Just google 'weight loss diets' and you'll get millions of hits on every diet on the planet -- from the orthodox to the wacky. This leads us to the big question: Which one works best in the long run? A small study recently pitted three popular diets -- low-fat, low-carb and low-glycemic index diet -- against one another in the fighting ring. It's a matter of hot debate, and it's shed an important insight: That not all calories are created equal.

For the study, researchers had overweight young adults lose 10-15 percent of their body weight, then put them through all three diets in random order, each for four weeks. For each diet, the subjects' metabolism and hormone levels were tracked. Here's what they found:

Low-fat diet

The diet: This mainstream diet cuts dietary fat and emphasizes whole grain products and a variety of fruits and vegetables, and is composed of 60 percent of daily calories from carbohydrates, 20 percent from fat and 20 percent from protein.

What the study found: This diet caused the biggest drop in energy expenditure (metabolism). It also led to an unhealthy lipid pattern and insulin resistance.

Low-carbohydrate diet

The diet: Modeled after the Atkins diet, the low-carb diet is composed of 10 percent of daily calories from carbohydrates, 60 percent from fat and 30 percent from protein.

What the study found: The very low-carb diet produced the greatest improvements in metabolism. One caveat: This diet increased participants' cortisol levels, which can lead to insulin resistance and cardiovascular disease.

Low-glycemic index diet

The diet: Made up of minimally processed grains, vegetables, healthy fats, legumes and fruits, 40 percent of daily calories comes from carbohydrates, 40 percent from fat and 20 percent from protein. (Low-GI carbs digest slowly, helping to keep blood sugar and hormones stable after the meal.)

What the study found: The drop in metabolism was between that of the low-carb and low-fat diet.

Verdict on the three diets

When you go on a diet, you'll want to avoid a dip in metabolism as this slows down your weight loss effort. The study found that the low-fat diet resulted in the greatest reduction in resting energy expenditure, the low-glycemic index one saw an intermediate drop, while the low-carb diet had the least decrease.

But even though going low-carb preserved the most metabolism, there were health downsides, including chronic inflammation and cortisol, the key stress hormone.

Bottom-line? A low-glycemic index diet is best for weight loss and cardiovascular disease prevention, says study author Cara B. Ebbeling, Ph.D. Plus, it has another advantage: "Low-glycemic-index diets are easier to stick to on a day-to-day basis, compared to low-carb and low-fat diets, which many people find limiting," Ebbeling adds. "Unlike low-fat and very- low carbohydrate diets, a low-glycemic-index diet doesn't eliminate entire classes of food, likely making it easier to follow and more sustainable."