The B vitamins are crucial in normal metabolism, which involves all the chemical and physical processes in the body that use or convert energy, from breathing to digesting your food. Fatigue is a common symptom of B12 deficiency, which causes anemia. Too little iron can also cause fatigue and anemia. If you are deficient in either of these nutrients, taking them in supplements may help you feel more energized.
Fatigue is not among the most common symptoms of vitamin D deficiency, which include thinning of bones. A lack of the vitamin can cause muscle weakness, though, and supplements can help, particularly in older people at risk of falls.
You should be able to meet your nutritional needs and feel energetic by eating a well-balanced diet. But if you think you may be deficient in vitamins B or D, iron or any nutrient, talk to your doctor about whether you need to take supplements.