Years ago, scientists played around with a pound of squishy, slimy human fat and found that it contained 3,500 calories of energy.

But—sorry to break it to you—burning a pound of fat isn't as simple as burning through 3,500 calories.

Consider the following and infuriating (at least for thin guys) scenario: Two men go on an exercise and eating plan so that they consume 3,500 fewer calories per week than they burn. One man has five pounds to lose; the other has 50. At the end of one week, the leaner guy might lose about half a pound—and a third of the weight will be from muscle. Meanwhile, the obese guy will have lost more than three pounds, mostly from fat and water.

"There's tremendous variability in how a 3,500 caloric deficit affects different people," says Pamela Peeke, M.D., M.P.H., senior science adviser at Elements Behavioral Health and author of The Hunger Fix.

Why's that? Well, one huge factor determining the results of our dieters is body composition. "The more fat a person has to give, the quicker he will lose weight and weight from fat," Peeke explains. Meanwhile, when you get closer to your body weight, your body holds on to fat stores for dear life and sacrifices muscle over fat, she says. The body is perpetually afraid that it will starve; it's perhaps biology's least-sexy-ever survival mechanism.

Meanwhile, how you try to hit your caloric deficit (which is a necessity to lose weight) has a huge impact on whether you lose weight from muscle, fat, or just water.

The faster you try to achieve a deficit, the more weight you will lose from muscle as opposed to fat. As will be the case if you diet alone, she says. However, exercise—and most markedly, strength training—and protein consumption promote muscle growth so that you will not lose as much muscle. In fact, if you consume an adequate amount of protein (the Academy of Nutrition and Dietetics recommends getting 20 to 30 grams, four times a day and after exercise), you could potentially increase your lean-muscle mass while reducing your body-fat percentage.

What's more, if you are cutting calories from carbs, you will also lose water weight. In the body, every gram of glycogen (carbohydrates) in your body is stored with a few grams of water. So when you go low-carb, your metabolism breaks down those glycogen reserves for energy, and you end up peeing out the accompanying water. That's another reason why, calorie per calorie, obese people tend to drop weight drastically: They have a lot of water to lose.

You also need to realize that your calorie-cutting strategy does alter your metabolism—and what it takes to take in fewer calories than you're consuming over the long haul. Contrary to popular opinion, people's metabolic rates slightly decrease as they lose weight. That's because it takes more energy (a.k.a. calories) to fuel a 280-pound human than a 180-pound one, she says. And if you lose most of your weight from muscle, your metabolism will plummet—which is one more reason why extreme diets suck.

CALORIE MATH

Now that all that's settled, if you want to determine roughly how many calories your body burns a day, check out the Mayo Clinic's calorie calculator. Aim to take in 300 to 500 fewer calories per day to lose weight.