The chemist labels this nutrient organic because its structure is based on carbon, hydrogen and oxygen. The nutritionist pictures this nutrient in several forms. The sports enthusiast uses this nutrient for better and more sustained performance. The foodie thinks of most plant products and a few animal products, like milk. And some dieters fear this nutrient.
So, did you guess correctly? The answer is carbohydrate. Simple carbohydrates are called sugars and taste sweet. Complex carbohydrates are mainly starches and dietary fibers, and do not taste sweet. Your body breaks starches apart into sugars, which can be taken into the body and used just like sugars from your foods.
Dietary fibers, however, cannot get into your body. Dietary fiber is from plant tissue. There are two main types. Insoluble fiber looks the same going in as coming out ... think bran muffin. Soluble fiber includes plant gums and pectins (like what Mom used in making jelly). Fruits, vegetables and whole-grain products are great sources of both. Dietary fibers help maintain the health of the digestive tract. They give the muscular system something to work against -- it's like resistance training for other muscles. Fiber also make your feces softer and cuts the risk for that pesky problem, constipation.
You can store only enough carbohydrate in your body to last about a day. So it is important to replenish your carbohydrate (glycogen) stores by eating good sources of carbohydrate at each meal, if possible. Your body uses a mixture of a simple sugar (glucose) and fat to provide you with energy throughout the day. If your body doesn't have enough carbohydrate available, it cannot use fat properly, as any athlete knows.
What about the current (and recurrent) craze of low-carbohydrate dieting? This style of eating is either high protein, high fat or both. In both cases, your body makes by-products that can be toxic to your kidneys and brain. Your body tries to get rid of these compounds through the urine. Thus, you appear to lose weight because you're losing mainly water. You can become dehydrated, feel weak, have problems thinking, and have bad breath. It is particularly dangerous for pregnant women and people with diabetes. Like any other "un-normal" way of eating, gaining weight back can be a problem once you start eating normally again.
If you really eat low-carb, that means little or no potatoes, pasta, noodles, bread, rice, fruits, vegetables, milk, legumes, and, of course, nothing with sugar. How boring! And, how nutritionally silly! This restrictive way of eating eliminates most of the foods on the Food Guide Pyramid, including chocolate! It's much better to eat smaller servings of all foods and get a bit more exercise each day. You gained the weight slowly, over time, and you should think about losing it the same way.
As an adult, you need 130 grams of carbohydrate each day just to keep your brain working properly. So, more IS better ... about 300 grams a day if you eat 2,000 kilocalories a day ... to meet current recommendations. You simply can't get enough carbohydrate if you cut out all those delicious and nutritious foods I've listed above.