Scientists classify starch and most dietary fiber as carbohydrates. They are each made of simple sugars hooked together into large, complex molecules called polysaccharides.
Polysaccharides vary based on the types of simple sugars that make them up and the way they are hooked together.
The most common simple sugar in most kinds of fiber and starch is glucose or blood sugar.
Depending on how glucose is put together into polysaccharides, it can be either easily digested or not digestible at all.
The most easily digested polysaccharide made from glucose is starch.
It is the common dietary carbohydrate in starchy foods. Its digestibility hinges on enzymes that we make for breaking down glucose to glucose connections.
The main enzyme for digesting starch is amylase.
This enzyme is secreted from salivary glands that begin starch digestion even before we swallow it. Amylase is also secreted from the pancreas into the small intestine, where starch digestion continues to completion.
The most common high-starch foods include:
- grains (cereal and non-cereal),
- legumes (beans, peas),
- tubers (potatoes, sweet potatoes, Jerusalem artichokes, jicama, cassava, taro), and
- roots (beets, carrots, radish).
In addition, all flours and processed foods made from these sources consist mainly of starch.
Fibers that represent the other extreme of glucose polysaccharides include the completely indigestible fiber from plants called cellulose.
Cellulose is better known as the fiber that makes up cotton.
Although we don’t eat cotton, we do eat the equivalent fiber in every veggie or fruit that contains cellulose.
Celery is a good example of a high-cellulose veggie. That’s what makes celery such a great source of bulk fiber. It passes through the GI tract relatively intact.
Cellulose-rich foods comprise a long list favorites. Typical ones include:
- Brussels sprouts,
- collard greens,
- spinach, and
- all lettuces.
Their cellulose content is what makes them high-fiber foods.
The enzyme for breaking down cellulose is called cellulose. Humans do not make this enzyme.
Cellulose and starch represent the extremes between totally indigestible and highly digestible carbohydrates. Many other types of polysaccharides span the spectrum between them. Some are made of simple sugars besides glucose, and some consist of mixtures of glucose with different kinds of sugars.
The comparative digestibility across such a wide range of polysaccharides blurs the distinction between what we call fiber and what we call starch. Some of these “tweeners” are known by different names such as soluble fibers or resistant starch.
One of the most important properties of these different types of polysaccharides is their role as foods for our friendly gut bacteria.
Such bacterial foods are now referred to as “prebiotics.”
They are every bit as important to our health as cellulose and starch.
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