常感疲劳可能是维生素B12缺乏的表现之一，因为维生素B12是人体制造红细胞、产生能量和合成脱氧核糖核酸所必须的营养物质。据称在美国约有1/6的成人缺乏维生素B12（Your body needs vitamin B12 to make red blood cells, produce energy and synthesize DNA. Marginal deficiency is quite common, affecting about 1 in 6 adults in the US） 。

即使是轻度的维生素B12缺乏亦可引起轻度贫血的症状，诸如疲乏、气短、牙龈出血、注意力难集中，皮肤疮白和腹泻或便秘等（Even marginal deficiency can cause early symptoms of mild anemia: Fatigue, lack of energy or lightheadedness， Shortness of breath mostly during exercise，Swollen, red tongue or bleeding gums，Problems concentrating，Pale skin，Diarrhea or constipation）。

慢性维生素B12缺乏可损害神经功能（Chronic B12 deficiency is a serious disorder that can lead to nerve damage. Symptoms can include memory loss, numbness and tingling in the hands and feet, confusion and disorientation, loss of balance, depression and heart disease.）

通常年龄增大、膳食不当和基因变异是导致维生素B12缺乏的主要原因，如50岁以上成人、素食为主者和存在某些基因变异的个体易患维生素B12缺乏而在日常生活中需要增补（Age, dietary factors and genetics can increase risk for vitamin B12 deficiency. The best sources of B12 in the diet are seafood and animal products. People who are older than 50, vegetarians and vegans and those with genetic risk variants can easily become vitamin B12 deficient. For these people, supplementation may be recommended.）。

The TCN2 gene helps transport vitamin B12 from the blood into your cells. A risk variant of this gene, carried by about 77% of Africans, 57% of Caucasians and 46% of Asians, is associated with low serum B12 and hyperhomocysteinemia (a risk factor for cardiovascular disease). Genetic variants in CUNA and FUT2 genes affect the absorption of vitamin B12 and can also lead to deficiency.