If you're lacking these key nutrients, you may feel tired, dizzy, and more.Credit: ilona titova / Getty ImagesKey TakeawaysVitamin D is one of the hardest nutrients to get enough of from food and sunlight alone, so it’s smart to ask your healthcare provider about testing if you’re concerned.Magnesium and iron are also common shortfalls, and you can often boost both by eating more leafy greens, beans, lentils, nuts, seeds, whole grains, seafood, …