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Fiber: The big picture

Updated: Sep 22, 2022


Many of you have asked me to give you the big picture on Fiber. What is it? How does it relate to other carbs? So here it is. The picture below summarizes it. But with every summary, there are some subtle things that get lost, which I will highlight.


All carbohydrates are chains of sugar molecules. The simplest is obviously a single sugar molecule such as fructose or glucose. When you attach fructose to a glucose molecule, you get sucrose (table sugar). As you add more and more sugars you get to simple starches or simple fibers and beyond that to complex starches and complex fibers. The difference between a starch and fermentable or non-fermentable fiber molecules is the type of bond between the sugar units. This bond determines how or if we can break it down. This can be accomplished directly (i.e., our stomach and enzymes), indirectly (via our gut bacteria) or not at all (e.g., cellulose). One easy way to see this in action is to take some white piece of bread and start chewing it. Over roughly a minute, the enzymes in your mouth will have degraded the starch in the bread down to the sugar units, and you will feel a growing sweet taste in your mouth.


Broadly, from a health perspective, you can group all carbohydrates into simple and complex carbohydrates. The more complex, the more your body has to work to break down the carbohydrate chains, and the healthier the result. Our enzymes and stomach acid can over time degrade non-fiber carbohydrates. Fiber, however, cannot be degraded by us directly. For that, we have our gut bacteria, which can degrade many, but not all fibers. These gut-friendly fibers (prebiotics), are food for our gut bacteria. Some fibers cannot be degraded by our gut bacteria. For example, cellulose will simply pass through our system.


Most fibers found in nature tend to be complex. These can be grouped into soluble (you can dissolve them in water) and insoluble (they don't dissolve in water). Most Insoluble fibers are also non-fermentable (i.e., our gut bacteria can't break them down). They nevertheless have many health benefits such as stimulating mucus formation in our intestines. Soluble fibers are mostly fermentable and constitute the main food for our gut bacteria.


So in summary, complex carbohydrates, even though they are made of sugar units are fundamentally healthy for us because they feed the gut bacteria, bind bile acids and help reduce cholesterol, make us feel fuller, thus helping to maintain a healthy weight. Our gut bacteria, in turn, engage with many of our body systems to add years of additional healthy life to us.


So when you hear about a low-carb diet, you need to be careful not to throw out the baby with the bathwater. The amount of fiber and complex carbs are powerfully health enhancing. There is no other nutrient on earth, natural or human-made that rivals the healthy life-prolonging benefits. Now you know.


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