It may sound too good to be true but scientists claim they have created a pill that will make you fit without having to exercise.

Researchers at Harvard Medical School have discovered a hormone found naturally in the muscles that triggers the calorie-burning benefits of exercise and believe it could be used as a weapon in the battle against obesity.

Levels of the hormone, irisin, increase in the body during exercise, boosting energy expenditure as well as controlling blood glucose levels.

The Harvard scientists believe the hormone could lead to treatments for obesity as well as diseases such as diabetes and cancer. They also said the drug would benefit patients who could be treated with exercise therapy but are too weak.

However, doctors have warned that this will not be the cue to cancel your gym membership as although the hormone can aid weight loss is does not improve muscle strength.

Irisin triggers the production of ‘good fat’ or ‘brown fat’, which burns off weight and improves glucose tolerance. This type of fat naturally disappears as we age and is replaced by ‘bad fat’ or ‘white fat’, which typically collects around the waist.

Experiments found irisin helps to convert ‘white fat’ to ‘brown fat’, thus increasing energy expenditure and improving resistance to obesity-linked diabetes.

The study, published online in the journal, Nature, found that irisin has “powerful effects” on adipose tissue – deposits of white fat that store excess calories.

When obese mice were injected with a modest amount of irisin over a short period, their glucose control improved and a small amount of weight was lost.

Lead researcher Professor Bruce Spiegelman says the weight loss results could improve with more extensive irisin treatments.

He said: "Whether longer treatments with irisin and/or higher doses would cause more weight loss remains to be determined.”

To compare the benefits of irisin with exercise, the drug was injected into sedentary mice that were obese and pre-diabetic.

Within 10 days of treatments the mice lost a small amount of weight and had improved control of blood sugar and insulin levels, which could prevent the onset of diabetes.

It is hoped the findings will help in understanding the biological mechanisms that convert physical exercise into beneficial changes in the body. However, Professor Spiegelman stresses that irisin supplements would not act as a 'miracle pill' to replace regular exercise.

“We’re not trying to replace diet and exercise. That is still important,” he added.

Because irisin is a natural substance and the mouse and human forms of the substance are identical, Professor Spiegelman believes it should be possible to begin clinical testing within two years, the Telegraph reports.

Richard Scrivenor, health, fitness and nutrition lecturer at Premier Training International, does not believe irisin is necessary in the fight against obesity.

He told The Huffington Post: "White fat or adipose did not just appear from nowhere one day in your life; it accumulated as a result of your internal physiology telling it to via hormone messengers promoting fat storage."

He added: "If we manipulate our physiology all day every day, promoting optimal internal health by eating, resting, sleeping and exercising, there would be no need to be considering irisin."

While a pill may one day offer help with shedding pounds, exercise offers a multitude of additional benefits, both emotionally and physically. See our gallery of the health benefits of running - more satisfying than swallowing a pill.

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• Health Benefits Of Running

• Helps You Sleep

  Having trouble nodding off? It stands to reason the more energy you expend during the day the more likely you are to be able to fall asleep easily without feeling restless. Beware of exercising too close to bedtime though as this can leave you feeling over-stimulated and wide awake.

• Aids Concentration

  Running boosts blood flow and oxygen to the brain. It also promotes the growth of new brain cells. Studies suggest the effects are strongest in the hippocampus, the region of the brain associated with learning and memory.

• Good For Your Bones

  As a weight-bearing exercise, running helps to build strength in your bones and can reduce the risk of bone disease, osteoporosis.

• Anti-Ageing

  Running makes the skin appear plumper and smoother by boosting the production of collagen in the skin cells and promotes a clearer complexion by transporting nutrients and flushing out waste. By stimulating circulation and drawing blood to the surface of the skin it promotes a rosy glow while toning the muscles in the face to counteract sagging.

• Makes You Happy

  Forget reaching for the chocolate to cheer yourself up. Instead, get your running shoes on. A natural anti-depressant, running stimulates the brain's pituitary gland to release endorphins, the body's natural feel-good chemicals. Plus you won't get any of those post-chocolate guilt pangs - just a feeling of achievement.

• Reduces Stress

  As well as boosting your mood, running increases your ability to cope with everyday stress. Go for a run in the morning before work and you'll be in a much better frame of mind to cope with the day's ups and downs.

• Burns Fat

  Running changes the speed with which your body is able to burn fat by altering your metabolism. The average person burns approximately 300 calories per half hour of 10-minute-mile running, making it one of the best energy and fat burning exercises there is.

• Improves Muscle Tone

  Not only will running help you lose inches, it will help you convert excess fat to muscle, leaving you with a leaner and more defined silhouette - including the upper body as well as your calves, thighs and bottom.

• Boosts Immune System

  A 2010 study of 1,000 people, published in the British Journal of Sports Medicine, found that regular exercise nearly halved the chances of catching cold viruses, or at the least, made the infection less severe.