I'll first explain the concept in more detail, and then give you an example of how it actually works.

First of all, every day our bodies use up a certain number of calories just to "keep the lights on." What do I mean by that? It takes energy (calories) to keep your lungs breathing in and out, to keep your heart beating, in fact, to keep your body warm (98.6 F or 37C). Our bodies are using up energy to keep these biological systems working, whether we are eating or not. Scientists have come up with a name for the energy that a person needs just to "keep the lights on"- it's called the Basal Metabolic Rate or BMR. Not everyone's BMR is the same - it varies with your height, weight, age and gender. So for example, a 25 year old 5' 5" female weighing 125 pounds will have a BMR of about 1,386 calories per day. A 25 year old football player who is 6' 4" and weighs 300 pounds will have an estimated BMR of 2,152 calories per day.

Ok, so now we know that our bodies are using up a certain amount of calories, even if we do nothing but sit on the sofa all day (BMR). To this we need to add a second category of calorie burner - our daily Activity. Every time we "move" - walk up the stairs, push a shopping cart down the grocery aisle, work around the house, etc., we use up/expend/burn more "activity" calories. Walking the dog, going for a light jog or running on a treadmill - these activities all burn more calories.

The third and last category of daily calorie burners is one that I can almost guarantee you have never heard of before. It is called the Thermic Effect of Food. What in the world does that mean? To explain, let's say you go out and order a large steak with baked potato and all the trimmings. As you slowly ingest this meal, guess what your body has to do? It has to perform major "work" to slowly break down and digest the food you are eating. It is actually burning or using up calories for digestion. This is called the Thermic Effect of Food.

Ok, we are getting close to revealing the one basic concept that is at the foundation of weight gain and loss. We have already discussed the three ways we burn calories every day (BMR, Activity and Thermal Effect). What else do we need to consider? Yes, of course, it's the other side of the equation - that is, how many calories we take in every day - what we eat and drink. For this, we need to add up all the calories consumed in our meals - breakfast, lunch, dinner, snacks and drinks.

Now we're ready to understand the one simple rule for weight gain or loss. Let's use a real life example. Let's say Suzy is a 30 year old female, about 5' 6" tall and weighs 130 pounds. Her BMR is 1,390 calories per day. Suzy maintains a fairly active lifestyle, so let's say her physical activity calorie burn is 610 calories. Finally, her Thermic Effect calorie burn is about 200 calories. So, if we add up all three ways that Suzy burns calories, we get 1,390 + 610 + 200 = 2,200 calories burned / used up / expended each day.

If Suzy eats or consumes about the same number of calories, she will, on average, maintain her present weight (2,200 calories burned = 2,200 calories consumed). If she eats slightly more calories, she will gain weight (2,200 burned, 2,300 consumed). If she eats slightly less calories, she will lose weight (2,200 burned, 2,100 consumed). In other words, if you want to lose weight, you need to create a calorie "deficit." If you want to gain weight, you need to create a calorie "surplus."

Most of us would like to lose a little weight, so as you can see from the math, you can create a caloric deficit two ways, really. You can eat the same and exercise more, or you can eat less and maintain your present activity level. Ideally, you will take in slightly less calories and become slightly more active every day - thus working both sides of the equation at the same time, but please note - you should always consult a physician before making any changes to your dietary or lifestyle habits.

Here is something that you might think is controversial at first, but if you think about it for awhile, you will understand an important distinction about how calories work. Calorie surpluses or deficits, as explained above, are the biggest factor in determining your weight gain or loss over the long term, no debate - end of story. As an extreme example, you could devise a weight loss plan that includes eating nothing but cotton candy and funnel cakes all day, as long as you were strict about counting calories and creating a deficit. (Please do not do this; these are two of the worst foods I can think of, and I'm only using them to make a point.) If you burned more calories than you consumed eating these two disgusting foods ? you would lose weight. On the other hand, your nutrient intake would, of course, be a disaster. So you see nutrition is really a separate and distinct issue from weight gain or loss. Therefore, why not make the effort to choose nutrient dense foods, like fruits and vegetables?

Remember that the deficit/surplus concept works over the long term. Your daily weight fluctuation can be affected by many temporary factors such as water retention, undigested food, etc. The best way to lose weight permanently is to create a small-daily caloric deficit eating nutrient-dense foods. Since a pound of fat is equivalent to about 3,500 calories, one strategy would be to create a deficit of 500 calories per day X 7 days = 3,500 calories per week. This equates to losing about a pound a week. If you think that's too small of a goal, remember that there are 52 weeks in a year. Another hint: you don't have to rely solely on eating less food to create the caloric deficit ? you can also work the other side of the equation by becoming more active in your daily lifestyle. (Again, consult a physician before making any changes to your dietary or lifestyle habits).