Sugar, Starch, and Fiber – A Carbohydrate Breakdown

Carbohydrates always seem to be under debate. One moment they’re good for you, then they’re bad for you, then they’re somewhere in between, etc.

It’s made even more complicated by the fact that there are many different kinds of carbohydrates, and they all have different effects on the body.

Oh, and every person’s body is different too, making the whole understanding of all this stuff even more complex.

My goal here is to provide a little clarification regarding these nutrients, and help you understand a little better about what you put in your body each time you eat.

What is a carbohydrate?

A carbohydrate molecule (also called a sugar or saccharide molecule) is a chain of ring structure molecules made up of carbon, hydrogen, and oxygen (check out this wikipedia page for a visual representation). Different carbohydrates are classified depending on how many rings (or sugar-units) are present in the linked up structure. See the table below for further classifications:

Type Monosaccharides Disaccharides Polysaccharides
Structure 1 sugar unit 2 sugar units >2 sugar units
Chemical Examples Glucose









Food Examples Not often found alone in foods Table Sugar/HFCS



We could spend a long time talking about the metabolism of carbohydrates and such, but I prefer to keep this simple with good take home points.


Sugars (in food this are mainly disaccharides, as monosaccharides aren’t generally found by themselves in nature except for cellular mechanisms) are easily digested because of their size. This leads to quick digestion in the gut and absorption into the bloodstream, generally leading to large blood glucose and insulin spikes. For more, check out the articles here and here.

Note: this is an oversimplification, I admit it. The connection between simple sugars and insulin is very interesting and still being researched, with new findings contributing more and more to our understanding of how these molecules work together in regards to energy, metabolism, and weight gain/loss. I think what is most important to understand about sugars, though, is that they are fast digesting, and lead to many bad effects on our body, especially when they make up a major portion of a meal.


Think of polysaccharides as long chains of sugar molecules. Depending on the number of these molecules and how they are connected, they have different effects in our digestive system.

One way to classify polysaccharides is based off the way they are digested/broken down in the body: starch vs fiber.

The simple difference between these two classifications is that starch is digestible while fiber is not. We can delve even further as shown in the chart below.



Rapidly Digested Starch – This is what flour and other processed grains are made of. The issue with these carbohydrates is that they are quickly broken down into single glucose molecules, meaning there is quick absorption of glucose from the gut into the bloodstream. This has similar issues to sugars – fast blood glucose spikes and quick onset of hunger.

Generally, the more refined the grain, the faster it breaks down in the gut. Hence, whole wheat flour is technically better than white flour, though I would argue both are rapidly digested.

Slowly Digested Starch – Also called Resistant Starch, this is all the rage in the scientific and athletic performance communities. These starch molecules digest slowly in the gut, leading to a slow release of glucose into the bloodstream. This leads to stable energy and appetite suppression. Green bananas in particular are loaded with resistant starch, and many endurance athletes use resistant starch supplements for extended stable energy levels.

Soluble Fiber – Though we humans cannot digest soluble fiber, our gut bacteria can. And when they do, they release short-chain fatty acids. Before you go running off saying fat is bad, check out this articleAlso, these short-chain fatty acids have been shown to have many beneficial properties including improving the body’s response to stress and decreasing inflammation. Sources of this fiber include vegetables, fruits (especially with the peel), nuts, beans, and whole grains (real whole grains)

Insoluble Fiber – This stuff doesn’t get digested at all. Rather it just comes out the other end, helping to increase the mass of bowel movements. Doctors often tell people that are constipated to eat more fiber, referring mostly to insoluble fiber for its “bulking” abilities (I’ve never really understood why making poop bigger helps constipation, just doesn’t really makes sense). Additionally, scientific studies have shown no real improvement in constipation symptoms from addition of extra fiber. Insoluble fiber is seen in high proportion in cereals/grains as well as in more balanced proportions with soluble fiber in fruits, vegetables, and beans.

Important Point: While these different forms of carbohydrates all have specific ways they affect the human body, every person is different.

Some people are more tolerant of glucose and rapidly-digesting starch than others. Some don’t seem to need carbohydrates much at all and do fantastically on a very low carb diet. Compare those to the ones that can eat as much pizza and cake as they like and never gain a pound. This likely has a major genetic component, potentially related to ancestry – super interesting stuff we’ll have to dig into in the future.

Similarly, each person’s microbiome is unique. This also affects the way different people respond to starch and fiber. Microbiome research is just now starting to flourish, we’ll know more and more as time goes on.

Disease processes like metabolic syndrome/type 2 diabetes also affect the way people respond to these different carbohydrate forms.

The main point here is that you need to learn your body and understand what foods you respond to and what foods mess with your body. Personally I do well with a lot of vegetable based fiber/starch. But I respond very poorly to simple starches as well as beans (even though they have lots of fiber). That’s just me, you’d be different – just pay attention to your body next time you eat something.

One other thing, fiber has gotten a lot of press for being very good for you. Food companies have caught on to this and often add supplemental fiber to their processed foods, enough so they can add the “high-fiber” logo on their packaging. But often hidden behind that added fiber is a bunch of sugar and simple-starches. It’s always better to get your fiber from real foods.

About the Author

Chris Goodrich, MD

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