Every January many tens of thousands of people make the decision to lose weight and get fit. This might simply be because they feel they have added a little excess weight during the Christmas period or for some other long term goal, like running the London Marathon.

So they visit their local health and sign up for a gym membership to use the treadmills or they put their trainers on and head out the door for an outdoor run.

But, research shows that running can actually make you fat. Surely not I hear you cry. Well, I'm afraid it just might.

Firstly, let me stress that running is an excellent, if not the best way, to improve the cardiovascular system and increase fitness. Running is a high impact activity which stresses the body and forces it to adapt by becoming more efficient at transporting oxygen around the body - allowing you to train harder and longer over time, which is the true mark of fitness.

However, running also increases the level of the hormone cortisol in the body. Cortisol is a  hormone that has been shown to increase the ageing process as well as distribute excess fat around the waist. This means that too much treadmill or outdoor running can inflate the levels of cortisol in body, making it counter-productive, perhaps, when it comes to weight loss.

Well, let me clarify this point a little. Weight loss may occur. However, this weight loss may be in the form of muscle mass as cortisol can push the body into a catabolic state where it uses muscle mass as fuel rather than body fat. This then means the metabolism is reduced, hence the body's ability to burn calories reduces - leading to increases of body fat levels.

When it comes to weight loss what all gym users and exercisers are looking for is a reduction in body fat levels rather than actually body weight; though with reductions in fat levels often comes weight loss.

So, many leading personal trainers, including personal trainer to the stars James Duigan and Steve Bond, who between them have trained the likes of Foo Fighters, Pink, Elle Macpherson and Hugh Grant, suggest using high intensity interval training when it comes to running. This means inter-changing sprints at full speed with bouts of jogging. This will increase muscle tone in the legs, increase fitness quicker and allow the body to burn more calories, leading to lower levels of body fat, especially around the waist.