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Vitamin D: 30
When you're deficient in vitamin D, you may be at increased risk for heart disease, cancer, multiple sclerosis, and immune disorders, not to mention osteoporosis. To make sure you're getting enough, take a blood test for vitamin D: Your level should be greater than 30.

To boost vitamin D: If you can't get 15 minutes of sun exposure daily, take a supplement containing at least 1,000 IU of D3, the most potent form of the vitamin, or chug a tablespoon of cod liver oil every morning.