Calories for weight loss

When it comes to losing weight, the magic number is 3,500, say countless nutrition experts, diet books and magazine articles. That's the number of calories it takes to lose a pound fat. In other words, eat 500 calories less than you need every day for seven days and you should emerge one pound lighter.

The theory sounds great. Unfortunately, it doesn't work that well in practice.

Many people find that through they manage to trim their calories in accordance with this theory, the numbers on the scale don't seem to equal their efforts.

"The (value) is fundamentally flawed in that it assumes two things. One, that the components of energy intake and energy expenditure are static; and two, that they don't interact" notes Joseph Donnelly, professor of health, sport and exercise science at the University of Kansas.

It's easy to control how much a lab rat eats and the energy it burns. But you need to contend with literally thousands of outside factors that impact your behavior and your body. Unless you've decided to spend your days and nights living in a bubble at a research lab, your calories in and calories out will vary, sometimes drastically, each day.

Causes of the variation include resting metabolism, exercise, lifestyle activity and the thermic effect of food. Even something as minor as the timing of your meals can affect your metabolism.

Of course, that doesn't mean operating on the 3,500-calorie notion is completely hopeless. But, it's only a rough guideline.

In theory, if you eat 84,000 calories more than you expend, you'll gain 24 pounds." But, this doesn't happen. Your body's response to a change in calorie intake is dynamic rather than static.

The same thing happens when you try to lose weight by eating less. Eating 500 fewer calories each day equals 3500 less calories a week. In theory, you'll lose 52 pounds in a year. But it doesn't work out like this.

What is a calorie anyway?

Most people think that every gram of protein or carbohydrate contains four calories, while every gram of fat contains nine calories. But it isn't true.

If you were to ask a chemist how many calories were in a Big Mac, the answer would be 570,000. However, if you asked how many kilocalories the Big Mac contained, the number would be 570 — an amount more familiar to most people.

You see, one kilocalorie is really the equivalent of 1,000 calories. So, when someone tells you that one gram of fat contains 9 calories, what they really mean is that it contains 9 kilocalories. Whenever you see the word "calorie" used, remember that the real term is actually a kilocalorie.

A kilocalorie (you'll also see it written as kcal, or kcalorie) or Calorie (with an upper case "C") is the amount of heat needed to raise the temperature of a kilogram (a liter) of water by one degree Celsius.

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Reference
Weyer, C., Pratley, R.E., Salbe, A.D., Bogardus, C., Ravussin, E., & Tataranni, P.A. (2000). Energy expenditure, fat oxidation, and body weight regulation: a study of metabolic adaptation to long-term weight change. Journal of Clinical Endocrinology and Metabolism, 85, 1087-1094