"Fatigue is a classic sign of B12 deficiency, which usually occurs in people who don't eat very much animal protein," says Danine Fruge, MD, associate medical director of the Pritikin Longevity Center & Spa in Miami. Chewing a lot of antacids to relieve heartburn can also lead to B12 deficiency, because antacids interfere with B12 absorption.

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.