"Fast digesting" and "slow digesting" are not strictly correct. What's being discussed are the simple carbohydrates (mono/disaccharides) and complex carbohydrates (polysaccharides).
Simple (monosaccharides)
- Glucose
- Fructose
- Galactose
Simple (disaccharides)
- Sucrose (glucose + fructose)
- Lactose (glucose + galactose)
- Maltose (glucose + glucose)
Everything else is a polysaccharide and thus "complex".
The fast versus slow difference refers to how much effort it takes for the body to break everything down into monosaccharides for absorption. The simplicity of the mono and disaccharides means that less effort is required. In the case of glucose, you can absorb small amounts directly in the mouth, whereas polysaccharides are typically not broken down enough until they reach the small intestine or even the large intestine.
Fructose and galactose are typically converted into glucose, while the disaccharides are split into their component parts and the non-glucose part is then typically converted into glucose. Ultimately glucose is the preferred end result, and a hierarchy of conversion effort makes the complex versus simple moniker obvious.
That's your crash course on the carbs.
The way to tell between a simple and a complex carb on a nutrition label is with the carbohydrate, fiber, and sugar listings. Sugars are simple carbs, fibers are complex carbs, and the remaining grams from the total after subtracting those two are typically starches. Note that the listed fiber may only be the insoluble kind, and the remaining total would be both starches and soluble fibers. But either way you're looking at a complex carb.
So looking at a label for spaghetti, I see 31g of total carbs, 2g of fiber, and 1g of sugar. This pasta is a complex carb source because it's overwhelmingly composed of polysaccharides. Compare that with an apple where the total carbs would be around 25g with 4g of fiber and 19g of sugar. This fruit is a simple carb source because it's overwhelmingly composed of mono and disaccharides.
Are you diabetic? If not, you have no need to worry about insulin spikes and blood sugar.
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