Q I have run several marathons and after every one I have had an unpleasant taste in my mouth for about a week afterwards. What’s going on?
A The way our bodies use energy during a marathon might explain this unpleasant taste. Carbohydrate, in the form of blood glucose, is your body’s first choice of fuel, but when carbohydrates are dramatically reduced, sufficient glucose is no longer available to meet your body’s energy needs. At this point you begin to burn fat for energy, turning it into a source of fuel called ketones.
When your body burns fat for fuel, it goes into a temporary form of ketosis, which might cause the unpleasant taste in your mouth. Dietary ketosis is a normal, beneficial part of human metabolism where excess ketones are discarded in urine, but this temporary form of ketosis that you experience during an endurance event may create an excess of ketones that produce the unpleasant taste. Drinking plenty of water will help to dilute the concentration of ketones and eating a healthy diet will encourage your body to use carbohydrate again for fuel. In the short term chewing fresh parsley will freshen your mouth.
An unpleasant taste can also be triggered by dehydration. Having a dry mouth causes oxygen deprivation in the saliva, and increases the number of anaerobic bacteria that inhabit the tongue, so you may lose the good bacteria that help prevent bad breath.
If your body is in a state of dehydration after running a marathon, the saliva that helps to cleanse the teeth of bacteria and keeps the tongue hydrated will be significantly reduced. This lack of saliva dries the mouth and prevents bacteria from being washed away, which can cause halitosis. Ensuring that you rehydrate after the marathon should prevent this unpleasant taste.
— Lindsay Curtis, nutritionist and member of the British Association for Nutritional Therapy