. . "Originally, they were all lumped together as vitamin B, but later on it was realized that there were many different forms of the vitamin, which is why there are different names and numbers for vitamin B. There are actually 8 vitamins in the group, but the most important ones in terms of tiredness and fatigue are: VitaminNameDeficiency causesB1ThiamineFatigue, memory loss, weakness, depression, cog" . .