How your doctor knows: Your GP will ask about what you eat, whether you're getting enough sleep, and the medications you take. If you don't eat many (or any) meat or dairy foods or take supplements containing B12, you sleep 7 to 8 hours each night, and you're physically active, odds are good that your low energy is due to a B12 deficiency.
The food fix
Have two servings of nonfat dairy foods, such as fat-free milk or nonfat yogurt, and 3 to 4 ounces of lean protein daily. Good sources of B12 include seafood such as fish, clams, oysters, and mussels, as well as lean beef and pork, chicken, and fortified cereal.
Supplement solution: Take 500 to 1,000 mcg of vitamin B12 in tablet form every day to raise and maintain your B12 levels.