Up Your Vitamin Intake

We have a worldwide epidemic of vitamin D deficiency, says Northrup. Everybody needs vitamin D, which can be found in foods like sockeye salmon, eggs, and milk.

Hyla Cass, MD, an integrative medical practitioner and author of 8 Weeks to Vibrant Health, adds that certain prescription drugs like acid blockers can deprive the body of nutrients like vitamin D.

Surveys show that Americans don't get enough vitamin C, says Elisabetta Politi, RD, MPH, CDE, nutrition director at the Duke Diet & Fitness Center.

Citrus fruits are a good source of vitamin C. "It's a myth that vitamin C prevents the cold," she says. "But having an appropriate amount of vitamin C from fruits and vegetables can boost immunity."