Today on AskMetafilter, someone wrote in with this gem_
If I ate 1lb of fat each day for a week would I gain more weight than if I'd eaten 1lb of celery each day for a week, and if so why.
Oy, and they wonder why there's an obesity epidemic in America. This guy obviously needed a lesson in the basics of metabolism, which I was bored enough to provide_
It sounds like you need to understand a little more about how calories and energy are used and stored by your body.
A calorie is simply the amount of energy required to raise the temperature or 1 gram of water by 1 degree Celsius. Thus, it's a measure of how much energy is in a substance. Food calories are actually kiloCalories (colloquially known as Calories with a big C).
When you eat something, your body breaks the chemical bonds in your food, and then uses that energy to do all sorts of useful stuff_ keeping your heart pumping or muscles running, providing ATP for chemical processes, releasing heat to keep you warm, etc.
The molecules comprising your food can be broken down into just a few different types of molecules. Carbohydrates have simple sugars as their basic building blocks and include complex structures like starches. Proteins are built from amino acids and Nucleic acids are built up from nucleotides. Then there are fats, which are more accurately called lipids.
Being composed of different things, each of these molecule types have different amounts of energy stored in them. Lipds are, by far, the most energy rich, which is why sugars (fructose, sucrose, et al) have far fewer calories than fats (oils, butter, etc). In simple terms, there are just fewer high-energy bonds to break per gram of sugar, than per gram of fat.
Okay, so that's why different foods have different amounts of calories. Now, what about storing fat in our beer guts? Well, for most of our evolutionary history, food has not always been this abundant and cheap. As a result, when we're taking in loads of calories during a glut, our bodies are able to take some of that energy and store it away for lean times. The body does this primarily by adding fat. (remember, they're a very efficient and energy rich way to store energy). Later, if we're short on food, those fat reserves can be tapped, and metabolized so that our body doesn't shut down completely every time we get hungry.
You can think of the whole process as an equation_
Calories eaten - Calories burned = net Energy gain.
If your net is positive, you'll begin storing energy (and gain weight). If it's negative, you'll begin to burn stored energy and thus, lose weight. This conversion to fat in the body happens at a rate of roughly 3500 Calories per pound of stored fat.
So, imagine you're currently eating 2000 Calories per day, and your weight is constant. If you start eating 250 less calories per day, you'll lose weight at the rate of about 1 pound every two weeks. Remember, there are two terms in that equation - energy in, and energy out. So, if you want to speed up the process, you could start exercising, and if you burn an extra 250 calories per day, you'll lose the weight twice as fast.
So yes, eating a pound of celery will result in far less weight gain than eating a pound of fat. It's not about the weight of the food, it's about the caloric content.