Jan's big weakness is chocolate. Well, he is german. And they all gain weight off season, jan's not the only one. And it's not like germans are known world-wide for being dainty pickers and light eaters. I think some are just better at self-denial.
Eddy Merckx said that he also had a tendency to pork up in the winter, so he would do six-day races. ALso for the money,of course. Maybe jan should do something like that and be more active in the winter.
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You can catch more flies with honey than with vinegar. That's great...if you want to attract vermin.