Getting burned.

Although you get a double sense of accomplishment when you ask the flirty guy who hangs out down at the local swimming hole to lather sunscreen on your back (he's also helping you to prevent skin cancer and premature aging of skin), you're robbing yourself of a much-needed survival tool. Vitamin D occurs naturally in some foods, especially fish and milk, but it's most efficiently produced in the body by exposure to sunlight, and shunning the sun could be giving you low levels of this nutrient. A lack of vitamin D has been associated with arthritis, multiple sclerosis, and colon and prostate cancers. So how can you get the sun's benefits without overexposing yourself to its harmful rays? Some experts recommend upping your vitamin D intake to 1,000 international units each day--five times the recommended daily allowance--through both foods high in the nutrient and supplements.