The two most common basic forms of sugar are glucose and fructose.

Glucose is an essential sugar, not very sweet, and makes up starch. Table sugar (sucrose) is 50% glucose. It is absorbed by the bloodstream rapidly through the intestine. This is good, because tissues require a steady flow of glucose. Howver, excess amounts are toxic. Insulin is used to control the amount of serum glucose. It is released into the bloodstream whenever glucose levels go up. In the liver, insulin causes the liver to take up approximately the first 20% of the glucose entering the blood. This is glycogen replacement. The other 80% is used by various tissues in the body for energy. Because excess glucose is toxic, if there is an excess, insulin causes it to be immediately absorbed by the fat cells and stored as fat.

 Fructose is 1.5 times sweeter than glucose. It is present in large amounts in fruit and honey, and makes half of sucrose. Unlike glucose, which can be used by all the body's tissues, fructose can only be processed by the liver. When this organ has reached its maximum ability to convert it converts excess to fat, storing some in the liver and releasing the rest into the blood stream as low density lipoproteins. Excess fat in the liver impairs its ability to regulate the release of glucose into the blood stream and contributes to fatty liver disease. Excess fat or triglycerides in the blood stream cause a host of cardiovascular and metabolic problems.

Importantly, as blood glucose rises insulin rises. It mediates the uptake of glucose into the various body tissues for energy. It also causes fat in the blood stream to be taken up by the fat cells and simultaneously inhibits fat cells from releasing triglycerides into the blood stream. Thus, whenever insulin is elevated, the individual is getting fatter. Insulin is only produced when you consume glucose.

Fat stored as visceral fat, around internal organs of the body cavity, is particularly bad. Visceral fat is much more sensitive to hormones, making them more metabolically active. They are capable of storing and releasing fat more rapidly than other fat cells. When they release fatty acids, which they do all the time, they release it straight into the liver. An excess of visceral fat puts its owner at a much higher risk for metabolic disease than merely having a high BMI.

The rate of sugar ingestion and absorption is important. Modest rates of glucose release into the blood stream are processed by the liver and body tissues for energy. Modest amounts  of fructose are handled by the liver. The issues come with consuming foods that are too rapidly processed and released into the blood system and consuming to excess.

 

https://en.wikipedia.org/wiki/Glycemic_index

Glycemic index the glycemic index of a food is defined as the incremental area under the two-hour blood glucose response curve (AUC) following a 12-hour fast and ingestion of a food with a certain quantity of available carbohydrate (usually 50 g). The AUC of the test food is divided by the AUC of the standard (either glucose or white bread, giving two different definitions) and multiplied by 100.

https://en.wikipedia.org/wiki/Glycemic_load

The glycemic load (GL) of food is a number that estimates how much the food will raise a person's blood glucose level after eating it. One unit of glycemic load approximates the effect of consuming one gram of glucose.[1] Glycemic load accounts for how much carbohydrate is in the food and how much each gram of carbohydrate in the food raises blood glucose levels. Glycemic load is based on the glycemic index (GI), and is calculated by multiplying the grams of available carbohydrate in the food times the food's GI and then dividing by 100.