Metformin has three effects:
- makes muscles burn their reserves of glycogen
- depresses uptake of carbs from the gut
- depresses release of reserves from the liver
This is great if you're not exercising, because it mimics to some extent the effect of exercise. But it's not what you need for cycling, in fact it's the opposite. If I ride after taking the stuff, and in particular just after breakfast, my legs get sore faster and I lack oomph. No fun there.
In addition, since carb uptake is depressed, more sugars and starches remain in the intestine and are passed on down to Skid Row, where the symbiotic bacteria have a party. This results in the release of much more gas than usual, and in copious volumes of aerated faeces. Taking extra carbs while riding so as to overcome the uptake depression enhances the effect.
Because it builds up in the body over time, my doc told me to drop the metformin a couple of days before long-distance events, but for normal rides I usually just skip it on the day.
