Table sugar, sucrose, is a disaccharide composed of a molecule of glucose and a molecule of fructose. When we eat it, an enzyme called sucrase cleaves the molecule into its components, which are metabolised separately. All cells in the body can metabolise glucose, but fructose can be handled only in the liver. Fructose, apart from lacking the immediate toxicity of ethanol, has otherwise a very similar effect on the body to that of ethanol; it is metabolised by the same metabolic pathway in the liver that handles ethanol, and therefore causes the same liver problems when consumed in any quantity. It also has the same effect on the brain’s reward center (the nucleus accumbens), so it is addictive to many people.
Elevated serum glucose, the result of eating table sugar or too much other carbohydrate, causes various other kinds of trouble. Excessive glucose in the blood stream, hyperglycaemia, causes damage and even death, so the body responds by secreting insulin to drive the excess out of the blood stream. This excess glucose gets shunted into muscles to be metabolised, or else is converted into fatty acids and stored in our adipose tissue. Excessive amounts of fructose overwhelm the liver’s metabolic pathway and are then handled in the liver by a process called de novo lipogenesis, which eventually leads to fatty liver disease, steatohepatitis, cirrhosis, and ultimately, death.
The rise in serum insulin caused by a spike in serum glucose brings its own problems. Not only does it stimulate systemic inflammation, but hyperinsulinaemia also interferes with the body’s ability to produce nitric oxide (NO), which is what keeps our arteries relaxed and flexible.
When someone eats a high-carb diet, these effects all accumulate, causing all kinds of chronic problems. I suspect that if a person has been eating a ketogenic diet long enough to reverse some of these problems, a sudden high dose of sugar could cause effects more noticeable than they were back in the days when that person was experiencing constant, ongoing damage. Whether on a ketogenic diet or on a regular high-carb diet, it is wise to avoid sugar, because of the double-whammy it gives the body. Excess glucose is bad enough, but excessive fructose causes even more serious problems.