Complex carbs are generally starches, which are big chains of glucose molecules. The body is very good at breaking starch down quickly, which means functionally it’s similar to just eating a whole bunch of glucose.
“Sugar” generally means “sucrose”, and that means both glucose and fructose. To overgeneralize, fructose is more problematic because it is metabolized in the liver and is associated with fatty liver disease.
That means that fructose is worse for diabetics but it’s not like a lot of complex carbohydrates are any good for them anyway.
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