The answer to the question of whether or not we NEED carbs is not b/c our body can produce carbohydrates. As everyone has previously stated CHO's are necessary for brain function. In extreme cases the body can rely on Ketones; but since they are acidic they are not exactly the best for our grey matter.
But maybe I should go over the various roles of different fuel substrates to qualitatively answer this question.
You have 3 major sources of energy; 1) Fat, 2) Carbohydrates, and 3) Protein. Well I GUESS you can include alcohol too (with a bit over 7kcals/gram), but I'm going to ignore it for the sake of simplicity.
There's a reason I numbered them going from fat to protein first b/c fat is the body's preferred energy source, CHO's second, and Protein last. Don't yell at me yet...I know this is not ALWAYS true. I'm talking in terms of a sedentary non-exercising individual. Exercise changes the equation a bit too and the body ALWAYS wants some carbs for proper brain function (it can't utilize fat); and I mean a monosaccharide.
So first let examine substrate utilization at rest, neglecting the brain.
When the body is at rest it's most efficient source of energy is Oxidative Phosphorylation; the aerobic breaking down of fats WITH oxygen. So to start from the beginning we eat a fat; it enters the GI tract; bile salts from the pancreas break it down into something called micelles and three glycerol molecules (bile salts act like soaps and just break down the fats into smaller chunks). The micelles then enter the lymphatic system through other processes which I won't write about since I'm not familiar enough with them). The fats will either end up in adipocytes or muscle cells.
Now let?s say the individual goes from being sedentary to moving. For the first few seconds of locomotion the body is NOT ready to start oxidizing fat in the mitochondria so it relies of the CP-ATP system (Creatine Phosphate-ATP). The amount of energy the body uses vs. what it produces until equilibrium is reached is then referred to as the O2 deficit I believe. If you want to see a graph I can probably dig one up.
Now if this exercise is low-intensity to moderate (under 60%) of VO2 max the primary source of energy will be fats. Why is this? Fats are the most efficient at being broken down. Hell when churning out carbs you have to invest 2ATP molecules to churn our 4ATP molecules (net gain=2ATP). So basically exercises that rely on less than 60% of VO2 max rely primarily on type 1; red muscle, slow twitch fibers.
Why are they called red fibers? Well since the slow twitch fibers break down fat they require a large supply of oxygen for oxidative phosphorylation; thus they need a large blood supply to supply them with this oxygen and have an extensive capillary system.
They also have a high concentration of mitochondria b/c of this.
Now we move up in exercise intensity; to sprinting movements or weight lifting movements.
The body can NOT create ATP fast enough aerobically in this instance so it relies on anaerobic metabolism; e.g. Glycolisis (breaking down of carbohydrates). This can happen in a few ways: 1) Breakdown of intracellular muscle glycogen, 2) Increased glucose uptake into the muscle cell.
1) Breakdown of glycogen - Glycogen is basically a long chain of glucose molecules held together by water. For every gram of glycogen there are about 3 grams of H20 attached to it. This is why people on low CHO diets lose weight so fast (Assuming a well-fed individual has 3000kcals of CHO energy in both the liver and muscle which equates to 750 grams of CHO...3 grams of water...equals about 6lbs of water weight between bonds). When the muscle needs CHO..E.g. glucose it will break down the glycogen into individual glucose molecules which can then be metabolized.
2) So many of you might be saying well exercise increases Growth Hormone and glucagon (both catabolic hormones that release CHO from the liver and muscle tissue) and suppresses insulin levels which shuttles CHO into a) muscle cells b) fat cells c) the liver. Why then does glucose uptake increase during exercise?
Surprise but CHO uptake is not only regulated by insulin. There are something called GLUT transporters that are not insulin dependent; in this case GLUT-4 is the most important. Glut-4 expression increases during and after exercise helping increase glucose uptake.
So what is the fat of this glucose once it has been broken down: 1) Lacate, 2) Pyruvate
Lactate can either be reconverted to something else..and pyruvate will fuel the TCA cycle to break down fat. Do you remember when pyruvate supplements were the next big thing? It took me years before I could find any reason they would work but basically there is a phrase that goes "Fat Burns in a carbohydrate fire."
This essentially means that CHO breakdown creates the substrates necessary for fat oxidation; the more substrate; the more fat that can be oxidized.
So how does training influence this?
Well any type of endurance training increases the amount of glycogen that can be stored intramuscularly; it also increases the amount of CHO that can be taken up during High Intensity Exercise (Between 55% and 75% of VO2 max)..
Also it causes a shift in the "curve" that compares Fat oxidation vs. glycolysis for the body to be more favorable of fat oxidation vs. Glycolysis and higher percentages of VO2 max.
Now going onto proteins!
You're body doesn't like to use protein as an energy source. Why is this? It's very energy-taxing. Your body can not use protein directly as an energy source; it first needs to be de-aminate (removal of a nitrate group attached to an "R" group). The fate of proteins is then unknown; it can be converted to many things; but at a cost. Not to mention that protein is what produces urea in your piss.
True; protein can go through gluconeogenesis to become a CHO but why should the body do this when it already has CHO stored?
Not to mention that your body can't store protein. Remember the problem with creating a night time protein formula? Well protein blood levels stay elevated for about a max of 4 hours (depending on the type of protein you digested) b/c once they are converted to individual amino acids and brought into the bloodstream they will either go about their business repairing tissues, or replacing lymphocytes (Dr. LL stated in the seminar that the macrophagic cost of skeletal muscle trauma is about 30g protein/day) or they can be converted into something else (e.g. gluconeogenisis)
The take home message is that Protein is not a preferred fuel substrate.
Fat is the preferred fuel substrate for most activities, and CHO is the preferred substrate when fat cannot be oxidized fast enough.