So this was supposed to be the original post but somehow Vitamin D took over my train of thought.... Anyway... sugar.. since this is training oriented I'll start there.
1)If you are sick more you can't train as effectively... and being sick is depressing.... and not fun... and actually don't train when you are sick, unless it is super chill and your are not that sick.
2) High sugar diet=More sick
There, now that you are convinced I will delve into better evidence on why to ease up on the sugar...
Remember Vitamin D and how it helps out your innate immunity? Sugar does the opposite... In the previous post I described how cells of the adaptive immune system can specifically recognized a specific pathogen. The innate immune system can't do that... It's why you get vaccinated so when some boss pathogen that you were vaccinated against busts past your innate immune system the adaptive is there to exploit its weaknesses since it "knows" it (has a receptor that will bind specifically to it). The innate does however have a general array of receptors that recognize "PAMPs" or Pathogen-Associated Molecular Patterns. PAMPs are general proteins found on the surface of many types of pathogens. So while not very specific and so not quite as effective as killing the bad guys, innate immune cells can still stop an invading virus or bacteria in it's tracks by binding to a PAMP.
One such receptor that binds to a PAMP is the Mannose-binding lectin (MBL). In the linked study(http://file.scirp.org/Html/7716.html), it was found that dietary sugar down-regalutes this MBL/PAMP invader halting mechanism!!!!
In another study... (http://onlinelibrary.wiley.com/doi/10.1002/ptr.2509/abstract), it was found that sugars can even inhibit the method by which macrophages (part of innate immunity) kill an engulfed pathogen. They can't form a reactive oxygen species so pretty much your internal lazer gun is out of batteries (don't take my word for that and instead look at this graphic...http://bloodjournal.hematologylibrary.org/content/92/9/3007)
So you get the idea.. I'm sure there are numerous other articles that look into that. It also doesn't mean cut out sugar totally though because without it your brain wouldn't work and yes your immune system wouldn't work. How much then? I don't know, I'll leave that to your own research and good judgement, aka processed food prob not the best).