Carbohydrates serve several goals in the human body:
The most basic role of carbohydrate is to supply energy. The human body gets its energy from splitting off glucose(1) molecules into smaller compounds and oxidizing them to form water, which frees quite a large amount of energy.
(1) can be found in the blood and extracellular fluids, and can be made from glycogen(2)
(2) glycogen is stored in the liver and muscles and in smaller amounts in the other organs and tissues of the body.
Carbohydrates provide the major fuel for muscular exercise. The muscles use the glycogen present in the muscle cells and glucose in the bloodstream. However, glycogen from the muscles is more efficiently used than glucose because the breakdown of glycogen for use does not require energy input at the time, whereas a certain amount of energy is used to bring the blood sugar into the metabolic system of the muscles.
The body works much more efficiently from carbohydrate intake than from broken-down body protein and fats because protein and fat molecules, when used as fuel, yield less than their total caloric value in the form the muscles can use. The remaining portion is used for the conversion of these molecules into suitable fuel. This conversion takes place in the liver and adipose tissue, which supply the body’s organs with fuel via the bloodstream.
Ultra-low-carb diets can be harmful to your thyroid gland, which can damage both your results and your health. When you decrease your carbohydrate intake for too long on a low-calorie diet, your thyroid and the hormones it produces/stimulates can be negatively impacted. Since certain thyroid hormones regulate your rate of metabolism - the amount of daily energy you spend at rest - your results can slow and your weight might plateau.
If you're finding that you're really struggling to make it through your workouts - or you're just having trouble getting through your day, it's vital that you take a look at the carbohydrate content in your diet.
Some dieters don't mind reducing their workouts and doing the bare minimum to maintain lean muscle mass. If you're only performing a few compound lifts per workout, then you won't necessarily need a shipload of carbs to complete your training. Roughly 10 grams of carbs for every four sets lifted should be sufficient to keep your muscle glycogen levels high enough for resistance training.
However, if you want to maintain fairly intense workouts throughout your fat-loss program, you'll need carbs for training energy. Since carbohydrates are directly related to your ability to recover after working out, you'll find that you grow weaker and weaker following each and every training session. If you crave intense workouts and push yourself to the limit, an extremely low-carb diet is not going to be fun. If you can forgo extremely intense, long workouts, then you can be more flexible with your carb intake. Just don't cut them out if you need to train insane.
The brain uses more glucose than any other organ or tissue in the body at rest. Thus, when carbs are dropped, you're going to feel it.
When carbohydrates are reduced for an extended period of time, your body will begin to produce ketone bodies. Ketone bodies are your brain's alternate fuel source. Some people seem to feel perfectly fine running off ketone bodies, while others feel miserable. They're unable to concentrate, tired all the time, and they just don't feel 'present' as normal. How people deal running on a tank full of ketones tends to vary, so you might have to dramatically decrease your carbs to know for certain.
Some people proclaim that getting into ketosis is the best thing they've ever done and that they have endless energy, but this is definitely not the norm. For every person that feels great, you'll find one or more who are struggling to sustain such a limited diet. If you decide to try low-carb dieting, be careful: it can influence your focus, concentration, and your performance at school or work.