Vit D supplementation can be useful for some people but only really needs to be a relatively blunt instrument in terms of dosage.
Thiamine is important especially for people who drink alcohol, but it is widely supplemented in food.
Im interested in the subject and supportive of any effort to improve peoples health before problems occur, but in a market which is 99% bullshit, misinformation and basically outright fraud, the bar is pretty high to claim to be legit and not be associated with all the scammers. Unfortunately the publicly available information and this post about this company don't even come close.
Good luck but you will need to significantly up your game to come across as legitimate and actually useful.
This is the first time in a while ive questioned the selction of a YC company.