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.
As a reminder, always consult your doctor for medical advice and treatment before starting any program.