A workmate purchased a nice MTB a few months ago and goes for semi-regular rides. He is quite unfit, hasn't done any exercise previous to buying the bike (aged mid 30s) and is overweight (about 14 stone 5.5ft tall) and until recently was a pack a day smoker! He has given up smokes now he reckons though he still has the occasional one. But he has (IMO) a shocking junk food habit and doesn't seem to understand that this effects his cycling ability not to mention his general health. He'll have a bunch of fried food including hamburgers, chips, KFC, Maccas etc for morning tea (often skips breakfast) and more of the same junk for lunch and probably dinner as well. I'm not a nutrition expert but I like to think I have a pretty reasonable idea what is good for you and what is bad, especially as it relates to cycling performance (I ride/race road bikes).
He has a bit of a thing for high performance cars too (as do I) so I tried to compare his junk food habit to crap fuel in a finely tuned race car. Say the race car is tuned for high octane C16 or Avgas fuel and you put crappy ordinary unleaded fuel in it it will at best run **** and at worst damage the engine (melt pistons etc).
Also if you keep putting fuel in and you haven't used up all the fuel that's already in the tank then eventually it will swell up and break (OK that's not exactly true but you get my point...). Like the human body. If you keep feeding it 'fuel' that isn't being used it converts to fat.
He still doesn't get it (or doesn't want to get it maybe...). He saw a $8000 CF MTB in the shop the other week and figures if he buys this he will be able to fly up hills and be king of the trails. I tried to tell him it doesn't work this way, he may see a very slight improvement in performance but he needs more training and better nutrition way more than he needs an $8000 bike (his current bike cost $1600). He ignored my advice and is paying the new bike off and has seriously told me he doesn't intend to ride it off road incase he damages it so it will be for the road only!
Aaaaarrrrrgggggggghhhh! He drives me nuts!