We would need to find out if anybody who does not have keratoconus has ever eaten cake, but even if it was found that they had, all we would have is evidence that eating cake and chocolate was not a sufficient condition for KC.
then we would need some people with KC to volunteer to go without cake or chocolate to see if their condition improved. I should like to volunteer to be part of the control group: people with KC who eat choc and cake to compare withthose who abstain.
We might just get written up in a future book endorsed by the BMA

Andrew