The Truth About Carbs
An in-depth breakdown of carbohydrates and their impact on training, energy, and recovery.
Carbohydrates play a crucial role in fueling the human body during everyday functions, and an even greater role in sports. Coaches, especially distance coaches, often tell their athletes to eat plenty of carbs or to “carb load” before a big training session. But with so many types of carbohydrates, food sources, and opinions on “good” vs. “bad” carbs, the message often gets lost. So let’s break down what carbohydrates are, how they function in the body, and why they matter so much for endurance athletes. Once you understand how to use them, you can fuel smarter and run stronger.
What Is a Carbohydrate?
A carbohydrate is one of the three main macronutrients essential for the human body. While bread and pasta are obvious examples, carbs are also the main ingredient in soda, candy, fruit, and yogurt. Chemically, a carbohydrate is an organic sugar molecule, or saccharide, built from carbon, hydrogen, and oxygen. These structures often take a ring shape.
At their core, carbs are sugar molecules, and they come in different sizes:
The most basic form of a carbohydrate is a monosaccharide (mono = one, saccharide = sugar). These include:
Glucose – the body’s main fuel source (C₆H₁₂O₆)
Fructose – found in fruit
Galactose – found in dairy
All carbs we eat eventually break down into glucose, which is vital for energy. For runners, glucose powers muscle contraction, brain function, and physical endurance.
From there, sugars can link up to form:
Disaccharides – two sugars joined together (e.g. sucrose = glucose + fructose, lactose = glucose + galactose)
Polysaccharides – long chains of sugars, including:
Starch (plant-based, digestible)
Fiber (plant-based, mostly indigestible)
Glycogen (animal-based, stored in muscle and liver)
Types of Carbohydrates
Simple vs. Complex Carbs
Carbs are typically categorized as simple or complex depending on their structure and digestion speed.
Simple Carbs: Mono- and disaccharides that digest quickly and raise blood sugar rapidly. Found in:
Candy, soda, baked goods
Fruit and dairy (naturally occurring)
These are often referred to as “empty calories” because they frequently come from sources with little to no nutritional value (micronutrients such as vitamins, electrolytes) beyond energy.
Complex Carbs: Polysaccharides like starches and fibers that digest slowly and provide sustained energy. Found in:
Whole grains, legumes, starchy vegetables (like potatoes, corn, and squash)
Starches: Amylose vs. Amylopectin
Here’s where things get a little more nuanced. Not all starches are created equal:
Amylose: A linear starch molecule that digests slowly due to limited enzyme access.
Amylopectin: A branched starch molecule that digests faster due to more surface area.
Foods high in amylose include legumes, whole grains, and green bananas. Foods high in amylopectin include white rice, potatoes, and many processed starches.
Fiber: Soluble vs. Insoluble
Fiber is a complex carb that the body can’t fully digest:
Soluble fiber absorbs water, forming a gel that slows digestion and feeds healthy gut bacteria. Found in oats, beans, and some fruits.
Insoluble fiber adds bulk to stool and speeds up digestion. Found in whole grains, vegetables, and bran.
Fiber helps stabilize blood sugar, improves gut health, and supports recovery.
Why Carbohydrates Matter
Carbohydrates are broken down into glucose, which powers virtually every cell in the body — especially muscles and the brain. Glucose is the preferred energy source during moderate to high-intensity exercise because it produces more usable energy (ATP) more quickly than fat or protein.
ATP (adenosine triphosphate) is the “currency” of energy for our cells. Glucose is the fastest and most efficient way to make ATP.
Protein can be converted into glucose via gluconeogenesis, but this only happens during carb-depleted states and is not efficient.
Fat cannot be turned into glucose, and fat metabolism is slower and less efficient.
The body stores glucose as glycogen in the muscles (~500g) and liver (~100g). During exercise, muscles rely on glycogen as a quick-access energy source. When glycogen runs out, you hit the wall, this is called bonking.
Glycogen supercompensation (carb loading) is the process of maximizing glycogen storage before a major effort. By eating more carbs than usual (10–12g carbs/kg body weight/day), the body can increase muscle glycogen from ~500g to as much as ~750g.
This extra fuel is critical for long-distance events. Since glycogen binds with water (3–4g water per 1g glycogen), you may notice slight weight gain, but it’s just hydrated fuel, not fat.
Supercompensation takes about 2–3 days because glycogen storage is a gradual process that depends on enzymes like GLUT4 to shuttle glucose into muscle cells.
How Much Do Runners Need?
The average adult needs 45–65% of daily calories from carbohydrates. That’s about 225–325g/day based on a 2,000-calorie diet. But endurance athletes need significantly more:
Daily training: 5–10g carbs per kg of body weight
Supercompensation: 10–12g/kg in the 2–3 days before a race or hard effort
Example: A 140-lb (63.5kg) runner needs:
318–635g/day for training
635–760g/day for supercompensation
What About Simple Sugars During a Run?
There’s one time when simple carbs are your best friend: during training or racing. That’s where energy gels come in. Products like GU or Maurten deliver ~20–25g of simple carbs (mostly glucose and fructose) in a fast-absorbing form.
These sugars spike blood sugar and provide quick fuel right when you need it. Some gels also contain electrolytes and caffeine for added benefit. Think of them as portable glucose packets; convenient, efficient, and race-day ready.
Carbohydrates play a crucial role in fueling the human body during everyday functions, and an even greater role in sports. Coaches, especially distance coaches, often tell their athletes to eat plenty of carbs or to “carb load” before a big training session. But with so many types of carbohydrates, food sources, and opinions on “good” vs. “bad” carbs, the message often gets lost. So let’s break down what carbohydrates are, how they function in the body, and why they matter so much for endurance athletes. Once you understand how to use them, you can fuel smarter and run stronger.
What Is a Carbohydrate?
A carbohydrate is one of the three main macronutrients essential for the human body. While bread and pasta are obvious examples, carbs are also the main ingredient in soda, candy, fruit, and yogurt. Chemically, a carbohydrate is an organic sugar molecule, or saccharide, built from carbon, hydrogen, and oxygen. These structures often take a ring shape.
At their core, carbs are sugar molecules, and they come in different sizes:
The most basic form of a carbohydrate is a monosaccharide (mono = one, saccharide = sugar). These include:
Glucose – the body’s main fuel source (C₆H₁₂O₆)
Fructose – found in fruit
Galactose – found in dairy
All carbs we eat eventually break down into glucose, which is vital for energy. For runners, glucose powers muscle contraction, brain function, and physical endurance.
From there, sugars can link up to form:
Disaccharides – two sugars joined together (e.g. sucrose = glucose + fructose, lactose = glucose + galactose)
Polysaccharides – long chains of sugars, including:
Starch (plant-based, digestible)
Fiber (plant-based, mostly indigestible)
Glycogen (animal-based, stored in muscle and liver)
Types of Carbohydrates
Simple vs. Complex Carbs
Carbs are typically categorized as simple or complex depending on their structure and digestion speed.
Simple Carbs: Mono- and disaccharides that digest quickly and raise blood sugar rapidly. Found in:
Candy, soda, baked goods
Fruit and dairy (naturally occurring)
These are often referred to as “empty calories” because they frequently come from sources with little to no nutritional value (micronutrients such as vitamins, electrolytes) beyond energy.
Complex Carbs: Polysaccharides like starches and fibers that digest slowly and provide sustained energy. Found in:
Whole grains, legumes, starchy vegetables (like potatoes, corn, and squash)
Starches: Amylose vs. Amylopectin
Here’s where things get a little more nuanced. Not all starches are created equal:
Amylose: A linear starch molecule that digests slowly due to limited enzyme access.
Amylopectin: A branched starch molecule that digests faster due to more surface area.
Foods high in amylose include legumes, whole grains, and green bananas. Foods high in amylopectin include white rice, potatoes, and many processed starches.
Fiber: Soluble vs. Insoluble
Fiber is a complex carb that the body can’t fully digest:
Soluble fiber absorbs water, forming a gel that slows digestion and feeds healthy gut bacteria. Found in oats, beans, and some fruits.
Insoluble fiber adds bulk to stool and speeds up digestion. Found in whole grains, vegetables, and bran.
Fiber helps stabilize blood sugar, improves gut health, and supports recovery.
Why Carbohydrates Matter
Carbohydrates are broken down into glucose, which powers virtually every cell in the body — especially muscles and the brain. Glucose is the preferred energy source during moderate to high-intensity exercise because it produces more usable energy (ATP) more quickly than fat or protein.
ATP (adenosine triphosphate) is the “currency” of energy for our cells. Glucose is the fastest and most efficient way to make ATP.
Protein can be converted into glucose via gluconeogenesis, but this only happens during carb-depleted states and is not efficient.
Fat cannot be turned into glucose, and fat metabolism is slower and less efficient.
The body stores glucose as glycogen in the muscles (~500g) and liver (~100g). During exercise, muscles rely on glycogen as a quick-access energy source. When glycogen runs out, you hit the wall, this is called bonking.
Glycogen supercompensation (carb loading) is the process of maximizing glycogen storage before a major effort. By eating more carbs than usual (10–12g carbs/kg body weight/day), the body can increase muscle glycogen from ~500g to as much as ~750g.
This extra fuel is critical for long-distance events. Since glycogen binds with water (3–4g water per 1g glycogen), you may notice slight weight gain, but it’s just hydrated fuel, not fat.
Supercompensation takes about 2–3 days because glycogen storage is a gradual process that depends on enzymes like GLUT4 to shuttle glucose into muscle cells.
How Much Do Runners Need?
The average adult needs 45–65% of daily calories from carbohydrates. That’s about 225–325g/day based on a 2,000-calorie diet. But endurance athletes need significantly more:
Daily training: 5–10g carbs per kg of body weight
Supercompensation: 10–12g/kg in the 2–3 days before a race or hard effort
Example: A 140-lb (63.5kg) runner needs:
318–635g/day for training
635–760g/day for supercompensation
What About Simple Sugars During a Run?
There’s one time when simple carbs are your best friend: during training or racing. That’s where energy gels come in. Products like GU or Maurten deliver ~20–25g of simple carbs (mostly glucose and fructose) in a fast-absorbing form.
These sugars spike blood sugar and provide quick fuel right when you need it. Some gels also contain electrolytes and caffeine for added benefit. Think of them as portable glucose packets; convenient, efficient, and race-day ready.
Carbohydrates play a crucial role in fueling the human body during everyday functions, and an even greater role in sports. Coaches, especially distance coaches, often tell their athletes to eat plenty of carbs or to “carb load” before a big training session. But with so many types of carbohydrates, food sources, and opinions on “good” vs. “bad” carbs, the message often gets lost. So let’s break down what carbohydrates are, how they function in the body, and why they matter so much for endurance athletes. Once you understand how to use them, you can fuel smarter and run stronger.
What Is a Carbohydrate?
A carbohydrate is one of the three main macronutrients essential for the human body. While bread and pasta are obvious examples, carbs are also the main ingredient in soda, candy, fruit, and yogurt. Chemically, a carbohydrate is an organic sugar molecule, or saccharide, built from carbon, hydrogen, and oxygen. These structures often take a ring shape.
At their core, carbs are sugar molecules, and they come in different sizes:
The most basic form of a carbohydrate is a monosaccharide (mono = one, saccharide = sugar). These include:
Glucose – the body’s main fuel source (C₆H₁₂O₆)
Fructose – found in fruit
Galactose – found in dairy
All carbs we eat eventually break down into glucose, which is vital for energy. For runners, glucose powers muscle contraction, brain function, and physical endurance.
From there, sugars can link up to form:
Disaccharides – two sugars joined together (e.g. sucrose = glucose + fructose, lactose = glucose + galactose)
Polysaccharides – long chains of sugars, including:
Starch (plant-based, digestible)
Fiber (plant-based, mostly indigestible)
Glycogen (animal-based, stored in muscle and liver)
Types of Carbohydrates
Simple vs. Complex Carbs
Carbs are typically categorized as simple or complex depending on their structure and digestion speed.
Simple Carbs: Mono- and disaccharides that digest quickly and raise blood sugar rapidly. Found in:
Candy, soda, baked goods
Fruit and dairy (naturally occurring)
These are often referred to as “empty calories” because they frequently come from sources with little to no nutritional value (micronutrients such as vitamins, electrolytes) beyond energy.
Complex Carbs: Polysaccharides like starches and fibers that digest slowly and provide sustained energy. Found in:
Whole grains, legumes, starchy vegetables (like potatoes, corn, and squash)
Starches: Amylose vs. Amylopectin
Here’s where things get a little more nuanced. Not all starches are created equal:
Amylose: A linear starch molecule that digests slowly due to limited enzyme access.
Amylopectin: A branched starch molecule that digests faster due to more surface area.
Foods high in amylose include legumes, whole grains, and green bananas. Foods high in amylopectin include white rice, potatoes, and many processed starches.
Fiber: Soluble vs. Insoluble
Fiber is a complex carb that the body can’t fully digest:
Soluble fiber absorbs water, forming a gel that slows digestion and feeds healthy gut bacteria. Found in oats, beans, and some fruits.
Insoluble fiber adds bulk to stool and speeds up digestion. Found in whole grains, vegetables, and bran.
Fiber helps stabilize blood sugar, improves gut health, and supports recovery.
Why Carbohydrates Matter
Carbohydrates are broken down into glucose, which powers virtually every cell in the body — especially muscles and the brain. Glucose is the preferred energy source during moderate to high-intensity exercise because it produces more usable energy (ATP) more quickly than fat or protein.
ATP (adenosine triphosphate) is the “currency” of energy for our cells. Glucose is the fastest and most efficient way to make ATP.
Protein can be converted into glucose via gluconeogenesis, but this only happens during carb-depleted states and is not efficient.
Fat cannot be turned into glucose, and fat metabolism is slower and less efficient.
The body stores glucose as glycogen in the muscles (~500g) and liver (~100g). During exercise, muscles rely on glycogen as a quick-access energy source. When glycogen runs out, you hit the wall, this is called bonking.
Glycogen supercompensation (carb loading) is the process of maximizing glycogen storage before a major effort. By eating more carbs than usual (10–12g carbs/kg body weight/day), the body can increase muscle glycogen from ~500g to as much as ~750g.
This extra fuel is critical for long-distance events. Since glycogen binds with water (3–4g water per 1g glycogen), you may notice slight weight gain, but it’s just hydrated fuel, not fat.
Supercompensation takes about 2–3 days because glycogen storage is a gradual process that depends on enzymes like GLUT4 to shuttle glucose into muscle cells.
How Much Do Runners Need?
The average adult needs 45–65% of daily calories from carbohydrates. That’s about 225–325g/day based on a 2,000-calorie diet. But endurance athletes need significantly more:
Daily training: 5–10g carbs per kg of body weight
Supercompensation: 10–12g/kg in the 2–3 days before a race or hard effort
Example: A 140-lb (63.5kg) runner needs:
318–635g/day for training
635–760g/day for supercompensation
What About Simple Sugars During a Run?
There’s one time when simple carbs are your best friend: during training or racing. That’s where energy gels come in. Products like GU or Maurten deliver ~20–25g of simple carbs (mostly glucose and fructose) in a fast-absorbing form.
These sugars spike blood sugar and provide quick fuel right when you need it. Some gels also contain electrolytes and caffeine for added benefit. Think of them as portable glucose packets; convenient, efficient, and race-day ready.