Do carb blockers work? How much of carb can the body stop absorbing when we use these supplements?
Yes they exist and yes the work. Carbohydrates are all made of little sugar units hooked together They are big and complex like starch or simpler and smaller like sucrose or very simple and small like single units of glucose and fructose. The only way that carbs can enter your body is in the very simple form such as glucose and fructose.
So one way to prevent, or "block" carbs from entering your bloodstream is to prevent them from being broken down into the simpler form in the first place. For instance, by inhibiting the conversion of starch to glucose. Or inhibiting the conversion of sucrose (table sugar) into glucose and fructose. There are many natural chemicals in some foods that inhibit this process, such as chlorogenic acid which inhibits amylase (which allows the conversion of starch to glucose). Another name for chlorogenic acid is Green Coffee Bean Extract. Pure chlorogenic acid is VERY good at inhibiting the conversion of starch to simple sugars.
Another way to "block" carbs is to close the "doors" in our gut lining that allow the simple sugars such as glucose and fructose to go from the gut to the bloodstream. These doorways are called Glucose Transporter 2 (or GLUT2 for short). If you can inhibit GLUT2, then you can inhibit simple sugars from getting into your bloodstream. Just like with chlorogenic acid, there are foods that contain GLUT2 transporter inhibitors such as myricetin which is found in dark berries.
With large enough concentrations of phytochemical such as these, you can nearly stop carb absorption. But three things become readily apparent. One, it is almost impossible to consume enough to completely block carbs. Realistically the upper limit is 30%. Two, at those concentrations there can be side effects such as nausea and esophageal burning. And the most important of all, three, these unabsorbed carbohydrates continue down the intestines and are readily fermented by the microorganisms that live there, resulting in gas, flatulence. Lots of it. You'll spend a lot or time on the toilette. You've traded fats for farts!