Not all carbohydrate are sugars but all sugars are carbohydrates. That is a very common mistake that is made (and sugars are NOT converted in the stomach to glucose but rather in the small intestine) . To understand the difference some basic chemistry is required. Sugars are relative simple molecules that can be viewed , in essence, as a five or six sided ring with hydroxyl (OH) groups stuck at at the apexes. There are a whole bunch of one ring sugars around but the two diabetics will commonly come across are glucose (of course) and fructose. Note that the suffix -ose is used for sugars. (As an aside biochemists actually prefer to use the term "saccharide" rather than sugar and that is what I will use from here). These simple sugars are known as monosaccharides.
Stick two monosaccharide units together and you get a disaccharide. (imagine like a pair of handcuffs)
For example: lactose (milk sugar) is made up of a glucose and a galactose unit; sucrose (table sugar) is a glucose plus a fructose molecule, maltose - two glucose units
These simple(ish) compounds are all soluble in water .... (very very important !)
Trisaccharides are not common but I will mention one: raffinose. Now raffinose consists of three separate monosaccharides: glucose, fructose and galactose.
The thing about raffinose (very common in beans, sprouts etc etc) is that humans don't possess a particular enzyme that cracks the link to galactose, so the molecule passes down the GI tract to the lower intestine where it is fermented by bacteria ... producing gas! That is the reason why those veggies make you fart!
Beyond tri, saccharides are classified as "oligo" and "poly".
An oligo saccharide consists of a few monosaccharides (typically up to 10) linked together and a "poly" is above 10.
Going to ignore oligo's and move to polysaccharides which are less and less soluble in water and are used as storage materials in living systems . There are two which are important in diabetes: starch and glycogen. These are NOT sugars but consist of multiple monosaccharide units (typically glucose) linked together in a complex branch structure.
Starch consists of two polysachharides: amylose and amylopectin. Amylose is linear (imagine a railway freight train) where as amylopectin is branched. The proportion of amylose to amylopectin is critically important when we discuss the GI Index in a mo ...
Simple saccharides (mono and di) are very quickly absorbed into the body (very soluble) whereas polysaccharides need to be processed (broken down) first.
This brings us onto the GI Index which is a measurement of how quickly a particular food can raise blood glucose levels
It is the rate/ease of conversion of a saccharide to glucose that gives rise to the GI Index.... If a substance requires a fair amount of processing to form glucose then it will have a low GI index; if not a high.
It is this that explains some anomalies of which the fact that fructose has a low GI index is the most obvious. Fructose must be absorbed into the body, sent to the liver and only then converted into glucose, whereas maltose has a sky high one! Sucrose (glucose + fructose) intermediate.
Back to starch.
Although I said that amylose is linear, that is not the whole story. It is curled up (imagine a worm) and, resists breakdown by enzymes and also by boiling, hence has a low GI. Amylopectin is much more more easily broken down in such a mannerand will have a high GI ... Additionally anything which slows down the attack of starch by enzymes, boiling etc will cause a GI to be lowered - that is why wholefoods have a lower GI than processed.
So to sum up:
A food that has a high proportion on amylose to amylopectin will have a low GI index. Anything which protects the starch from breakdown such as fibre, husk etc will also cause the GI index to be lowered.....