For years, carbohydrates have stood as the go-to fuel for any type of activity. With the recent popularization of the low carb-high fat and ketogenic diets, more research has been conducted on whether an emphasis on fat for fuel or is a better way to go—due to the benefit of fat oxidation. In other words, training our bodies to better utilize fat stores for energy. 

In this article, we’ll elucidate the role of carbs and fat when it comes to various types of training, how much you should consume and when. We’ll then shift our focus to the most recent research on the fat adapted and ketogenic diets and deduce whether there are any performance benefits derived from forcing our bodies into ketosis—in the short and long term (to better optimize fat utilization).

Carbohydrates and Fat’s Role in Training

Carbohydrates (CHO) are our body’s main (and preferred) source of energy and are the least calorie dense of the macronutrients, weighing in at 4k/cal. The carbohydrates we consume are broken down into glucose in the body, which provides ready fuel for working muscles during exercise.

Fat, on the other hand, is a slower, less preferred source of energy. It is the most calorie dense of the macros (weighing in at 9k/cal). When we consume dietary fat, our bodies go through the process of fat oxidation which breaks down fatty acids and turns those calories into energy. 

During exercise, our bodies will burn both glycogen and fat as fuel, but will choose its energy source based on the type of activity, the intensity and duration. Let’s deduce each.

Type of Activity and Intensity 

In high intensity exercise, our muscle’s first resource is glycogen that’s derived from carbohydrates. When the intensity of our workouts increases, we use more of our type II muscle fibres (which have less oxygen than our type I fibres) and as a result, our bodies are more reliant on carbohydrates to produce lactic acid. The lactate produced is then transported through the bloodstream to be used as fuel. 

Fat, on the other hand, is stored in our muscles and primarily used by our bodies during low (< 40% VO2max) to moderate intensity exercise (~40-65% VO2max). Fat oxidation is slow moving compared to carbohydrate oxidation, but our capacity to store glycogen is limited. Our fat reserves are much, much larger—giving us a bigger fuel tank to pull energy from during exercise. If we’re on a longer, easy steady-state run (over an hour), fat will also be the primary source of fuel; helping us sustain our efforts for longer periods of time. 

The Crossover

There’s a point of exercise intensity when we hit our respective VO2 max threshold and our bodies make the switch from using fat as the primary source of fuel to carbohydrates and vice-versa. This switch is called the crossover. If we start out with sprinting (anaerobic exercise), our bodies will draw from our glycogen stores, but if we’re holding a slow jog (aerobic), the body will start relying on fat as fuel. The main reasoning is that with aerobic exercise, oxygen is now more accessible to be used to oxidize fat molecules. When you’re sprinting at full speed, breathing becomes laboured and less oxygen is available, but when you’re jogging or walking at a brisk pace, breathing is more controlled because more oxygen is available.

Exercise Duration

Exercise duration is another variable that alters when our bodies make the switch from using carbs to fat. For aerobic exercise over 30 minutes in duration, our bodies will start to rely on fat as the major source of fuel vs. glycogen.

Now that we know the physiological basics of how our bodies use carbs and fats as a source of energy, let’s move on to nutrition timing and how to utilize each macronutrient to improve performance. 

Nutrition Timing Strategies

The suggestions I’m about to present are based on scientific literature, but I always advise readers to first delve into self-experiment. While these studies may be a good rule of thumb to follow, nutrition timing strategies and recommended quantities are highly individualized. These suggestions are also for those who perform better in a fed state. If you work out in a fasted state, this section won’t apply so you can skip over it.

Pre-Workout

Running

If you’re doing any sort of aerobic exercise, consuming a mix of carbohydrates and fats with a bit of protein (while going heaviest on the carbs) is a good way to go. That way, you can fuel up on glycogen as the first source and use the fats as a back-up. Studies show the recommended intake of carbs pre-workout is 0.25-0.4 grams of CHO per pound of body weight. So for my weight of 139 lbs, I would consume a pre-workout meal of 35-55g of carbohydrates an hour (or less) prior to my workout. This is equivalent to either a full bagel, two slices of bread or a banana and half a bagel.

If I’m planning a 2-hour run, I’d double my intake and use the following formula:

0.25 x 139lbs x 2 = 70g
0.4 x 139 lbs x 2 = 111g

So, prior to my workout, I’d consume ~70-111g of carbohydrates (as long as my body and digestive track can handle it). Some people can’t digest that many carbs pre-workout (myself included). Either give yourself more time to digest or bring extra race nutrition in the form of gels, chews or sports drinks to consume intra-workout. In terms of timing, research has shown conflicting results. Some studies show performance benefits an hour before and some 3-4 hours, but most generally, they fall into the 1-3 hour range depending on how fast your body can digest. 

Some of us can digest much faster than others—we all have different metabolisms. For me, I need to eat at max an hour before my workout. Otherwise, I find myself looping back to the kitchen. But this depends on how much I ate the night before, the proximity to bedtime, what time I workout during the day, and so on. Generally speaking, it’s best to experiment with nutrition timing when it comes to carbs and measure how it impacts your performance and energy levels in the gym.

Weightlifting

Weightlifting doesn’t require nearly as much glycogen as aerobic exercise for energy—unless of course, you’re doing some crazy hybrid training like Orange Theory or F45. The average weightlifter doesn’t deplete as much glycogen when working out as, say, a runner, and can get away with eating fewer carbs and supplementing pre-workout fuel with more protein and/or fat. Some studies have shown that weight lifting results in only a 26-28% glycogen depletion. Whereas with running, after 90-120 minutes, it can result in a 100% depletion.

Gender Considerations 

Some more recent scientific evidence has shown that carb intakes affect performance between males and females differently. One study showed that women burn more lipid (fat) and less carbs than men during endurance exercise due to the role of the estrogen (specifically the 17-beta estradiol hormone). This means that women may benefit from consuming more fats than carbs pre-workout than men do.

Even though studies have shown carbs to be more beneficial to men than women, please don’t just take this at face value—some women perform better with a higher carb intake (myself included) and some men perform better with fat and low carb. I encourage you to try it out for yourself. 

Carb and ‘Fat’ Loading Strategies

Next, we’ll turn to some strategies involving loading up on certain macronutrients prior to an endurance event.

If you’ve trained for any sort of endurance event, you’ve likely come across the carb loading strategy. As you start to taper for an event and bring down your mileage in the weeks and days leading up to a race, you want to conserve energy and fill up your glycogen stores as much as possible. Our bodies have the capability to store up to ~1,600 calories worth of carbohydrates in our muscles and liver at a given time. Topping off our fuel tanks will give us that readily available energy to perform our best on race day.  

When undergoing a “carb load” cycle, you want to aim to get at least 70% or more of your daily calories from a carbohydrate source, while simultaneously reducing your training leading up to race day. Low glycemic index sources are the best for sustaining prolonged energy without negatively affecting glucose levels (and the subsequent swings in energy). but personally, I like to mix both low and high glycemic in the form of gnocchi, ice cream and chocolate the night before a big race. By consuming carbs, you’re maximizing glycogen stores but you don’t want to neglect the other vital macronutrient: protein. Our bodies also use amino acids while exercising. 

This strategy has worked really well for me over the years for marathons and ultras. While some athletes will carb load for up to 5 days leading up to an event, for me, “carbing up” within 24 hours of the event—and then 2 hours prior—has done the trick. I also use this strategy in the midst of my marathon training. Say I have a 30k or 18-mile run planned the next day. The night before, I’ll go heavy on the carbs and have found that I always have a surplus of energy on my runs. If you haven’t tried this strategy before, it’s been a tried and tested strategy for years—give it a go on your next big race 🙂

Fat Loading

On the opposite end of the spectrum is fat loading. Have you heard of it? Maybe, but probably not. The theory behind fat loading is that rather than consuming high amounts of calories from carbs, you load up on the fats instead. Apparently, consuming these extra fats will “protect” your glycogen stores and increase intramuscular stores of fat, which will improve fat oxidation. In other words, it could help improve your body’s ability to burn fat and in turn, increase performance. Makes sense, right?

There’s two ways to do this. The first is called acute fat loading where you consume 60-90% of your calories from fat approximately 3-4 hours before an event—typically from a single meal. The second is called chronic fat loading (AKA the ketogenic diet to low carb diets), where you’ll consume most of your calories from fat five or more days before an event.

So does the fat loading strategy actually improve performance in endurance sports? Sadly, no. Studies have shown that fat loading has proven ineffective and can even hinder performance if swapping out your carbs with fat. The most frequent finding showed that fat loading actually increases the rate of perceived exertion. Meaning, that endurance exercise will feel harder when fuelled by high fat at the expense of carbs. 

So, if you’re going to choose one of the “loading strategies” before a big race, definitely side with carbs.

Intra-Workout

Intra refers to during our workouts. Depending on how long we exercise for, ingesting calories (mostly from a carbohydrate source) can help reduce fatigue and extend our efforts. Aside from duration, whether we decide to fuel during our workout depends on a few other variables including the type of activity, intensity, and how well we can digest our calories. 

Running

When it comes to anything under the 45 minute mark, you can most likley get away with doing your run fasted or your pre-workout meal will probably suffice. Unless I’m tackling a longer duration run (13 miles or more), I don’t like to consume any calories – it makes me feel sick to my stomach and actually hinders my performance. I usually just have a protein bar (~150 cals) pre-workout or I’ll do mine in a fasted state (if I’m running 5-6.25 miles). 

Endurance Events

No surprise here, but several studies have shown consuming carbs during endurance events can enhance performance and most importantly, help prevent you from passing out. When it comes to fuelling during races (half marathons and marathons), a fast absorbing form of carbs (ie. gels, chews, sport drinks, etc.) taken within 10-30 minute intervals is typically recommended. I put together a full breakdown of how to fuel for different race distances in my Ultimate Nutrition Guide for Runners, co-written by ultramarathon experts. 

Prior to race day, it’s absolutely vital to practice your nutrition strategy during your long training runs. With longer distances, your body will be in extreme GI stress, making it a less than ideal time to start experimenting with different foods. Every person is different, so what you decide to ingest is completely up to you; you know your body better than anyone else.

In terms of intra-workout ingestion, as long as your glycogen stores are full, you typically don’t need to consume any additional carbs until about 75–90 minutes into the race.

The majority of us won’t spend over 45 minutes on strength training, but if you do, it’s advisable to also ingest some nutrition in the midst of your workout. Sports drinks, gels or candies are the usual go-tos for this, as we can ingest them quickly and don’t have the digestion issues to boot.

Weightlifting

Studies show that consuming carbohydrates alone or with a protein source during resistance training will increase our glycogen stores and can also offset muscle damage. If you do plan on working out longer than 45 minutes, here’s a helpful little guide I found on bodybuilding.com to follow as a rule of thumb:

Post-Workout

What and how much you consume post-workout depends on the activity you’re undertaking and the duration. Protein is the most important macro to focus on post-workout—it will help slow muscle protein breakdown and improve muscle protein synthesis, which will lead to better repair and recovery for muscle growth. Studies have shown some differing results when it comes to post-workout protein consumption but aiming for 20-40g is always a safe bet. In terms of timing, the “anabolic window” myth of consuming protein within 1 hour has been busted. Research has shown that the window has been extended to 4-6 hours (including your pre-workout meal). However, if you tend to workout fasted, it’s important to get protein in as quickly as possible (ideally within an hour after your workout). 

After a long run, you’ll also want to replenish your depleted glycogen stores quickly. Higher glycemic index foods may allow for a more rapid glycogen recovery versus lower glycemic index foods. The longer we wait to ingest a carb source post-workout, the less impact we’ll have on glycogen recovery. A good rule of thumb for carb intake is to consume ~0.5-0.7grams of carbs per pound for body weight within 30 minutes of exercise to help rebalance glycogen. 

Fats

A popular belief is that you should avoid fats post-workout as they supposedly slow down the digestion and absorption of other macronutrients, such as glycogen replenishment. However, studies have shown that ingesting fat has no effect on glycogen synthesis. While indeed it can slow down ingestion, it won’t hinder the ability for protein or carbs to replenish glycogen or inhibit muscle protein synthesis in any way. But when you are choosing a post-workout meal, it’s important to opt for the protein and carbs (if you were doing an aerobic workout).

Ketogenic and Fat Adapted on Performance

The second part of this post is for those who are on the ketogenic diet, ‘fat adapted’, or want to learn more about the performance aspect of fat adapted to our training. We won’t be covering the ketogenic diet on our fat loss or muscle building efforts, but rather, focusing on the performance aspects during endurance exercise. Let’s first deduce the difference between keto and ‘fat adapted’. 

Keto vs. Fat Adapted

Keto is an extreme form of carbohydrate restriction (<50 grams / ~ 200 calories per day). With the minimal ingestion of carbs, you’re forcing your body to produce ketones that are used by the body as fuel (vs. glycogen). This is known as ketosis. To get to this state, it usually takes ~2 weeks or so of severe carbohydrate restriction. During this transition time, people report feeling nauseous, lethargic, low energy, a “foggy mind”, and other undesirable side effects. There’s actually a name for this transition period—it’s called the keto flu. Our metabolic systems are truly incredible in their ability to use differing macros as fuel. While the “default” system is to grab the carb source first, we can adapt our bodies into using fats instead. 

Once you’ve sustained ketosis over a longer time frame and your body has made the switch to using fat as the main source of energy, then you’re deemed as becoming fat adapted. But a big stipulation here is that if you exceed your carbohydrate intake for the day, you will immediately come out of ketosis and your body will go back to its preferred energy source: glycogen (from carbs). This makes the keto diet one of the most rigid and restrictive diets out there—if you indulge in some additional beers or a pizza, many people on keto report feeling like shit.

Fat Adapted on Performance

Our bodies have a much larger capacity to store fat vs. carbohydrates. The hypothesis goes like this: by acclimatizing our bodies to fat oxidation, we are increasing our capacity to store fat (our back-up source during exercise). Endurance athletes can train their body to use fat stores instead of glycogen as the primary source of energy (rather than the other way around) and the consensus is that because of all these benefits, fat adaptation should help with performance in endurance. Further, because endurance athletes experience the dreaded “bonk” due to low blood glucose, if you’re using ketones as fuel, you can avoid this.

When we ingest carbohydrates, our bodies go through an 11-step process in order to turn carbs into readily available energy. In the alternative, when we ingest fat, the process of turning fat into ketones is just a quick, 3 step process. Further, ketone bodies are more “energy-intensive” which can provide a quick supply of energy. Ketones are also shown to have anti-inflammatory benefits and can reduce fatigue and exercise-induced muscle damage. While this all sounds good and dandy in theory, this long-term strategy has proven more of a hindrance than helpful. While I do have a personal bias towards the high carb strategy in my own training, I have gone down the low carb route before and it hasn’t done me any favours in improving performance. 

Rather than go “all in” on keto to try to reap the performance benefits, coach Chris Carmichael suggests that “athletes are better served by periodizing carbohydrate availability in order to maximize training quality and performance outcomes.” 

Training Low

The training low strategy suggests that by doing your training runs low on carbohydrates, you’re teaching your body to better use fat stores as fuel (the bigger tank). Glycogen reserves will last longer and you’ll be able to tap into that fuel source at the ready. Wonderful. You can accomplish this state by either training fasted or by not consuming any carbohydrates during your longer training runs. 

There are several drawbacks that make this strategy, well, not very fun during our training. First off, training low will feel much, much harder and less enjoyable—your rate of perceived exertion will go up and you’ll have less fuel to push you through the workout. While you’ll still be able to access some energy for high intensity bouts, the capacity for intense effort will be severely limited than, say, if you were fuelling on higher carbs. 

The results of this strategy are mixed. One study on elite race walkers showed worse performance and absolutely no benefit whatsoever to a low carb high fat strategy.

Some study participants showed better performance and others were on the ‘worse’ end of the spectrum, which mirrors an ethos I live and breathe: when it comes to fitness, the outcomes rely heavily on the individual. If you are going to test it out, research shows that the better route to take is to start out in a fasted state before the long run and consume carbs as you go.

Performance Limitations 

Physiologically, without access to carbs as a fuel source, we’ll lack the energy to tap into a high intensity during races. Although yes, we’ll be able to burn fat more efficiently, this comes at the cost of more oxygen needs. “It takes approximately 8% more oxygen to liberate energy from fat compared to carbohydrate, which means relying primarily on fat reduces economy”, writes coach Chris Carmichael. Our performance in training and on race day relies on more than our body’s ability to burn fat. Let’s look at some case examples. 

One study on trained athletes showed that going keto did indeed make the switch to using fat as the main fuel source (even at higher intensities) and saw some positive effects on body composition (weight-loss), improved recovery, better skin, and reduced inflammation. From a performance perspective (measured in time to exhaustion, VO2 max, peak power and ventilatory threshold), the ketogenic diet did not contribute whatsoever to performance. The participants all stated that they had lower energy levels at the outset, and they struggled during the higher intensity bouts of their training session. 

Another study with elite walkers showed that a low carb/high fat diet actually hindered performance due to reduced exercise economy.

Most research points out that while you can sustain energy fueled by fat, especially if you’ve trained your body to become fat adapted, your rate of perceived exertion (RPE) will likely go up and you’ll struggle with pushing yourself during training runs. Even in ultramarathons and long runs, the epitome of steady-state and low intensity, there will still be high intensity interspersed within the race where you’ll need to pull from glycogen stores.

KD and Exercise Induced-Fatigue

As aforementioned, there hasn’t been particularly favorable evidence towards the fat adapted diet and performance, but some studies have shown that the keto diet could help prevent or at least slow down exercise-induced fatigue.

When fatigue starts to set in during exercise it’s usually due to the combination of the following factors: the depletion of glycogen, the accumulation of lactate in our muscles, and oxidative stress. One paper looked at how keto can actually help produce more lactate and while it may not be the primary impetus behind fatigue, it’s a contributing factor.

KD and Exercise Induced-Fatigue

As aforementioned, there hasn’t been particularly favorable evidence towards the fat adapted diet and performance, but some studies have shown that the keto diet could help prevent or at least slow down exercise-induced fatigue.

When fatigue starts to set in during exercise it’s usually due to the combination of the following factors: the depletion of glycogen, the accumulation of lactate in our muscles, and oxidative stress. One paper looked at how keto can actually help produce more lactate and while it may not be the primary impetus behind fatigue, it’s a contributing factor. 

In practice, there were a few studies that showed the KD helped improve lactate thresholds (helping us sustain harder efforts for longer). One study looked at trained cyclers on KD and reported an increase in lactate threshold at the end of the experiment. Another study deduced that high performance athletes saw a decrease in lactate concentrations with the keto diet.

There hasn’t been a ton of research published in this area and with the slight pros, the study participants also reported several negative side effects including lower energy in their workouts, trouble pushing themselves during high intensity intervals, etc. The stipulations surrounding the findings of most of the studies stated that there aren’t enough long term studies and comprehensive evidence suggesting that KD can prevent fatigue faster than say “carb adapted”.

GI Issues

Since fat is the most calorie dense of the macros, we don’t need as much of it to hit our calorie needs during a workout. Endurance events cause major GI stress, so in theory, it makes sense that we’re simply not consuming the same food volume if we opt for fat (9k/cal) vs. carbs (4k/cal). For this reason, keto may sound appealing but in practice, there hasn’t been conclusive evidence to state whether fat can actually prevent GI issues. It’s best to get your body used to nutrition (whether that’s carb or fat) prior to race day by practicing your nutrition strategy on long training runs. Even if we meticulously plan our nutrition however, fat or carbs, we’re still prone to GI issues on race day. 

While I tried to remain objective while writing this article and giving HFLC and keto the benefit of the doubt, the overwhelming research has shown that for endurance athletes, carbohydrates are the way to go for pre-workout and intra workout fuel optimal performance (especially during high intensity aerobic exercise).If you’re more into strength training, you can probably get away with fuelling more with fats and protein.

Conclusion

While I tried to remain objective while writing this article and giving HFLC and keto the benefit of the doubt, the overwhelming research has shown that for endurance athletes, carbohydrates are the way to go for pre-workout and intra workout fuel optimal performance (especially during high intensity aerobic exercise).If you’re more into strength training, you can probably get away with fuelling more with fats and protein.

While all this information is great and dandy, the best way to determine your own nutrition plan is to practice in your training, keep track of your energy levels and performance, and devise a nutrition plan and timing strategy that’s uniquely yours.

Sources

Wildman, Robert PhD, RD¹; Kerksick, Chad PhD, ATC, CSCS*D, NSCA-CPT*D²; Campbell, Bill PhD, CSCS³ Carbohydrates, Physical Training, and Sport Performance, Strength and Conditioning Journal: February 2010 – Volume 32 – Issue 1 – p 21-29 doi: 10.1519/SSC.0b013e3181bdb161

Regulation of Fat Metabolism During Exercise: https://www.gssiweb.org/en/sports-science-exchange/Article/regulation-of-fat-metabolism-during-exercise

These are the Carbs You Should Eat Before a Workout: https://www.teamusa.org/USA-Triathlon/News/Blogs/Fuel-Station/2018/September/04/These-are-the-Carbs-You-Should-Eat-Before-a-Workout