Iron

• Iron is necessary for the production of hemoglobin, which transports oxygen from the lungs to the rest of the body. Even a marginal iron deficiency will leave you feeling lethargic. Women---especially female athletes---are at greater risk for iron deficiency, the most common deficiency in the United States.
  
  

Magnesium

• Muscles need magnesium to properly contract, making adequate intake of this nutrient crucial for muscle growth and for developing power. Magnesium also helps the body absorb calcium, which you need to maintain strong bones.
  
  

Glutamine

• The amino acid glutamine helps pull water and glycogen into muscle cells. It also helps ensure that amino acids don't abandon muscle growth for some other physiological task, according to "Muscle & Fitness."
  
  

Potassium

• Intense physical activity and excessive fluid loss, from sweating for example, can deplete potassium levels. Your body needs potassium to maintain fluid balance and to convert carbohydrates and protein into energy the body can use.
  
  

Vitamin D

• Vitamin D is necessary for muscle function and for building strong bones---but Americans are increasingly deficient in this key nutrient. Symptoms of deficiency include lethargy and depression.