A study performed in Germany and the US in 2 groups of young men, where one group took vitamin C and vitamin E during 4 weeks of intensive exercise and the other took nothing. Has shown that those who didn't take them had more oxidative stress but improved insulin sensitivity and blood sugar control.
The researchers concluded that exercise causes an adaptive response and thus improves diabetic control but the supplements undid this benefit.
There have been many studies in the last few years showing that supplements are not that beneficial and its better to obtain our nutrients from whole foods.
So eat a tasty balanced diet have fun try new recipes and there's really no need for pills, in fact if you have diabetes and are exercising maybe you need to re-think taking them