You could check https://en.m.wikipedia.org/wiki/List_of_vegans and https://en.m.wikipedia.org/wiki/List_of_vegetarians

I see Brian Greene (theoretical physicist), Douglas Hofstadter (cognitive scientist and physicist), George Church (geneticist) and Christine Korsgaard (philosopher) on the list of vegans, although you could check if they were producing good work while vegan. I haven't checked the list of vegetarians, but there are probably plenty of famous examples there. Edward Witten (Fields Medalist mathematical/theoretical physicist) is/was vegetarian.

There are also plenty of famous vegan artists and entrepreneurs, and their work often takes originality or novelty.

I'd be pretty cautious about putting much weight on those experiences of yourself or your friend that you've mentioned. Doctors see people everyday who swear that crystals cured their arthritis or that medication A works for them but not medication B (when B is just A with a different brand name). I've learnt to become very skeptical of the patterns I recognise in my own health. I've had experiences where I could have sworn that A was causing B that later turned out to be wrong.

As it is I don't see why your prior would be "vegan diets make thinking worse" rather than "vegan diets don't affect thinking" (my own suspicion) or even "vegan diets make thinking better" (I'm sure someone out there has their own anecdotes supporting this)

Agree with other commenters that we shouldn’t put too much weight on anecdotes, but just to provide a counter-anecdote to yours I’ve been ~99% vegan for over 3 years and it seems like my thinking ability and intellectual output has if anything improved during that time.

My best guess is that it varies based on the person and situation, but for the majority of people (including probably me) a decently planned vegan diet has ~no effect on thinking ability.

Having been on a "close to vegan" diet for many years with quite catastrophic results I would like to point out a couple of subtleties in this sphere. 

The deficiencies associated with any diet can take years to appear. For example in a zero B12 diet it can take 6 years for a deficiency to appear. During this time the body is in effect "eating itself" i.e. sunstituting one form of animal food (oneself) for food eaten.

Often, as in my case, the initial switch in diet was associated with an improved sense of well-being due to not eating junk food any more.  This is sometimes referred to as the vegan honeymoon.

There is often a large gap between the level of supply of nutrients that will avert frank deficiency and an optimal supply. But it is often assumed for example that if you don't have scurvy that you have plenty of vitamin C.

I learned the  hard way that much of the scientific literature is worthless or worse (anti-knowledge). In particular you cannot learn anything reliable from reading only abstracts. This may be obvious to some but is not obvious to many. Financial and ideological biases loom large and a lot of research is only published because of the "publish or perish" mandate. Purely observational studies are almost all useless, and "corrections for confounders" are not reliable. 

 

No idea how to go about finding information on this, but by my personal priors I would weight various kinds of evidence as follows:

• ~0 weight on any anecdotal evidence,
• low weight on studies and clinical trials,
• moderate weight on arguments by biological plausibility.

Being related to diet, my prior is that people are usually over thinking it. However I have always agreed that it seems unlikely that a fully vegan diet has no nutritional downsides without supplementation.

I've done a cursory search, just wikipedia, here are my thoughts on the biological plausibility of deficiencies among vegans in your two suggestions, creatine and choline:

• creatine
  • is only found in animal tissue
  • is absorbed in digestion
  • is also metabolized from amino acids found in plants
  • ->
  • seems like it could plausibly be something that vegans are deficient in
• choline
  • is an essential dietary component, not being metabolized sufficiently in enough quantity
  • deficiency is rare in humans
  • is found in the same quantity per gram in wheat as in chicken
  • ->
  • seems unlikely that it is something that vegans are deficient in

Is your standard "genius-level", or even "genius-level technical work"? Or being highly productive in intellectual work? I think plenty of philosophers and EAs who have done good research, including at prominent EA orgs, have been vegan or at least vegetarian and will probably meet the last standard.

Some who (I think) have been veg (not sure if vegan specifically or if they're still veg): Peter Singer (~vegan), Will MacAskill, Brian Tomasik (lacto-veg), multiple research staff at Rethink Priorities (where I work) even outside animal welfare, Sam Bankman-Fried (vegan), Rob Wiblin and Howie Lempel (and others?) at 80,000 Hours, I'd guess some research staff at Open Phil and not just those focused on animal welfare. Vegetarianism and veganism are very common in EA (https://rethinkpriorities.org/publications/eas2019-community-demographics-characteristics), so we shouldn't be surprised to find good examples (but not necessarily geniuses), and if we didn't, that could be a bad sign.

SBF is vegan, and while his day job isn't "intellectual" in the conventional sense, it takes a lot of intellect