Calcium
What it Does: Besides building strong bones and teeth, calcium aids in muscle contraction and proper blood clotting. This is a big one for runners, as calcium helps prevent stress fractures.
Should You Be Worried? Probably. Menopausal women and those who can't maintain regular menstrual cycles are often calcium deficient. If you are not getting enough calcium from your diet, your body steals from your bones, and this can lead to serious injury.
Where to Find It: Milk, yogurt and cheese are calcium jackpots. If you don't do dairy, fortified orange juice, canned salmon with bones and dark, leafy greens are good choices.
Vitamin B12
What it Does: B12 maintains red blood cells, while keeping nerve fibers healthy.
Should You Be Worried? Maybe. No active form of vitamin B12 is found naturally in plant foods, so strict vegans may develop deficiencies.
Where to Find It: Animal products, dairy and eggs. Vegan options include fortified sources, such as some soy milks.
Iron
What it Does: Iron is essential for forming the oxygen-carrying compounds in blood and muscle. Without enough iron to produce red blood cells, anemia results, leading to fatigue, poor recovery and subpar performance.
Should You Be Worried? Probably. Iron-deficiency anemia is fairly common in active women, as well as vegetarians. However, a blood test will easily reveal a deficiency. To avoid iron depletion, monitor your iron status during periods of hard training or high mileage.
Where to Find It: Animal foods, such as red meat and dark poultry, provide the most readily absorbable form of iron. Iron can also be found in dried fruit, dark greens, beans, whole grains and soy foods, but it is more difficult to absorb.
Potassium
What it Does: Potassium partners with sodium to balance the fluid and electrolyte levels in your body. As a runner it is particularly important, since steady fluid levels help to regulate your heartbeat and prevent muscles from cramping.
Should You Be Worried? Maybe. Potassium supplements are rarely necessary as so many common foods supply this mineral. However, it's still important to be conscious of maintaining a balanced diet.
Where to Find It: Bananas, baked potatoes, winter squash, milk and yogurt, cantaloupe, pinto beans and spinach are all excellent sources.
Sodium
What it Does: Sodium is the major electrolyte in sweat—and the one lost in the greatest concentration. Although sodium often gets a bad rap, it is necessary to maintain normal blood pressure and muscular function.
Should You Be Worried? Probably. Sodium consumption is crucial to consider during long or difficult runs. If you run for more than an hour at a time or train in hot and humid climates, your sodium levels are likely to tank.
Where to Find It: To help prevent hyponatremia (diluted blood sodium level), eat salty foods (olives, soups, cheese, pretzels, etc.) in the few days leading up to long efforts. Drink sports beverages with sodium during periods of intense or prolonged exercise.