essential nutrients

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Potassium is a vital mineral for the function of all cells, tissues and organs in the human body—i.e., life as we know it. "We also know that potassium is important for controlling blood pressure, and there's a lot of evidence that it reduces your risk of stroke," says Walter C. Willett, MD, chair of the nutrition department at the Harvard School for Public Health. And, as many athletes are aware, it can help prevent muscle cramps and post-workout soreness.

You need: 4,700 mg a day

You get: National survey data shows over 90 percent of Americans don't get the recommended daily allowance, says Willett, who is also the author of Eat, Drink, and Be Healthy: The Harvard Medical School Guide to Healthy Eating.

Good sources: Eat a variety of fruits and vegetables that have higher levels, like broccoli, cantaloupe, winter squash, sweet potatoes and, yes, bananas (these foods should be eaten raw, steamed or baked but not boiled, since boiling depletes potassium.)
essential nutrients

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This trace mineral, required for proper development and growth, is crucial for thyroid function and keeping your metabolism on track. Iodine isn't produced by the body, so it should be an essential part of your diet.

You need: 150 mcg a day; 250 mcg if pregnant (new research reveals that even a mild deficiency can have a long-term effect on brain development); 290 mcg if nursing.

You get: While iodine deficiencies are much less common than they were before the mineral was added to table salt in the 1920s, Willett says that a growing number of Americans are not getting enough. This is due, in part, to more people setting aside regular table salt in favor of non-iodized kosher or sea salt, or avoiding dairy products (iodine is often present in cow or goat feed, which is passed to us via their milk).

Good sources: If you tend to use non-iodized table salt, make sure you're eating enough seafood (seaweed, cod, shrimp, canned tuna), dairy foods, enriched grains and eggs. You could also consider taking a supplement.
essential nutrients

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Vitamin D
Long underestimated, vitamin D is much more than just a bone fortifier; there's strong evidence that adequate levels can reduce our risk for many cancers, says Willett, and can protect against heart disease and type 2 diabetes. One study found that people with very low levels of vitamin D in their blood are 26 percent more likely to die from any cause (heart disease, cancer, infection, etc.) than those with recommended amounts. The challenge is that very few foods in nature contain vitamin D.

You need: 600 IU a day; 800 IU if over 80.

You get: Willett says that about half of Americans, and 80 percent of those with darker skin, don't get enough (higher amounts of melanin in the skin interfere with vitamin D absorption).

Good sources: The easiest way to get vitamin D used to be from the sun, but now our risk of skin cancer is high enough that all dermatologists (and most nutritionists, including Willett) advise instead to rely on food or supplements. Your best bet is the flesh of fatty fish (such as salmon, tuna and mackerel). A tablespoon of cod liver oil has 340 times the RDA (though many folks cannot stomach it). Smaller amounts are naturally found in beef liver, cheese and egg yolks. You can also find vitamin D in fortified milk, yogurt and orange juice . (Talk to your doctor about supplements).
essential nutrients

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Vitamin B12
Vitamin B12 is an energy booster that also strengthens the immune system. It's necessary for the growth of red blood cells, too, says Willett. But nature's upper is only found naturally in animal products.

You need: 2.4 mcg (2.6 mcg if you're pregnant, 2.8 mcg if you're nursing).

You get: If you're a vegan or vegetarian, or you go for days without meat, fish or eggs, it's likely you're not getting enough vitamin B12. Adults over 50 also tend to have a harder time absorbing this vitamin, says Willett (a problem, especially since a deficiency accelerates cognitive decline, as do those with some intestinal and gastric conditions.

Good sources: Pescatarians, you're in luck: Clams are off the charts in B12, and rainbow trout and salmon are pretty high, too. If you don't eat any animal products at all, look for fortified cereals, nutritional yeast and supplements with 100 percent of the recommended daily value.
essential nutrients

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Magnesium is involved in hundreds of bodily functions, from keeping your muscles, nerves and heart functioning to regulating your immune system. Boosting your intake could help keep your mood steady, as research has found an association between low levels and depression and anxiety (not to mention migraines). A magnesium deficiency may also contribute to heart attacks and hypertension.

You need: 320 mg for women; 420 mg for men.

You get: Although we hear a lot about the importance of magnesium, studies show that more than half of us don't get enoughwe're 100 mg short, on average, of the recommended amount.

Good sources: Nuts are magnesium powerhouses (almonds have the highest amount, at 80 mg per ounce, but cashews are a close second). Also look for green vegetables like spinach and okra, as well as soybeans, legumes, seeds and unrefined whole grains.

Next: Secret substitutions to make the foods you love even better
As a reminder, always consult your doctor for medical advice and treatment before starting any program.