Will a person be held accountable for taking a nutritional supplement that causes excessive flatulence, leading to an inability to maintain ablution, knowing that it is not essential?

Frequent gas is not a sin, and you are not required to maintain ablution except for performing prayer. If you can perform ablution correctly, there is no sin upon you. You are not obligated to stop taking the dietary supplement unless it prevents you from achieving purity and performing prayer. In that case, you would be considered like one with incontinence (sahib al-salas), and you would perform ablution after the time for prayer begins and then pray the obligatory prayers and voluntary ones. It is forbidden for you to consume the supplement if your intention is to nullify your purity, just as it is forbidden to travel with the intention of breaking the fast. However, if it is for your need, there is no sin upon you, and you should perform ablution after the prayer time begins.