Carbohydrates are major sources of energy and have some specific roles in the human body. They are the primary sources of fuel for the human brain. Although ketone bodies can supply most of the energy to the brain during a starvation state, the axons cannot use anything but glucose. This makes carbohydrates vital to the function of every single neuron in the brain and in the rest of the nervous system. In addition, carbohydrates are the only sources of fuel that are acceptable to the kidneys and the red blood cells.
Now most people will find the necessity of carbohydrates to the red blood cells to be surprising because the red blood cells carry oxygen. However, their job is to transport the oxygen and not to use it up for themselves so they have to derive their energy from anaerobic glycolysis. However, the brain is by far the largest consumer of carbohydrates. The combination of the brain, kidneys, and red blood cells consume an average of about 130 to 150 grams (~500 to 600 calories) of carbohydrates per day.
If the body is deprived of adequate carbohydrates, it will use the processes of gluconeogenesis (making of new glucose) and ketogenesis (making of ketones) to provide energy for vital functions. It is important to note that fats are not good fuels for making glucose because the glycerol group is the only part of a triglyceride that can be used to manufacture glucose. The fatty acids themselves can only be oxidized or converted to ketones.
The major source of raw materials for making new glucose will come from the amino acids in muscle protein. All amino acids have an amine group, a carboxyl (acid), and a side chain which determines the type and nature of the amino acid. The different amino acids are peptide bonded in various combinations to make the different proteins in our bodies.
Whenever the body breaks down protein, the various amino acids are released. Some of these amino acids are suitable for making glucose (glucogenic), some are suitable for making ketone bodies (ketogenic), and some can be used for either purpose. In either case, the body has to remove the amine group from them before it can use them. The removed amine group will join other ions to become ammonia. However, because ammonia is toxic, the body will use the uric acid cycle to convert the ammonia into uric acid.
This process takes place in the liver at the same time that the gluconeogenesis and the ketosis are taking place in the liver. Although uric acid is less toxic than ammonia, uric acid can still cause problems such as gout. The kidneys will perform extra work to flush out this uric acid via the urine. Because the kidneys need carbohydrates, they will be low on fuel while performing extra work to eliminate this waste. Loss of lean tissue to supply the amino acids and loss of water due to urine production account for much of the weight loss on low carb diets. Also, fatty acids and amino acids are being converted into ketone bodies to make up for the energy shortage. In other words, of the fat loss that actually occurs, much of it happens because by cutting out carbohydrates, the person is basically cutting calories.
It is important to know that the type of gluconeogenesis due to low carb diets occurs for a different reason and uses different raw materials from the type that occurs between sets of intense exercise. The type resulting from exercise uses lactic acid as the raw material and aerobic lipolysis as the source of energy. The type that results from low carb diets uses deaminated and decarboxylated amino acids as the raw materials, and is a starvation process. In addition, it receives the energy from ketone bodies which result from ketogenesis which is another starvation process. As a result, low carb diets are starvation diets by definition, and they cause the body to cannibalize its own living tissue (muscle) when it is starved of a vital nutrient (carbohydrates). This is consistent with the definition of a starvation diet.
Another interesting fact is that the body will inhibit glycolysis when carbohydrate (glycogen) stores are low. Because carbohydrates (due to glycolysis) produce usable energy (ATP) faster than any other nutrient, this explains the degraded athletic performance and lack of energy experienced by subjects on low carbohydrate diets. It is no surprise that triathletes, marathoners, power lifters, or other high level strength or endurance athletes would perform poorly on low carb diets. Also, the potential risks of low carb diets include overworked kidneys, gout, dehydration, mineral depletion, and hypoglycemia.