Iron is essential because it is a key part of hemoglobin which our red blood cells use to carry oxygen. People deficient in iron can be fatigued. They can have brain fog, little energy, and depression. Most of us get too little iron, but vegetarians and vegans are especially at risk. Red meat, some fish, and dried beans are some of the best sources of iron, but you can also get iron from dark green, leafy vegetables, like spinach.