When it comes to weight loss, we've historically been told to avoid carbohydrates like a pair of Crocs. More recently, certain carbs (we're looking at you, millet and quinoa) have become as fashionable as a crepe-soled suede summer boot. Which is it? Turns out that the most effective way to shed pounds and add muscle is to do what many elite bodybuilders have been doing for years—alternate low-carb, moderate-carb, and high-carb days. The finely calibrated practice is called carb cycling, and it's "the only diet option that lets you achieve both those goals," says Christine Gerbstadt, M.D., R.D., author of Doctor's Detox Diet.

And now, thanks largely to the release of the carb-cycling bible Choose to Lose: The 7-Day Carb Cycle Solution, by Chris Powell, and its recent follow-up, Choose More, Lose More for Life, the secret is making its way through the body-conscious and nutritionally savvy set. "I have fitness models coming to me in their twenties and thirties who use carb cycling to lean out fast for a photo shoot," says Powell, the trainer on Extreme Weight Loss.

Here's how it works: Low-carb days force the body into catabolic stages—in which it breaks down tissue. Then, on high-carb days, the food intake nourishes the muscles to ensure that only fat tissue—and not muscle—will be burned during the next catabolic phase. The processing of these carbs keeps the body's metabolism in top form. "It's difficult to quantify this, because it's such an individual thing," Powell says, "but it is significant. Saying that carbs aren't necessary to weight loss is like saying the world is flat."

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The Diet at a Glance

LOW-CARB DAY
1,325 calories, 42 grams of carbs

7 a.m.: Scrambled eggs, veggies
10 a.m.: Mozzarella, almonds
1 p.m.: Chicken sandwich, spinach salad
3 p.m.: Plain Greek yogurt
5 p.m.: Whey-protein smoothie
7 p.m.: Grilled salmon, asparagus

MODERATE-CARB DAY
1,800 calories, 180 grams of carbs

7 a.m.: Quinoa, 1 scrambled egg
10 a.m.: Dried apricots, almonds
Noon: Turkey-cheese-vegetable wrap
3 p.m.: Plain yogurt, blueberries
5 p.m.: Strawberry-milk-protein smoothie
7 p.m.: Halibut, asparagus, salad

HIGH-CARB DAY
2,700 calories, 450 grams of carbs

7 a.m.: Steel-cut oats, 1 scrambled egg, veggies
10 a.m.: Dried apricots, almonds
1 p.m.: Ham sandwich, vegetables
3 p.m.: Gluten-free pretzel sticks, low-fat yogurt, blueberries
4 p.m.: Casein protein isolate
5 p.m.: Banana-strawberry-protein smoothie
7 p.m.: Black cod, kasha, noodles, salad

Source: Dietitian Susan M. Kleiner, author of Power Eating

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Go With These Grains

Oatmeal: Calories: 306; carbs: 55g; protein: 11g; fiber: 8g (all figures based on one-cup servings).
Perk: Has anti-inflammatory properties that combat bloat.
Tip: Stick to steel-cut, which is less processed and better for blood sugar.

Quinoa: Calories: 222; carbs: 39g; protein: 8g; fiber: 5g.
Perk: Contains every essential amino acid—important for muscle recovery after a workout—and no gluten.
Tip: Grind into a flour to make your own pasta or buy quinoa flour or dry quinoa pasta from your local organic market.

Barley: Calories: 651; carbs: 135g; protein: 23g; fiber: 32g.
Perk: Barley has antiaging properties—chalk it up to the antioxidants. And a little goes a long way, nutritionally.
Tip: Stick to the hulled version—it has twice the minerals—or drink barley juice.

Bulgur: Calories: 151; carbs: 34g; protein: 6g; fiber: 8g.
Perk: Keeps your digestive tract on track by promoting fast elimination.
Tip: For use in cold salads like tabbouleh, soak for approximately 30 minutes.