Carbohydrates vs. Fat: How Endurance Athletes Fuel Their Training

Custom Keto Diet

Written by Doug Hay

The longstanding debate over whether you should burn fat or carbohydrates for workout and race day fuel is still raging in endurance circles, but here’s what I believe: 

There’s a time a place for both methods, you just need to know when and how to use them. 

I’m a competitive vegan triathlete, who, after years of terrible GI issues during training, switched to fueling almost entirely with whole foods. And through a lot of trial and error (and science… I have a M.S. in Nutrition), I’ve come to understand what fuel types work best for different workouts, races, and training regimens. 

In this post, I break down the different types of fuel — carbohydrates, fat, and even protein — and when and how to use them. If you follow this strategy, you’ll help eliminate bonks and sustain even energy levels throughout your session. 

How to Fuel Your Workouts and Training

1. Fueling with Clean Carbohydrates 

When it comes to fueling with carbohydrates, I like to split them into two categories: Low-calorie and high-calorie.

Low-Calorie Carbohydrates

Let’s start with low-calorie carbohydrates because they’re the most easily digestible and turn into energy almost immediately. These simple carbohydrates allow your body to use glycogen for quick energy when you need a near-instant boost or to prevent the dreaded bonk.

The downside is that it will be used up within 30-45 minutes, requiring frequent replenishment during longer workouts.  

Most processed energy products like gels, gummies, or drinks rely on these simple carbohydrates for energy, but there are plenty of whole food options as well. 

Example foods:  Fruit (dried fruit, bananas, Medjool dates), sweet potatoes.

How to Use:  Eat low cal, high carb foods when you feel a bonk coming on! You can also do this during your workout or immediately before to help provide you with a quick “revival” burst of energy.  

When to Use: Use every 30-45 minutes during long workouts or before short workouts in place of a full meal. 

Best For: High-intensity workouts, races, or as supplemental fuel during extended efforts.

High-Calorie Carbohydrates

This energy is similar to the one above, but the carbohydrates are slightly more complex. Since they are higher in calories, your body will not use the energy for glycogen as quickly because there will be more calories (energy) for your body to burn through.  

These types of foods are great for people who are going out for a slightly longer workout and only want to use low-calorie carbohydrates as a back up for anti-bonking.

Example Foods: Plain bread, cereals, other grains such as oats or oatmeal.

How to Use: Eat high-cal carb foods before a medium or long workout, and to keep you going during the extra long days! 

When to Use: Eat high carb, high calorie foods 1-3 hours before a workout longer than 90 minutes and every 2 hours during the workout. This is good to use during a workout if your stomach is sensitive and you don’t want to add fat content to mess with your GI tract. 

Best For: Endurance events longer than 90 minutes. 

2. Fueling with Carbohydrates and Fat

For lower intensity endurance efforts — hiking, ultrarunning, or backpacking — a mix of both carbohydrates and fat will be most effective. The high amount of calories from the fat helps you sustain your effort, and the stored glycogen from the carbohydrates acts as a backup. 

Fat and carbohydrates are the two highest calorie foods, so pairing these two together will provide for adequate fuel for your very long days. The glycogen stores will burn first, while the stores of fat will be ready for later in the day.

Example Foods: Toast and nut butter, granola with nut butter or mostly nut base, grain-free nut-based snacks, other sources of healthy fats paired with either low-calorie or high-calorie carbohydrates.

How to Use:  Long endurance days over 4 hours that require a bit more than just endurance — activities that will break into your muscle stores. 

When to Use: 1-3 hours before a workout, and every 1-2 hours during. 

Best For:  Hiking, biking, backpacking, trail running, or Spartan type obstacle courses. 

3. Fueling with a Balanced Mix of Protein, Carbohydrates, and Fat

What about protein

The idea behind fueling with a balanced mix of carbohydrates, fat, and protein is to increase the calorie intake while also increasing the nutrient intake.  

Not only will you be providing glycogen storage, but you will also help to replenish any torn muscle tissue and fatigue the following day.  

These meals may not be super high in calories, but the balance provides satiation and likely you will only need low-calorie, quick-burning energy for a backup.

I mostly use this method when training for a triathlon with back-to-back-to-back training blocks.

Example foods: Protein bars, energy bars, trail mixes with equal balances of nuts, fruit, oats. Smoothies, or full balanced meals.  

How to use: Before a medium or long workout, particularly between back-to-back sessions or a high-intensity race effort.

When to Use: Best 1-3 hours before a workout over 90 minutes, and every 2 hours throughout your workout. Also right after a workout for recovery or between back-to-back efforts. 

Best For: Peak training, race day, high-intensity workouts over 90 minutes, recovery. 

4. Fueling with Fat 

Fueling with only fat has become an increasingly popular topic over the past several years. The idea is pretty simple: 

Instead of relying on carbohydrates for energy, your body turns to slow, evenly burning fats, of which your body has plenty. Some ultrarunners and endurance athletes have had success with this method, but it takes training the body and running in a carb-depleted state.

For most of us, burning fat for fuel is best for low-intensity, cold weather activities like winter sports, mountaineering, or swimming. 

When the human body is exercising in the cold, not only is it burning extra heat calories (glycogen) from exercising, but it is also trying to warm the body up. Thus, it is burning fat stores like crazy.

Example foods: Avocados, nuts, seeds, nut butter, healthy oils. 

How to use:  Long endurance days over 4-6 hours. 

When to Use: Every 1-2 hours during a workout and in between sessions. 

Best For: Weight lifting, rock climbing, swimming (especially in colder water), mountaineering, and other mostly muscle and strength training sports that last longer than 90 minutes.

There’s No Right or Wrong Way to Fuel a Workout

Sure, some fueling techniques are more effective for specific types of training, but fueling for athletics can be very personal.

What works for me might not work for you, and you may have a ton of success with a technique my body couldn’t handle. 

So what should you do? 

  1. Start with the simple outline above to act as a guide. 
  2. Experiment. Try different techniques during different outings to see how your body reacts. 

It takes a lot of practice — and patience — to master the art of fueling your efforts.

But when you get it right, your workouts will feel like magic.

About the Author: Izzy Fischer is an Ironman 70.3 World Championship qualifier and Nutrition M.S., and happens to be No Meat Athlete’s newest team member.



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