High glucose levels from carbohydrate intake trigger insulin secretion. This is necessary to bring the glucose into the cell, and to get rid of it from the bloodstream where it causes damage to the tissues by glycation.
Within the cell, glucose can be either fermented without oxygen or oxidised with oxygen. Lower oxygen levels (and very high short term metabolic needs) promote fermentation. Higher oxygen levels (and lower metabolic ATP production rates) favour oxidation. More fermentation leads to greater accumulation of lactic acid, which further decreases oxygen levels. Red blood cells do not have mitochondria and therefore can only produce ATP by fermenting glucose.
Lower glucose leads to lower insulin. This triggers the release of fatty acids and glycogen into the bloodstream. If sustained, low glucose leads to the production in the liver of ketones primarily to fuel the brain whose cells can either use glucose, ketones, or medium chain fatty acids because longer molecules cannot pass the blood-brain barrier.
Everything we have described up to now about fat loss is universal in animals. But each animal is different, and each person is different. As far as fat loss is concerned, the individuality of people is related to their predispositions to insulin resistance and carbohydrate tolerance, (or actually, intolerance). Every person is differently intolerant to carbohydrates and differently predisposed to insulin resistance.
This is why in a group eating the same diet, there are people who are thin, people who are chubby, people who are obese, and everything in between. Basically, the greater the predisposition to insulin resistance (and the more sedentary), the lower the tolerance to carbohydrates will be, and the fatter you will tend to get. In contrast, the lower the predisposition to insulin resistance (and the more active), the higher the tolerance to carbohydrates, and the thinner you will tend to be.
This translates into different thresholds in the amount of carbohydrate we can eat without negative metabolic consequences, and therefore, the amount under which we must stay in order to burn fat instead of storing it. As a guideline, if you want to burn primarily fat as your body's basic fuel, this maximum amount per day would be around 20--25 grams if you are fat; around 30--50 gram if you are neither fat nor thin, and could be as much as 80--100 grams if you are very thin.
But no matter what your personal threshold happens to be, it will always be the case that the lower the intake of carbohydrates, the lower the glucose and insulin will be, and the more efficiently your body will burn fat as fuel.
G. Belanger, PhD — Kalibra Co-Founder and CSO