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The deficiency of this vitamin puts the human body through a range of diseases including drastic weight loss extreme fatigue and loss of concentration. We are talking about vitamin B12 or cobalamin, the vitamin that plays a key role in DNA synthesis and several other major functions of the body. A deficiency in vitamin B12 can have widespread effects on health, impacting both physical and mental well-being. A human being needs to consume 2.4 micrograms (mcg) of vitamin B12 a day.