10 Ways Your Body Is Telling You You’re Running Low on Key Vitamins

“ Fatigue” Possible deficiency: Vitamin D Being tired all the time, despite getting adequate sleep, is a red flag that your D is chronically low. If your doctor suggests you'd be a good candidate for a vitamin D supplement, studies show that they may be able to improve fatigue symptoms. (Though your doctor would advise you to take a much smaller dose daily.) 'After people get more vitamin D, it's amazing to hear them say they have so much more energy,' says Small.