问一个B12的问题，请解惑

我刚买了5000mcg的液体B12，查了一下网说daily use建议2.4mcg, 怎么会差那么多？ 市场上也没有那么低剂量的B12啊？ 最低的事500mcg。 

Recommended dietary amounts (RDAs) are 2.4 micrograms daily for ages 14 years and older, 2.6 micrograms daily for pregnant females, and 2.8 micrograms daily for breastfeeding females. Those over 50 years of age should meet the RDA by eating foods reinforced with B12 or by taking a vitamin B12 supplement.