When exercisers complain they are not losing any weight, fitness instructors and medical professionals usually conclude that they are either lazy or kidding themselves about how much consistent effort they are putting in. But a growing body of research is suggesting that we don't all respond to exercise in the same way.

When researchers at Louisiana State University put 742 completely sedentary people on a progressively challenging 20-week training programme, and measured their responses, they found that "VO2 max" (a measure of your muscles' ability to take up and use oxygen) had improved by as much as 40% in some subjects, and not at all in others. It was the same story with other markers of fitness, such as cardiac output (the amount of blood pumped by the heart each minute), insulin resistance, blood pressure during exercise and pulse rate.

"The discrepancies in the individual response to exercise are remarkable, even when the training people undergo is identical," says Dr Claude Bouchard, the study leader and an exercise physiologist and geneticist at the Pennington biomedical research centre. "For any given indicator of fitness, there appear to be people who have a quick and large response (high-responders), people who have a slighter response (low-responders) and some who see no change at all (non-responders)." Bouchard estimates that as far as cardiorespiratory or "aerobic" fitness is concerned, 10-15% of us fall into this last category.

Does this mean all the pounding away on the treadmill is a waste of time? Dr John Buckley, an exercise physiologist at the University of Chester, believes not. "From the 10,000 or so exercise assessments and follow-ups I've carried out in the past 18 years, I've observed that just because an individual is a non- responder in one aspect of fitness, this is not necessarily carried over to other aspects of fitness response."

And indeed, when Bouchard's team analysed the data from the people they had trained, they found almost no correlation at all between the responses of different markers of fitness to training. "There was no 'universal' non-responder," he says. Everyone should get at least some benefit from exercise. It just might not be the benefit you were hoping for, as Nicola Donald, who is 35 and lives in Glasgow, can testify. "I've been exercising regularly for 11 years and I've never lost weight as a result of it," she says. "I'm always at the gym - I'll typically do two 40-minute runs and three or four quite challenging hour-long classes, such as circuit training or Body Pump, each week. But I could work out for ever and I would never get a physique like some people I see in the gym - my body just doesn't seem to respond beyond a certain level."

Unsurprisingly, Bouchard's research has also shown that the ability to lose adipose (fat) tissue also varies from one person to the next. "Weight loss is one of the key motivators for exercise," says Buckley. "Around 70% of my clients have a weight loss goal." What if their workouts aren't having the desired effect? "I advise them that it is important to focus on all aspects of their health and not judge their lives and wellbeing by a single set of figures on the scales."

The possibility that stepping up your activity level won't necessarily result in shedding pounds is as disturbing to health professionals - who have spent years persuading us that it will - as it is to anyone who has been sweating it out in the gym to no avail. "We've always made the supposition that because there is a clear association between physical activity and fatness, the lack of physical activity is the cause of the fatness. But it could equally be the other way round," says Terence Wilkin, professor of endocrinology and metabolism at the Peninsula medical school in Devon. Wilkin is leader of the ongoing EarlyBird study, which is monitoring the metabolism, activity levels and body composition of 300 five-year-olds as they mature to 16. So far he has seen no evidence that activity levels of 12-year-olds have a bearing on their body mass index or risk of obesity. "Our findings tend to support the notion that obesity in the child comes from somewhere other than their physical activity level," he says.

Wilkin believes that results from the EarlyBird study, along with research restricting and enforcing activity on rodents, point to the existence of a natural, pre-determined "set point" that controls how active each of us will be. "It doesn't seem to be opportunity that dictates how active a child is, but a setting that lies deep in a part of the mid-brain called the hypothalamus. We have nicknamed it the 'activitystat'."

This activitystat, like a thermostat, will adjust your energy expenditure down when it thinks you have been too active, and up when it thinks you haven't been active enough. "Try as you might to override it, it will revert to type at the earliest opportunity," says Wilkin.

So what determines how high your activitystat is set? Bouchard's findings indicate that this is yet another attribute you can pin on your parents (or thank them for). When he put 10 pairs of sedentary identical twins on a controlled exercise programme for 20 weeks he found a huge variability in responsiveness from pair to pair, but very little difference within pairs. Both twins, in each fitness marker, would be low-, high-, or non-responders. And it's not just trainability that appears to run in the family. "Our findings suggest very strongly that the inheritability of responsiveness to exercise is at least as great for body weight, blood pressure and cholesterol levels," says Bouchard. "We found 2.5 times more variance between families than within families."

None of this, however, is an excuse to throw in the towel. "While you may not be losing weight or markedly improving your fitness," says Bouchard, "you may be reducing your blood pressure or improving your cholesterol level."

"The level of fitness to which you can aspire will be influenced by your genetics, but you do have some control over it," says Buckley. "It's often said that top athletes are 'born', not 'made' - but they have to realise their genetic potential through training hard and staying motivated." And that goes for us lesser mortals, too.

Are you a non-responder?

If you don't seem to be reaping the benefits of your workouts, ask yourself ...

· Have I been training regularly - at least three days a week - and consistently?

· Am I working hard enough? "Sometimes people just aren't reaching at least a moderate intensity to gain benefits," says Bouchard.

· Am I progressively increasing my training load to ensure I keep making progress?

· Have I consulted a professional to ensure my programme is balanced, challenging and effective?

· Am I taking care not to overtrain?

If the answer to all these questions is yes, you may be less responsive to exercise. But don't give up - you get a lot more than a fitter heart and a lower resting heart rate from exercise. Monitor as many variables as possible as you continue with your exercise programme - from your mood to energy levels, your blood cholesterol to body fat percentage, muscle tone to resting heart rate. Focus on other positives such as staving off back pain and reducing stress.