Metabolic Effects of lowering Sugar intake.
Metabolic Effects of Lowering Carbohydrate & sugar on Human Metabolism.
Once Sugar and glycogen is depleted Ketone bodies are generated or supplemented and flow from the liver to extra-hepatic tissues (e.g., brain) for use as a fuel. This spares glucose metabolism within muscle fibers in a mechanism similar to the sparing of glucose by oxidation of fatty acids as an alternative fuel. In comparison with glucose, the ketone bodies are actually a very good respiratory fuel. Indeed, there is no clear requirement for dietary carbohydrates for human adults. Interestingly, the effects of ketone body metabolism suggest that mild ketosis may offer therapeutic potential in a variety of different common and rare disease states. Also, the recent landmark study showed that a very-low-carbohydrate diet resulted in a significant reduction in fat mass and a concomitant increase in lean body mass in normal-weight men. Contrary to popular belief, insulin is not needed for glucose uptake and utilization in man. Finally, both muscle fat and carbohydrate burn in an amino acid flame.
The brain will use ketone bodies whenever provided with them (i.e., whenever blood ketone body levels rise). The blood-brain barrier transporter for ketone bodies is induced during starvation or very low carbohydrate intake, further promoting the flow of ketone bodies . This transporter has a Km that exceeds the concentrations of circulating ketone bodies that occur during starvation or very low carbohydrate intake, and a Vmax well in excess of energy demands . Therefore, ketone body delivery to brain will never be limited by this transporter. However, continued use of some glucose appears obligatory  and is supplied by way of hepatic gluconeogenesis. Finally, because of the inactivation of pyruvate dehydrogenase (by the low insulin concentration), the glucose that is used by tissues outside the brain is largely only partially broken down to pyruvate and lactate, which can then be recycled in the liver trough gluconeogensis . Therefore, red blood cells, for instance, which have an obligatory requirement for glucose, are not depleting the body of glucose. Interestingly, Volek et al. recently reported that a very-low-carbohydrate diet resulted in a significant reduction in fat mass and a concomitant increase in lean body mass in normal-weight men . They hypothesized that elevated β-hydroxybutyrate concentrations may have played a minor role in preventing catabolism of lean tissue but other anabolic hormones were likely involved (e.g., growth hormone).
Original Paper in https://jissn.biomedcentral.com/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2129159/