The current level of folic acid fortification of grain products leads to an intake of about 100 extra micrograms of folic acid per day. why doesn't the fortification program mandate higher levels of folic acid to more effectively meet requirements?

Folic acid is part of vitamin B complex. Lipid-soluble vitamins like A, D, E, K will be stored in the fat cells. But vitamin B and C are water-soluble vitamins that could not be stored. Only a number of the vitamin is used and the excess will be secreted, so there is no reason to have higher consumption. 

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