Since our bodies are said to be about 75% water, this evening I wondered how much I lost through sweat on an average run. The results were a little surprising.

I weighed myself before I set off: 13 stone 2.2 pounds (I’ve put a couple of pounds back on recently). I then ran my usual Hampstead Heath Extension circuit (43.11 - not too bad). Then I got home, stretched and jumped on the scales: 12 stone 12.8 pounds.

That’s almost three and a half pounds lost through sweat in less than three quarters of an hour. I know it was hot this evening, but wow, do I sweat. I think I need to re-run the experiment. It probably explains why I often get headaches later in the day, no matter how much I think I’ve re-hydrated.

Based on being made up of 75% water, this also means that if I carried on running without drinking I’d completely melt into a pool of skin and bone in around 30 hours. I am the incredible shrinking man.