As the moment of truth with your bathing suit inches closer, you might be cursing your sweet tooth for the belly bulge you can't seem to shed. But it could be what you're not eating that's actually responsible.

A 2000 study reported in the Archives of Internal Medicine reports that 77 percent of Americans are vitamin-D deficient. Numerous studies have shown D's crucial role in strengthening bones, fighting depression, and boosting immunity, but now, after decades of research and thousands of studies, experts may have finally proven that missing out on that one little letter could be a major factor in pushing the number on your scale higher and higher.

Heavy D, Slimmer You

Unlike most vitamins, D can be manufactured by your body, provided you're exposed to sunlight. But sunscreen—a must for preventing wrinkles and, of course, cancer—reduces your skin's ability to make D by 90 to 99 percent, depending on the level of SPF thoroughness of application. And even if you don't use sun protection (bad girl!), it's tough to catch the right rays. The sunlight you're most likely to get on a daily basis—in the early morning and late afternoon—is too weak to generate enough D. Not to mention that air pollution can filter out some of the UVB rays, so less of them are able to reach your skin at any time of day.

A far safer, saner, and more satisfying way to up your D level is to get more of the vitamin from your diet. A doctor can administer a simple blood test called 25-hydroxy vitamin D, or 25-OH D, which is the most reliable way to tell not only whether you are deficient but also how severe your deficiency is. And as you increase your consumption of D, your waistband could start to feel a whole lot looser. Every cell in your body needs D to function properly—including hated fat cells. Special receptors for D signal whether you should burn fat or simply store it; when D plugs into these receptors, it's like a key that revs your body's flab-melting mechanism.

Meanwhile, receptors in your brain need D to keep hunger and cravings in check, as well as to pump up levels of the mood-elevating chemical serotonin. (Good thing, since dieting often comes with a heaping side of crankiness.) D even optimizes your body's ability to absorb other important weight-loss nutrients, especially calcium. When your body lacks calcium, it can experience up to a fivefold increase in the fatty acid synthase, an enzyme that converts calories into fat. In a 2009 study published in the British Journal of Nutrition, obese women who were put on a 15-week diet and took 1,200 milligrams of calcium a day lost six times more weight than women who followed the diet alone. The end result: By fueling your body with the D-rich nutrients it needs to get out of a fat-storage state and into a fat-burning one, you could potentially speed weight loss by up to 70 percent.

Here's more welcome news: To get enough D, it helps to snack. You may have heard that eating between meals is a weight-loss wrecker, but don't believe it. Snacking, when done properly, is actually one of the best ways to reduce cravings and stay focused on your slim-down goal. Right now, while the mercury's high and the sun is shining, you're probably less tempted by the heavy, carb-laden comfort foods that are so enticing midwinter anyway. So take advantage of that and choose from the following list of potent fat-melting foods. All are loaded with vitamin D and other nutrients that preserve muscle mass, turn up the heat on metabolism, and encourage your body to incinerate fat, not hang on to it.

Make sure your meals are heavy on D. Then add any two of these snacks each day to help you get the amount you need (around 600 IU in the summer months), plus a dose of the recommended 1,000 milligrams of calcium.

Your Hunger D-fense

Here are a few tasty ways to up your intake of this essential vitamin.

1 D-fortified hard-boiled egg

(70 cal, 80 IU D, 27 mg calcium)

10 unsalted almonds plus 4 oz calcium-and D-fortified orange juice

(132 cal, 50 IU D, 209 mg calcium)

6 oz fat-free flavored yogurt

(80–100 cal, 80 IU D, 300 mg calcium)

1 medium tomato stuffed with 3 oz light canned tuna mixed with 1 Tbsp low-fat mayonnaise and 1 Tbsp chopped scallion

(138 cal, 154 IU D, 26 mg calcium)

16 oz iced cappuccino made with D-fortified nonfat milk

(80 cal, about 50–75 IU D, 150–250 mg calcium)

1/2 cup sugar-free, reduced-calorie banana pudding made with D-fortified nonfat milk

(70 cal, about 50 IU D, 150 mg calcium)

Orangetini: 2 oz D-fortified orange juice, 2 oz water, 1 oz vodka, 1/2 oz triple sec, 1/4 Tbsp grenadine, 1/4 Tbsp orange zest

(152 cal, 25 IU D, 92 mg calcium)

1/2 cup D-fortified cereal with 1/2 cup D-fortified milk and 1/4 cup blueberries

(139 cal, 117 IU D, 869 mg calcium)

3 oz canned wild salmon on 5 whole-grain crackers

(156 cal, 396 IU D, 235 mg calcium)

Alisa Bowman and the editors of Women's Health Alisa Bowman is a product developer and senior editor at Hearst.

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