Proteins are the building blocks for your muscles, organs, tissues, etc. Fats are necessary for long term energy storage, they carry certain forms of vitamins and nutrients (Vitamin E, for example) and your body needs fats for many of its metabolic processes.

Carbohydrates are the fuel that your body uses for energy. While it can use proteins and fats for that energy, which is why low-carb diets work, it prefers to use the carbs. They are easier to process for energy than either the fats or the proteins and your body gets more energy “bang for the buck” with carbs.

The Rings

On the molecular level carboydrates are ring molecules. A “simple” carb, like glucose, is one ring of six carbon atoms. Just like the ring on your finger. Table sugar is two of these rings. Starches and “complex carbohydrates” are made up of many of these rings. This is why your body takes longer to digest the complex carbs, it has to break them down into the individual rings first and then use them for energy.

This is also why a meal of simple carbs will blast your blast your blood sugar sky high. Your body can use them almost immediately. Have a glass of glucose or a soda and wham! Sugar is immediately pumped into the system and your blood suger levels leap. Insulin is secreted into the blood and the insulin causes the sugar levels to drop, along with your energy.

A meal of complex carbs takes longer to digest and as a result doesn’t slam sugars into your system, rather the complex carbs provide longer term energy. Fats and proteins take even longer to digest and convert into the carbs your body needs.

update: A dietician (in the comments) pointed out something with that last line, said line not being strictly accurate. Proteins are metabolized to glycogen then to glucose. Fats are broken down to ketones. Your body uses both, for energy,  just fine. See the comments for more.

No Carbs?

Extreme low-carb diets, such as they first part of the Atkins Diet, are not recommended for anything other than short term use. Even the Atkins diet brings you back into carbs after than first section, but they are the complex carbs. Simple carbs are almost completely abandoned.

As far as energy goes, though, your body only needs so much at one time and it can only store so much at one time. An athlete needs more than someone sitting in front of a computer all day, but even they have limits to how much they need right now. Those extra carbs are stored in two ways.

One way is conversion into glycogen, a type of starch, which is then stored in the muscles for near term energy reserves. The other way they are stored is fat. Your body can only store so much excess carbohydrate as glycogen, the rest goes to fat. (This is a little over-simplified, but close enough.)

In addition to being converted to fat, excess simple carbs over a long period of time can lead to a variety of health issues, diabetes and heart issues being among them.

So What’s a Simple Carb?

So what foods are the simple carbs? Any kind of sugar, rice, white flour, potatoes, Rice Krispies, Corn Flakes, etc. These are all “high glycemic index” (GI) foods. The GI is just a measure of how much a given food will raise your blood sugar as compared to a glass of glucose.

Fruits tend to be in the medium GI range and most veggies are pretty low. Go to Google.com and look for “high glycemic foods” or “glycemic index” and you’ll find charts with examples of various foods and their ratings.

So while you body does needs carbs for fuel it doesn’t need a ton of them and it works best if they come in slowly and steadily, rather than in a burst. You may have heard of the “eat several small meals” plans? This is one of the reasons they work. Your body has the time to properly process the carbs and so uses them, rather than storing them as fat.

Get Big Fast

Want to gain weight fast? Eat one big meal a day, preferably in the evening, and sleep afterwards. That’s how the Sumo wrestlers of Japan do it.

What to do?

There’s no “magic pill” to losing weight, but there are several plans which will speed up your weight loss quite nicely. One system, Strip That Fat, covers everything you need to know in detail. You can read the complete review of that system here.