If you're low on energy, you just might be low on Vitamin B12 as well. Studies have shown that as much as 25 percent of the U.S. population has a deficiency in this important vitamin.
Vegetarians and vegans may well be the hardest hit since Vitamin B12 can only be found in animal sources. It's important to take a supplement if meat, eggs, or dairy products are not a regular part of your diet.
As people get older, their bodies are less able to absorb B12, and they are more likely to develop a deficiency.
"The older you become the more your digestive system breaks down, especially if you have been eating the standard American diet. The use of antacids as well as anti-ulcer medications will also lower your stomach acid secretion and reduce your ability to absorb vitamin B 12. An infection with Helicobactor pylori, a common contributor to stomach ulcers, can also result in vitamin B12 deficiency."
http://www.wholefoodandmore.net/2010/03/vitamin-b12-for-more-than-energy.html