You have meticulously drawn up a weekly training plan in the lead-up to your race, diligently following it and clocking the mileage.

The weekend's long run is coming as you near the end of the work week. Alas, your plans are blighted when you wake up with your throat feeling like sandpaper and your nose leaky as a tap.

Undeterred, you carry on with your scheduled run, dismissing your symptoms as minor.

Should one continue exercising when one is ill?

THE NECK CHECK

A neck check is a quick way to determine if you should continue to train when unwell.

If your symptoms are above the neck, for example teary eyes, nasal congestion, runny nose, sore throat or a mild cough, then it is probably okay to continue.

However, if your symptoms occur below the neck, for example fever, chills, body aches, malaise, chest congestion, nausea/vomiting or diarrhoea, then I would strongly encourage you to give your body a much-needed rest.

RUNNING WITH A COLD

Exercise may be beneficial when one is suffering from an upper respiratory tract infection, also known as the common cold.

Symptoms include a runny or congested nose. Adrenaline, a hormone released during exercise, is a natural decongestant and helps in relieving nasal congestion as well as the widening of our airways.

Research suggests that heart and lung functions (and hence exercise tolerance) do not appear to be altered by an upper respiratory tract infection. This means that the common cold will not affect your ability to run at your usual intensity.

If running in a group, do be mindful of sneezing or coughing in close proximity to others.

RUNNING A FEVER? DO NOT RUN

Exercising with a fever is dangerous. Exercise further raises one's body temperature and heart rate, which are likely already elevated.

Heart rate increases by about 10 beats per minute for every 1-deg C rise. Running while febrile can result in excessively fast rates.

Viruses are a common cause of fever and side effects may include inflammation of heart muscle. This inflammation, coupled with a fast heart rate, presents much more stress and strain to the heart than the intensity of exercise would suggest. Ultimately, this can precipitate abnormal heart rhythms and, in severe cases, even result in sudden cardiac arrest and/or death.

PROTECTING IMMUNE SYSTEM

It has been well-established that regular exercise can boost one's immunity. On the other hand, too much exercise can have the opposite effect.

Prolonged high-intensity endurance exercise (for example, running a half or full marathon) can cause one's immunity to be weakened for up to 72 hours. The cause is not clear but one plausible reason could be the production of excessive free radicals and stress hormones which can suppress one's immune system.

If you find yourself frequently falling ill on the days after intense training, it may be helpful to take in more foods that are rich in antioxidants, such as green tea, dark chocolate, blueberries, strawberries and beetroot as part of your recovery diet, to give your immune system a boost. Sufficient rest and sleep are also needed.

Depending on how fatigued you feel, it is prudent to always listen to your body and schedule a rest day or two after a session of hard running.

•Dr Wang Mingchang is a sports medicine associate consultant with the NUH Sports Centre. He has completed 10 full marathons.