There are certain truths about exercise that we all hold to be self-evident: regular cardiovascular exercise makes you feel good because of the mood-boosting endorphins the brain releases during a workout; hitting the pavement or treadmill a few times a week compensates for the odd dietary indulgence; cardiovascular exercise strengthens our heart and lungs, warding off coronary heart disease; weight-bearing exercise is good for bones, helping to prevent osteoporosis.
Perhaps the most compelling reason that motivates reluctant gym-goers is that regular workouts hold the key to weight loss. Exercise is a sure-fire route to reducing body fat, right? Not necessarily, according to recent research, which appears to turn at least two of these received wisdoms on their heads. First, the idea that we can make up for that slab of carrot cake after dinner or supersize soft drink by sweating it out in the gym is on shaky ground. More importantly, the formula on which a multibillion-dollar global gym industry is built - that exercise equals weight loss - is far too simplistic.
The study throwing that equation into question caused a flurry of provocative headlines when it was published in the British Medical Journal last year (a New York Times blog asked: "Why Doesn't Exercise Lead to Weight Loss?"). Why all the fuss? Because the study, by a research team at the University of Technology, Queensland, found that 58 obese people completed 12 weeks of supervised aerobic training but lost an average of just seven pounds (3.2kg) - with many shedding barely half that. Further controversy followed a recent study from a team at the University of Colorado School of Medicine in Denver, which challenged the idea that exercisers benefit from "afterburn" following their workout. This theory, espoused by numerous fitness gurus, holds that we crank up our body's metabolism during exercise, and so continue burning calories once we're showered, dressed and going about our business. Not so, according to the research led by Dr Edward Melanson, an associate professor of endocrinology.
Each of Melanson's subjects spent 24 quiet hours in a calorimeter (a special laboratory room that measures the number of calories a person burns) followed later by another 24 hours that included an hour-long bout on an exercise bike. All of the subjects - divided into lean endurance athletes, people who were sedentary but lean and others who were sedentary and obese - ate three meals a day. To the researchers' surprise, none of the groups, including the athletes, experienced afterburn - they used no additional fat on the day they exercised.
"The message of our work is really simple," Melanson says. "It all comes down to energy balance: calories in versus calories out. People are only burning up to 300 calories in a typical 30-minute exercise session, which you replace with a bottle of Gatorade." That debunks the idea that a quick workout balances out that sneaky hunk of carrot cake, sadly. Another of the more surprising messages from studies such as this is that more body fat is burnt during low-intensity exercise than high-intensity exercise.
"If you work out at an easy intensity, you will burn a higher percentage of fat calories," says Dr Dan Carey, an assistant professor of exercise physiology at the University of St Thomas in Minnesota. For those hoping to reduce their body fat, Carey published formulas in The Journal of Strength and Conditioning Research last October that detailed the heart rates at which a person could maximise fat-burning. So does that mean fat-burning, rather than the standard exercise goal of calorie-burning, should be our aim? If so, should we all embark on low-intensity exercise regimes, opting for a brisk walk rather than a spinning class?
Carey says his theory is not as counter-intuitive as it would first appear. "The law of thermodynamics says that weight control is a relatively simple matter, however complicated we try to make it," he explains. "If you expend more calories than you consume, you will lose weight. It is really total calories expended, not fat calories expended, that will matter for weight loss." So, although more fat calories are burnt during lower-intensity exercise such as fairly sedate cycling on an exercise bike or a slow jog on a treadmill, to achieve weight loss (shedding overall body fat and thus slimming down typical "problem" areas such as the belly, hips, thighs and buttocks) you still need to crank up your workout to a higher intensity. That means challenging classes such as spinning, circuit training or interval training on the treadmill (alternating fast sprints with slow jogs, and steep inclines with flat, which is far more effective than "steady state" jogging, to which your body quickly adapts). Despite the hyperbolic headlines, the old exercise rules still apply.
But what of the University of Technology study that found lower-than-expected weight loss among the obese? Behind the lurid headlines is the critical factor of combining effective exercise with diet - the reason these obese subjects lost less weight than expected is that they didn't change their diets. And, as Melanson's study showed, exercise alone is not enough to compensate for a high-calorie diet.
"Are we really surprised that no changes to diet would lead to no changes in body weight?" asks the personal trainer Dax Moy (www.londonpersonaltrainingstudio.com). "Nutrition, as everyone knows, is the number-one factor in controlling weight. And nutritional change alongside higher intensity exercise training, including resistance, is the best proven way to improve body composition, manage body mass [weight] and improve other health measures."
Moy emphasises resistance work because another key factor in weight loss is increasing muscle mass. Muscle burns more calories than fat - double the amount, in fact, because muscle is "metabolically active" - so resistance work such as weight training or body-weight exercises such as squats, crunches, push-ups and pull-ups should be a component of any successful fitness regimen. Moy also expresses frustration that misinterpretation of these studies could put people off exercise when they sorely need it. "The trouble with studies like this is that they convince some people - arguably those who need physical activity the most - that they are unlikely to benefit from their exercise efforts," he says. "In doing so, many people will either quit or not start an exercise regimen that could genuinely benefit their health in more ways than just weight management."
This is another crucial point: exercise offers many benefits beyond fat loss. Regular, moderate-intensity cardiovascular exercise (enough to noticeably increase your heart and breathing rate) such as jogging and aerobics makes your heart and lungs work harder as they supply oxygenated blood to the muscles throughout your body. This lowers the level of "bad" LDL cholesterol in your blood, strengthens bones and wards off diabetes, high blood pressure and coronary heart disease.
And regular cardio workouts have a profound impact on mental health, combating stress, depression and anxiety (recent studies have found exercise to be as effective as antidepressants in tackling depression). As Moy explains, resistance training adds lean muscle, helping to burn more calories, while stronger abdominal, lower back and core muscles help stabilise the spine, keeping lower-back pain at bay. Not to mention the fact that a leaner, more toned physique helps you look and feel much better about yourself.
If weight loss is your goal, the key is to combine the right type of exercise with the right kind of diet. Jeni Pearce, a performance nutritionist at the English Institute of Sport (www.eis2win.co.uk), works with Britain's elite athletes to optimise their nutritional intake. She says that a common mistake among average gym-goers is overestimating the number of calories burnt during a workout. "People may think they've expended 500 calories during their hour on a treadmill because they feel they have worked really hard," she says. "But the reality is they may have only used 250-300."
It's very difficult to calculate calorie expenditure accurately, which is why exercise physiologists use the calorimeter. Pearce says the most reliable way to assess whether your eating and training are in balance is through body weight. "If your weight is increasing, you're either not exercising enough or eating too much, or a combination of the two," she says. It's not exactly rocket science, but then, despite our increasingly sophisticated understanding of how the human body works, shedding unwanted pounds is actually quite simple: burn more calories than you expend. That means combining sensible, long-term healthy eating with regular exercise.
"If you want to reduce body fat, or what I would call 'undesirable mass', it just takes time," Pearce says. "It's probably a lot more difficult and takes a lot longer than most people realise. You have to be in it for the long haul because you can't lose a kilo of body fat overnight." If you want to lose weight, the key is to ignore the excitable headlines and make sure you combine some form of resistance training with high-intensity exercise and a balanced, healthy diet that's high in protein and low in fat, especially saturated fat, which is hard to break down. Keep at it and you will definitely see results.