Iron deficiency is one of the most common nutritional deficiencies, causing mainly fatigue and poor blood health and brain function. Iron helps transport oxygen through the body and it is essential component of hemoglobin, which makes up red blood cells and participates in this oxygen transport. That makes adequate levels of iron directly linked to oxygen levels in our body, muscle activity and concentration. Without enough iron, a lack of healthy red blood cells might occur, a condition knows as anemia. Iron is further important for healthy skin and hair and boosting overall immunity. Best sources of iron are beef and pork liver, eggs, seafood, legumes, pumpkin seeds, nuts and spinach.