Thanks for your question!

While all B vitamins have their own unique functions in the body, B vitamins in general act as a coenzyme. Thiamin, Riboflavin, Niacin, Folate, Biotin, Pantothenic acid, B6, and B12 all act as part of coenzymes - which are small molecules that combine with an enzyme and activate it. Without a coenzyme, the enzyme is not able to do its job. Enzymes in the body help to affect absorption of key nutrients (including protein, fats, and carbohydrates), metabolism, and excretion of other nutrients. 

The big mistake people make is assuming B vitamins help supply the body with energy - which they do not, really. B vitamins help metabolize the energy-yielding nutrients such as protein, carbohydrates, and fats to provide fuel for energy to the body. Certain B vitamins help release energy to be used, while others help synthesize nutrients (such as protein) to produce energy, build/rebuild tissues, make hormones, and fight infections. In these ways, B vitamins indirect help absorb and deliver energy throughout the body. Also, cells that have a rapid turnover rate (meaning they do not live very long and need to be regenerated) utilize B vitamins to help in multiplication. This is especially useful for the red blood cells and the cells of the digestive tract. 

When present, a normal person should not be able to assess their B vitamin levels based on how they feel. However, when deficient, their absence is well-known. Because every cell is affected, symptoms associated with B vitamin deficiency include nausea, severe exhaustion, irritability, depression, forgetfulness, loss of appetite and weight, muscle pain, impaired immune response, swollen red tongue, loss of control of limbs, abnormal heart rhythm, skim problems, and cracked skin at the corners of the mouth (just to name a few). To prevent deficiencies of any one B vitamin, consume a diet rich in protein, dairy, enriched grains, legumes, fruits, and vegetables. Keep in mind, just because you may experience these symptoms, it does not necessarily mean you are deficient in any one B vitamin. Only blood laboratory results will indicate a deficiency. 

I hope this information helps. Happy Eating!