High Carbs vs. High Fat: Which Fuel Is Better For Athletes?

high fat vs high carb diet

Endurance athletes have long debated whether or not a high fat keto diet enhances performance.

CARBS ARE KING!” Shouts your ultra-endurance running partner.

“NO, FAT IS THE BEST FUEL SOURCE!” Says your personal trainer.

“I CAN SURVIVE ON SUNLIGHT AND AIR!” Screams that one person you’ve been meaning to unfollow on Instagram.

So, who is right?

I can say with total certainty that you can’t photosynthesize, so that leaves us left with the great carb vs fat debate.

ATP and Energy

To understand the discussion, it’s critical to have basic knowledge of how your body produces energy.

Energy is supplied to you and your cells by a chemical called adenosine triphosphate, or ATP. ATP is generated from fats, proteins, and carbohydrates.

We can eliminate protein in this debate. In healthy individuals, some protein is used to make ATP, but most is generally broken down to build essential components, like hormones, muscle tissue, and in the event of a surplus, fat.

If you ever find yourself starving, your body will convert protein into energy, and in the event of limited dietary protein, your body will break down muscle to get it. But because both a low-carb and high-carb diet also includes protein, this is unlikely to occur except for in the most extreme circumstances.

The ideal amount of protein for athletes varies widely, and although interesting, doesn’t play a major role in the low-carb vs. high-carb conversation, so we’ll leave this macronutrient for another article.

With protein being accounted for, your body has two fuel sources: carbs and fat.

Carbohydrates as fuel

carbohydrates as fuel

Carbohydrates are considered the main source of energy of a human diet. As an athlete, you’ve undoubtedly experienced the “boost” from an apple or sugary snack to avoid the dreaded bonk.

You’ve probably also noticed that other sources of carbohydrate, like endurance supplements or foods like cereal and potatoes, give you a longer boost of energy.

In both instances, you are experiencing the power of the almighty carbohydrate in two different forms: simple and complex.

The Difference Between Simple and Complex Carbs

The designation “simple” and “complex” describes the length of the molecule. Simple carbs are made of just one or two sugar molecules (monosaccharides and disaccharides).

These molecules include the monosaccharides glucose, fructose, and galactose, as well as the disaccharides maltose, sucrose, and lactose. Simple carbs are generally found in foods that are sweet, like candy, soda, and fruit, as well as vegetables and dairy products.

Simple carbohydrate breakdown:

Monosaccharides (one molecule)

  • Glucose
  • Fructose
  • Galactose

Disaccharides (two molecules)

  • Maltose
  • Sucrose
  • Lactose

Complex carbohydrate breakdown:

Complex carbs, or polysaccharides, are made of chains of glucose molecules. Complex carbs include starch, glycogen, and fiber. meaning they take longer to be broken down into glucose.

They include breads, pastas, and starchy vegetables like potatoes and legumes.

Polysaccharides (many molecules)

  • Starch
  • Glycogen
  • Fiber

Both simple and complex carbohydrates are used by your body as fuel. However, the distinction between the two is important because all carbs have to be broken down to monosaccharides before they can be absorbed in the body. That means that simple carbs are more readily available.

During digestion, carbs are broken down into monosaccharides, which are absorbed into the bloodstream. If you need immediate energy, it is dispensed to cells to create ATP. If you don’t need immediate energy, it is stored for later use.

Carbs can either be stored as the polysaccharide, glycogen, or as fat.

Two hormones, insulin and glucagon, regulate the amount of monosaccharides hanging out in your bloodstream.

When your blood sugar is high (indicating that you have more energy in your bloodstream than what’s immediately needed, insulin prompts cells to take up glucose and convert it into glycogen and/or fat.

When your blood sugar is low (indicating you need immediate energy), glucagon breaks down glycogen to release glucose into the bloodstream.

glucose and insulin during a 24 hour period

Now, that’s all well and good, but you’re wondering what happens in the absence of carbs, not when they are in abundance!


What happens after you deplete your glycogen stores?

In this case, your body needs to rely on fat.

Fats as Fuel

fats as fuel

In the absence of plenty of glucose, your body uses its stores of glycogen for fuel. But in the event that the glycogen is all used up, you have another trick: fat.

Chemically, fat doesn’t act at all like sugar, and can’t be broken down in the same way to make ATP. At least, not with limited glucose available (which is a conversation for another article).

Under these circumstances, fat is converted into compounds called ketones, which then produce ATP.

If carbs aren’t available for a long period of time, ketones accumulate in the blood, which creates a condition called ketosis.

What Is A Keto Diet?

A keto diet is a restrictive diet designed to induce ketosis.

A keto diet is often confused with a low-carb diet like South Beach or Atkins.

A typical low-carb diet consists of lots of protein and non-starchy vegetables. Usually, they limit or exclude sweets, grains, pastas, breads, fruits, and high-carb vegetables like potatoes and beans.

While keto is definitely low carb and excludes all the food groups listed above, not all low-carb diets are keto. The main difference?

Keto is low, low, LOW carb. Some keto fans reduce their carb intake to a mere 5% of daily calories.

A keto diet takes some diligent (some argue obsessive) macro tracking.

  • The ideal daily intake for each is a ratio of 70/20/10. That’s 70% fats, 20% protein, and 10% carbohydrates.
  • To induce ketosis, it’s generally recommended to eat less than 50 grams of carbs per day.
To put that into perspective, here’s the carb count of some common foods and products:
  • Dave’s Killer Bread: 22 g per slice
  • Barilla Penne pasta: 42 g per serving
  • Basmati rice: 36 g per ¼ cup
  • Gu energy gel: 22 g per packet
  • Sweet potato: 27 g per cup
  • Carrot: 6 g per medium carrot
  • Apple: 25 g per medium apple
  • Chicken breast: 0 g carbohydrate
  • Eggs: 0.6 g per large egg

The keto diet excludes foods like bread, pasta, sweets, potatoes, and fruit. Vegetables like potatoes and even carrots are often avoided. Those lost calories are replaced with fat.

A typical keto day might look something like this:


  • Coffee with ghee butter
  • Scrambled eggs, bacon, avocado, and cheese


  • Nuts
  • Kale chips


  • Steak salad with high fat dressing


  • Guacamole with veggies


  • Pork chop with cauliflower “rice”
  • A handful of blueberries

The keto diet can be pretty tasty if you’re really into avocados and butter.


...Will it make you faster?

Theorized benefits Of A High Fat Diet

There are a few theorized benefits of training your body to use fat.

Fat as vast energy source

As you now know, carbs are available in the bloodstream as glucose or stored as glycogen in muscles and in the liver. The amount of stored glycogen equals about 1680 kcal of available energy at any given time in the absence of additional carb sources (like a sugary drink).

Because of the limited amount of stored energy, athletes need to replenish their carbs after 1-2 hours of exercise and then continuously every few hours.

All that consumption redirects nutrients to aid digestion, which in theory, could reduce energy available for exercise.

Conversely, one pound of fat yields 3500 kcal, making it an incredible energy reserve. It’s possible that ketosis could also athletes to achieve longer training periods without the need to eat.

Improved recovery

Low-carb diets are proposed to lead to less inflammation and oxidative stress (due to less glycolysis), and this might help with performance and recovery.

Exercise does damage to muscles, and an “anti-inflammatory diet” could help athletes rebound quicker from hard efforts. Indeed, one study did show improved adaptations in mice.

Improved body composition

Namely by lowering body fat % and total fat mass. Indeed, the research does suggest that low-carb athletes do burn more fat during exercise.

Sounds like a decent deal, right? You get endless energy, a bonk-free life, and a super lean bod.

Actually, the argument for carbs is strong, too. Athletes have successfully relied on carbs for performance for the entire history of sports, but I’ll try to put my historical bias aside for a moment. Instead, let’s see what the research says.

High Fat diet for endurance athletes: Performance booster or placebo?

high fat diets for athletes

Now you have a basic understanding of how your body creates energy under normal circumstances using carbohydrates as the primary source of fuel. You also know that it has the ability to use fat instead via ketones. But the real question here isn’t what’s possible…

The question is which form of fuel is optimal for endurance performance.

Depending on who you ask, you’ll get varied answers across experts.

There are plenty of die-hard keto-promoting physicians, personal trainers, nutritionists, and athletes. There are even more who argue that the keto diet is either ineffectual or the gains are insignificant in relation to the sacrifices made to adhere to the strict diet.

Still others claim that ketosis is downright dangerous.

A recent review of the efficacy of a ketogenic diet on endurance performance sought to clarify its efficacy in improved endurance performance.

Keto diet and performance study breakdown

Researchers Caitlin Bailey and Erin Hennessy wanted to understand how ketosis impacted athletic performance in endurance athletes. They evaluated seven studies that looked at VO2max, or the maximum rate of oxygen consumption measured during incremental exercise.

Some studies also looked at additional measures of fitness, like time to exhaustion, race time, received exertion, and peak power.

The review looked at the following performance outcomes:

  • V02 max
  • Time to exhaustion
  • Race time
  • Perceived exertion
  • Peak power

Researchers only included studies that were peer-reviewed and evaluated endurance athletes specifically. In addition, all participants consumed less than 50 grams of daily carbohydrate, an amount generally considered to induce ketosis. Through biomarkers like ketones, all participants were confirmed to be in ketosis.

All research met the following requirements:

  • Research was peer-reviewed
  • Participants engaged in endurance exercise
  • Participants consumed < 50 g/daily carbohydrate
  • Participants achieved ketosis
Out of the seven articles reviewed, researchers found mixed results.
  • The majority (four out of seven) of articles reviewed found no significant increase in VO2max compared to the control group.
  • One found that the keto group experienced decreased time to exhaustion (meaning they became tired more quickly) while another found a higher rate of perceived exertion from ketosis.
  • Two out of seven studies found that a keto diet improved VO2 max. But both articles reported significant VO2 max increases across all diets tested. This suggests that outcomes were independent of dietary intervention.

Heatherly et al. did not report VO2 max outcomes, instead providing the percentage of baseline VO2 max achieved at various race paces.

They demonstrated that a low-carb keto diet was negatively correlated with the athletes’ aerobic efficiency.

The bottom line on low-carb, high fat diets for endurance performance

running on a keto diet

Despite the popularity of the keto diet for endurance athletes, this review should make you skeptical of its effectiveness.

It’s definitely clear that no one can say that the research shows low-carb diets are superior.

For one, there just isn’t that much research out there on ketosis and endurance performance. With only seven studies sampled, each with relatively few participants, it’s not surprising that we don’t have all the answers.

Further, there was a big diversity in the methodology and participants involved.

Research on keto diets is both limited and diverse, meaning that not only is there not enough research on keto, but the research that exists is extremely varied.

Studies are difficult to conduct and compare because there are so many considerations that impact the effectiveness of diets on performance.

For example, an elite athlete may experience different results compared to an enthusiast. Further, consider how variations in age, gender, and genetic makeup could impact the results.

Simply put, we don’t know whether a keto diet improves performance in endurance athletes.

And when you consider the mixed results of this study compared to the long history of successful high-carb diets, it might make you wonder if the sacrifice is worth the end result.

And what is that end result?

  • A deep-dive into the studies cited by Bailey et al shows that a keto diet increases the utilization of fat for energy, especially for those who can commit to the diet long-term. (Volek)
  • Yet Burke reveals that it takes about 8% more oxygen to get that energy out of fat (compared to carbs).

Translation: Relying on fat reduces economy.

Despite nearly doubling the oxidation of fat for fuel, a keto diet slowed down elite racewalkers.

Remember the discussion on how readily available carbs are as a fuel source? Well, without stored and serum carbohydrates available, you won’t be able to rapidly produce energy demands during bouts of short, high-intensity efforts.

Despite being an endurance athlete, there are still plenty of moments when you need a quick burst.

Further, it shouldn’t go unnoticed that transitioning into ketosis is not a great experience.

As carbs are the preferred energy source for your brain and your muscles, there will be a point when you have neither enough carbohydrate nor ketones to use as energy.

This experience is generally called “keto flu,” with symptoms including headache, foggy thinking, fatigue, irritability, nausea, constipation, and desperately wanting to eat a big bowl of spaghetti. During this time, training (and your personal life) could suffer.

And compliance isn’t just mentally difficult (no more beers with friends or slices of b-day cake), but impractical.

Nearly all foods made for athletes, like protein bars, energy gels, and sports drinks, rely on carbs as the main fuel source. Further, most aid stations have high-carb foods, too. Sometimes the best nutrition plan is the one that’s easiest to follow.

Finally, the keto diet isn’t risk-free. Did you know that the keto diet was form to reduce seizures in children? Most research on ketosis is around it’s efficacy in managing these seizures.

How it affects endurance athletes, on the other hand, is essentially unknown.

A low-carb high-fat diet could cause low blood pressure, kidney stones, constipation, nutrient deficiencies, and an increased risk of heart disease.

If you still want to explore the keto diet, consider trying it during your off season with the help of a physician.

About The Author:

Matt Mosman (MS, CISSN, CSCS) is a research scientist, endurance athlete, and the founder and Chief Endurance Officer at EndurElite. Matt holds his B.S. in Exercise Science from Creighton University and his M.S. in Exercise Physiology from the University of California. Matt and his family reside in Spearfish South Dakota, where they enjoy running, mountain biking, camping, and all the outdoor adventures Spearfish has to offer.


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