TL;DR: Fiber (a large, “slow” carbohydrate) and sugar (a simple, “fast” carbohydrate) are treated very differently by our bodies. Fiber helps reduce inflammation in our guts, while sugar helps provide energy during endurance exercise if we are moving faster than we can burn fat or make new glucose molecules in the liver. Eating sugar when we’re not moving leads to storing more fat on our bodies. (about 8.5 min read)

Metabolic tips for dealing with fiber vs sugar:

• Time your carbs with your movement - eat simple carbs when you’re moving so the sugar molecules get taken up by your muscles and not your fat cells.

• Get a minimum of 25g of fiber to help your liver maintain blood glucose levels when they start to fall (via glycogenolysis)

• Eat enough protein to help your liver maintain blood glucose levels when they start to fall (via gluconeogenesis)

Fiber has been getting a facelift recently. Not sure what I mean? Have you heard of “slow carbs” for athletes? Or “pre-biotics” for people who are trying to balance their gut microbiome to reduce bloating and irritation? Both of those are terms for fiber. I guess people really hate what they think “fiber” is, if there are two whole groups of marketers coming up with new words for it. Fiber is just a large carb.

First of all, we need to define what a “carb” is. Carb is short for “carbohydrate”. Chemists give the name “carbohydrate” to any molecule that has a carbon atom with a water molecule attached to it - the carbon is hydrated, carbohydrate.

Chemists are mostly concerned with the atoms that make up a molecule (here, carbon, oxygen, hydrogen) and how those molecules will react in different situations in a lab (chemists are a group of people who - no shade - don’t think about eating snacks all day so they’re less fun*). So for chemists, “carbohydrate” is descriptive enough and there’s no reason to distinguish between fiber (larger carbs, aka slow carbs) and sugar (simple carbs, aka fast carbs).

Biochemists are people who think about how molecules will react in our bodies. You may be “experimenting” with something in your life, but your body is still not a lab. The way that molecules behave inside your body is going to be determined by some rules of biology; for instance, the temperature that chemical reactions happen at inside your body is more or less constant. In a lab, you can light things on fire to catalyze a reaction (make two molecules interact or break down in some way). In your body, all of your chemical reactions (aka your “metabolism”) are going to happen at ~98°F (37°C). Even if you are a smoke show. Instead of extreme heat, your body uses proteins to move molecules and catalyze reactions.

Enzymes are what physically turn your food into your energy. Enzymes are proteins that carry out chemical reactions in your body. The reason why biochemists differentiate between fiber (large carbs) and sugar (simple carbs) is because different enzymes are responsible for breaking these molecules down. The different ways that fiber and sugar are processed by enzymes in your body has profound effects on your health.

“Slow” carbs vs. “fast” carbs: If you heard these terms before, congrats on being friends with an exercise physiologist. If you haven’t heard these terms and are about to start doing some endurance athlete stuff, then you will hear them and I want to get credit for telling you first.

“Slow” vs “fast” carbs is referring to how quickly different types of carbohydrates will get broken down so they can be moved from your guts into your bloodstream, and therefore, how fast they will raise your blood glucose levels (BGLs).

Fiber needs to be broken down into smaller pieces before it can be absorbed into your bloodstream, so it raises your BGLs slowly - it’s a “slow” carb. Simple carbs (sugar) do not need to be broken down before they can be absorbed (or, there’s only 1 chemical reaction that has to happen and it happens quickly, like in the case of sucrose and maltodextrin†). Simple carbs (sugars) raise your BGLs quickly - they are “fast” carbs.

When do you need to eat “fast carbs”? You need “fast carbs” (aka sugar) when you are running or cycling faster than your body can make new glucose molecules. This only happens when you’ve burned through your glycogen (animal starch stored in your liver and muscles) and you’re moving faster than your mitochondria can burn fat for energy.

Burning fat is also sometimes called “slow energy” but that is not because breaking fat down for energy is slow. It’s because breathing fast enough to get oxygen to all of your muscle cells has become slower than the rate at which your muscles using energy, and now your muscle cells have to find a way to make energy that doesn’t rely on using oxygen (which burning fat does). This is why a lot of professional cyclists and endurance runners are concerned about their hematocrit levels - it’s a way to measure how fast oxygen can get to their muscles so they can produce more energy, faster. And it’s also why those people eat Gu’s. I’ll talk more about this later.

But don’t you need to eat glucose to maintain your blood glucose levels? No. Your liver will produce glucose from other molecules (protein and fiber you broke down earlier) if your BGLs start getting low. Your liver makes glucose by breaking down glycogen (glycogenolysis) and making new glucose (gluconeogenesis). You do need to eat enough protein and fiber for your liver to do this.

Someone could maintain steady BGLs by eating enough protein alone and skipping all carbs (fiber included) completely if they wanted to. This is called the “ketogenic diet”** and it’s often followed by people with immune disorders or who are dealing with treatment-resistant epilepsy or migraines. Is the ketogenic (keto) diet right for you? I discuss that here, along with how adding fiber to your diet isn’t really at odds with maintaining ketogenesis.

This is what we mean when we say “time your carbs with your movement”. If you eat simple carbs (sugar) when you aren’t moving, your BGLs will raise quickly and your body will secrete a bunch of insulin to bring your BGLs back down to a safe and healthy level. If you aren’t moving or didn’t recently finish moving, your body will most likely store those simple carbs as fat. Eat carbohydrates when you know you’ll be moving so that you don’t store those sugars as excess fat.

Eating simple carbs (sugar) leads to more fat storage than eating fat. Excess dietary sugar leads to more fat accumulation than dietary fat and it also (eventually) leads to changes in your liver that raise your blood cholesterol levels more than dietary cholesterol. Processing dietary fat and cholesterol is a WHOLE THING that deserves its own discussion, so I will be discussing this in more detail in later posts. For now, if you are getting enough fiber and you are not eating simple carbs (aka sugar) that aren’t paired with movement, you don’t need to worry about how much fat you’re eating as long as you aren’t routinely over-eating.

ANYWAY. Stop saying “carbs” and start seeing fiber and sugar as different molecules. Your body sees them as different molecules. And you should too. Fiber is helping your gut stay healthy. Sugar is used in your body as a way to quickly transport a molecule that can be broken down into energy through your blood because it’s water-soluble and so it can be easily transported to different organs quickly. You do not need sugar in your diet to maintain steady BGLs (unless you’re doing some endurance athletics stuff) and you don’t have to avoid “carbs” when you’re trying to maintain steady BGLs or increase ketogenesis** if those carbs are fiber.

Metabolic tips for dealing with fiber vs sugar:

• Time your carbs with your movement - eat simple carbs to support endurance athlete stuff

• Get a minimum of 25g of fiber to help your liver maintain blood glucose levels when they start to fall (via glycogenolysis)

• Eat enough protein to help your liver maintain blood glucose levels when they start to fall (via gluconeogenesis)

*Some chemists are actually fun (Hi, Mya and Lisa). Biologists who study microbes helped us understand how to better ferment things, aka biologists figured out beer, wine, cheese, and rising bread. The famous (or infamous) physicist Dr. Richard Feynman said that biologists were his favorite scientists to hang out with because all they do is talk about snacks and sex (genetics). But to their credit, chemists helped us understand how to better distill things and favor certain chemical reactions over others, aka chemists figured out liquor and new cooking techniques.

A lot scientists will tell you not to hang out with a chemist who can’t cook bc its’ a tip-off that either they aren’t a good chemist (cooking is just doing the right chemical reactions in the right order) or they aren’t a fun chemist (cooking does take practice and ppl tend to be bad at things if they don’t practice doing them).

**Ketogenesis is the production of ketone bodies from the breakdown of fats. Increased ketone bodies in the blood, especially the molecule B-hydroxybutyrate (BHB), is associated with a more balanced metabolism and lower inflammation. This is because BHB inhibits a protein complex inside immune cells and stops them from overreacting. Read more about it here!

†Sucrose is the chemical name for table sugar.

For my nerds, sucrose a dimer of glucose and fructose (both monomers). Sucrose is very similar to High Fructose Corn Syrup (the most common HFCS is a mix of ~55% glc and 45% fru) and they have the same effect on your body. Maltose is a dimer of two glucoses, and maltodextrin is multiple glucoses together, but with a different bond than fiber. Maltose and maltodextrin all get broken down so quickly that they have basically the same effect on your metabolism as eating pure glucose.