessment at a doctor's office or health club. And, as we've frequently been told, the extra pounds we have to worry about most are the ones that collect around our middles, near our vital organs. Generally, body fat percentages over 25 percent for men and 35 percent for women are considered high - but there's still no clear answer about what amount of body fat is ideal for optimal health.

The reliability of BMI (Body Mass Index) in determining whether a person is obese is unclear. BMI is determined by dividing your weight (kilograms) by your height (metres squared). People who fall within the 18.5-24.9 range are considered normal weight by current guidelines - but people who have BMIs above 25 often eat well and exercise regularly, throwing some skepticism on the utility of the measure.

So what's the answer? Like most things, it's subject to debate. But doctors and fitness experts are recommending the same remedy offered for most things: eat better and exercise more.