1. You're Trying to Slim Down
You Need CALCIUM
You skipped dessert and hit the gym all summer long—and still haven't dropped those last 5 pounds. What gives? Chances are you're one of the 75 percent of women who fall short of the recommended 1,000 milligrams (mg) of calcium daily. A new study in the British Journal of Nutrition suggests that not getting enough of this mineral may make it harder to take the weight off: When researchers put overweight, calcium-deficient women on a low-calorie diet, they found that those who took a 1,200-mg calcium supplement daily shed 11 more pounds in four months than those who continued to get less than 800 mg a day. Researchers say calcium may regulate the secretion of leptin, a hormone that controls appetite.

Daily dose At least 1,200 mg a day in three doses of 500 mg or less. The body can absorb only that amount at once, says Somer; most multis contain between 100 and 450 mg. Avoid taking them with caffeine and wheat bran, both of which block absorption.

Food sources 1 cup calcium-fortified orange juice (350 mg), 3 ounces sardines (325 mg), 1 cup cooked soybeans (195 mg), 1 cup cottage cheese (187 mg).

2. You're on the Pill
You need VITAMIN B₆
Feeling tired and sluggish all the time? Your birth control may be to blame. In a study from Tufts University, 75 percent of oral contraceptive takers who weren't taking a multivitamin had low levels of energy-boosting vitamin B₆. "It may be because the vitamin is used to metabolize estrogen, the main component in many birth control pills," says lead researcher Martha Morris, Ph.D. Vitamin B₆ helps convert food into energy and maintain nerve function, so shortchanging yourself can lead to fatigue, irritability, and even depression.

Daily dose 2 mg, which you can get from most multivitamins. You can also switch from a regular vitamin to a prenatal one. "Each prenatal pill usually contains 2.6 milligrams of B₆ or more," says Morris. "But beware of megadoses, because regularly getting 100 milligrams or more of the vitamin can lead to nerve damage."

Food sources 1 baked potato (0.5 mg), 1 banana (0.4 mg), 1 cup red pepper slices (0.3 mg).