If magazines and health clubs are to be believed, the new year should coincide with a whole new you. Many gyms see up to a 50% increase in memberships in January and, in some places, up to two thirds of these memberships are cancelled by the end of the month. For those of us wanting to shed more than just a few pounds gained over the over-indulging season that is Christmas, we often think that sweating it out on a treadmill is the only way to go. 

Dr. Herman Pontzer of the City University of New York has shown that increasing exercise alone is not the best policy if you want to lose weight and keep it off. The general consensus is that by increasing your daily physical activity you will increase your energy output, which will result in weight loss. In short, if you put out more energy than you're taking in you'll lose weight. Many people believe that they can continue to eat what they usually would, but so long as they increase their physical activity, they should - in theory - lose weight.

Dr. Pontzer and colleagues have shown that this is not the case. In their study they looked at over 300 adults from five different geographical locations. The study looked at increasing physical activity across the whole group and assessing energy expenditure. The results of the study support the Constrained Energy Expenditure model.

There are two proposed models for energy expenditure, the additive and the constrained models. The additive model suggests that the more physical activity you do, the more energy you use whilst your body maintains a stable level of energy to be used for your body's normal physiological functions. 

The constrained model contrasts this by arguing that the body adapts to increased energy usage by reducing energy spent on other normal physiological functions. This means that instead of creating a steady, linear increase in energy output, the energy usage actually plateaus as the body becomes accustomed to the increase in activity.

"The body adapts to increased activity, so that the calories you burn each day don't really reflect your activity level," says Pontzer. "Our analyses compared different people with different activity increases, not changes in activity within a person's daily routine."

Future studies in this area will focus on looking at increases in activity per person and tracking individual energy expenditure over time.

Whilst there is no doubt of the health benefits that physical activity and exercise has on the body, it would appear that physical activity alone cannot sustain weight loss. For those of us hoping to start the new year a little lighter, this study highlights that reducing calorie intake-eating less- is the more productive way to go.

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