And so can weight.

The shortcomings of BMI are well-documented -- it doesn't differentiate between weight that comes from muscle mass and weight that comes from fat, so super-buff athletes and bodybuilder types are likely to be classified as obese. Of course that kind of exception is easy to spot but on the flip side, you can be right smack dab in the middle of your ideal weight range, and still be at high risk for all the same frightening conditions that so often claim obese people as their victims.

That's because from a health perspective, lack of physical activity can actually be a bigger risk factor than merely being overweight.

"Cardiovascular fitness is the type of fitness required to help prevent many diseases such as heart disease, cancers and diabetes," says Philo Saunders, a PhD scholar in the Department of Physiology at the Australian Institute of Sport.

And there's no direct correlation between weight and aerobic capacity, though current research by the Australian Institute of Health and Welfare shows that obese people are less likely to exercise moderately, possibly because the extra kilos can make it that much more difficult and obese people are often reluctant to work out in gyms or other public settings.

But, that said, thin and fit are certainly not synonymous.

"We have a lot of people coming in who fall into the normal weight range and we think 'this is going to be easy,' but then when we go to assess them it's a different story.

"It becomes really apparent in the push up tests, and often in cardio tests," says Megan Chisholm, a personal trainer and director of Definition Wollongong, a "first-timer" gym geared towards people who are not used to regular exercise. "Over the years I've had some really skinny clients who are really unfit. Their bone density tends to be a problem. Their cardiovascular fitness might not be that great."

And vice versa. A slightly overweight person who exercises regularly will likely be better off than a thin person who doesn't.

"You can be fat and fit or fat and unfit," says Trent Watson, a practising dietician at Newcastle University, "and the difference is one hour of exercise a day." This happens when people consume too much energy to reduce their weight, he says, but it doesn't negate the health benefits they'll receive if they exercise.

"That hour a day reduces the incidence of type II diabetes and heart disease two-fold, and increases longevity substantially," Watson says.

A number of studies in the United States back this idea. The Cooper Institute for Aerobics Research in Dallas, Texas, has been at the forefront of this type of research.

One study that tracked more than 25,000 men between 1970 and 1993, and another separate study that tracked 9925 women between 1970 and 1996 found that low fitness was an independent predictor of mortality across all body mass groups. Similar findings have come from research out of the University of California at Los Angeles and Baylor College of Medicine among other universities, and a number of books have been published on the subject as well, from Paul Campos's The Obesity Myth to Glenn Gaesser's Big Fat Lies.

Not that weight is irrelevant, but on its own it can be misleading.

"Changes in weight can be a better indicator of fitness than just looking at weight alone, but even that has its limitations because you can lose weight just by your diet and not change your cardiovascular fitness at all. Body fat's the same," Saunders says.

"They both can be good in conjunction with self fitness assessments if you're using them to measure the effectiveness of a program."

Studies on the long-term effects of dieting have found that as much as 50 per cent of the weight lost through dieting can occur at the expense of lean body mass, which slows your metabolism and can cause fatigue.

That's why most gyms and fitness centres use a series of tests to assess how fit their clients are.

Staff at the University of Wollongong Recreation and Aquatic Centre do check height, weight, body fat percentages and circumferences, but they also assess flexibility with the sit and reach test, muscular endurance (push-ups), abdominal strength, and cardiovascular fitness using an aerobic capacity test on an exercise bike to measure the heart rate-to-workload ratio, says exercise specialist Claire Fraser.

Exact tests vary from place to place but most use versions similar to these, although Chisholm skips the weight and fat percentage tests altogether.

"I hate them because people become obsessed with them. People come in and they may be losing centimetre after centimetre off their waist but might not be losing weight because scales don't measure lean body mass, so they're not happy. People come into the gym ready to be scammed, and if they're not losing pounds then that's their proof. They get too bogged down worrying about weight and they get demotivated."

Structured fitness tests like those mentioned are usually only done a few times a year, so often the best method of monitoring improvement is self-assessment.

You can monitor your own progress with a heart rate monitor or by keeping track of the time and frequency of your exercise, testing yourself on a measured course to see how your time and speed improves, or even just by keeping track of how you feel and noting if it takes less effort to run up that hill than it did a month ago.

There are also fitness assessment tests you can easily do yourself at home. (See below).

But there's another type of fitness these tests neglect to measure: metabolic fitness. This is how well your body responds to insulin. The more "insulin sensitive" you are, the better. Insulin sensitive bodies tend to have excellent glucose tolerance, normal blood pressures, and heart-healthy blood lipid profiles.

Therefore, insulin-sensitive people tend to be at lower risk of type II diabetes and heart disease than people who are "insulin resistant", according to author and physiologist Gaesser.

To get this sort of fitness assessed you'll have to see your GP for blood tests, but once again, there's plenty of research showing that exercise improves metabolic fitness as well.

"If you do low intensity exercise every day, at 50 per cent of your max, even if you're still overweight it can still have a very positive effect on your metabolic fitness and increase your insulin sensitivity," says Dr Helen O'Connor, a dietician in the department of Exercise and Sports Science at Sydney University.

Active Australia recommends 30 minutes of moderate activity most days. It can be incremental or all at once, and intensity isn't all that important to reaping the health benefits.

"Thirty minutes a day is enough not to gain any more weight and to reduce your health risks. It's the minimum, but it's not enough to actually lose weight. To lose weight you really need to exercise about an hour a day," O'Connor says. "But your health will improve even without that."

The world of health and fitness is filled with such paradoxes. For example, a study published by The Australian Institute of Health and Welfare in March found that men who were in their healthy weight range were more likely to smoke than their overweight counterparts. Obese men had a higher percentage of ex-smokers.

And overweight people were more likely to report usually consuming skim milk than people of healthy weight (10.5 per cent versus 7.8 per cent for men, 19.7 per cent versus 15.4 per cent for women).

Other research has found that the body fat measured in skin fold tests isn't necessarily a great predictor of health risks either. Abdominal fat poses a much higher risk for diabetes high blood pressure and heart disease.

So a person might not be very overweight, but if they are "apple shaped" and have most of their fat around their waist they have higher risk factors than someone with a higher per cent body fat on other parts of their bodies, O'Connor says.

In the end, it's a bit of check and balance. Any one risk factor isn't necessarily an overriding factor, so if you need improvement in one or two areas, don't despair.

But don't be complacent either.

Unless you have some seriously hermit-like tendencies you probably already know that exercise is the key to your health and when it comes right down to it, knowing the risks isn't enough.

"It's about compliance and adherence," says Dr David Pyne, a physiologist at the Australian Institute of Sport. "All the information is out there in the public debate. There's an awareness about it and most people could tell you what the basic tenants of a healthy diet are, but it comes down to compliance -- not just knowing the characteristics of a healthy lifestyle, but actually doing it and following it."

Even elite athletes need help.

At the AIS, dieticians have a number of initiatives that include supermarket tours (where athletes learn how to read product labels), cooking classes, and seminars on topics such as eating during extended travelling or how to manage in buffet-style communal dining -- where the instinct is to just pig out. Exercise is easier too with direct instruction, or with a partner, small groups or a personal trainer. It's hard to do it by yourself.

And if there's one thing that fitness experts, dieticians, doctors and physiologists agree on across the board, it's that getting fit requires an entire lifestyle change, especially if you are trying to lose weight in addition to the initial health benefits that exercise will bring.

"People might do weights three times a week and not see any real results but what they need to do is make exercise a habit. Something you do every day like taking the stairs instead of the lift, that sort of incidental exercise," Claire Fraser says.

Part of the problem is that the sedentary lifestyle so common these days often starts in our youth. We're raised this way, and statistics show that the patterns you develop at an early age are ones that affect you for the rest of your life, says David Simpson, CEO of the Bluearth Institute in Melbourne which runs Discovery School Experience, a program that sends coaches into primary schools across the nation in an effort to increase physical activity and promote a healthy lifestyle.

Simpson says with children starting organised sports at earlier ages there is increasing pressure not just to participate, but to excel. The change in emphasis from having fun to "must-win" has had an enormous impact on the number of kids who drop out of their sports by the time they reach their early teenage years. "And once that happens they're unlikely to come back later in life."

Bluearth's Discovery program uses inclusive, non-competitive games to reach everyone, especially kids who don't see themselves as sporty or don't have as well developed eye-hand co-ordination as some of their classmates might. They're the ones most likely to stop exercising.

"Right now with the Olympics on there's a mania for athletic performance and you see all these athletes excelling and associate it with fitness, but it's a tough standard to match. Fitness should be more of a lifetime activity," Simpson says.

But it's often not. Still, he and others interested in health promotion are optimistic that our concept of health and fitness is beginning to change.

"People are starting to realise that fitness is less about the classic Olympic athlete or the marathoner, and more about your physical wellbeing and being comfortable with yourself and enjoying it. It's less about dropping 15 pounds or having washboard abs, although that's still important to a lot of people," Simpson says. "If you're living a sedentary life you don't feel well."

Getting fit can be easier than we often think. "A lot of people think getting fit means they have to toil and toil in the gym grunting and all that, but if they have a solid program then they can exercise 30 minutes a day and achieve great results," Chisholm says.

"If you want to achieve fitness you have to incorporate it into your lifestyle. That doesn't mean you can't break out and have your treats -- I love my glass of red wine and cheese and bickies. But I exercise every day and eat fairly well. It's all about balance. The misconception people have about health and fitness is that it's all or nothing. It's not. That doesn't work because it's too hard to keep up the momentum."

The Bluearth Institute

http://bluearth.com

American College of Sports Medicine

http://www.acsm.org