Fuel for the Work Required | A Game Changer for Endurance Athletes?

Here at Resilient Nutrition we’re all about using cutting-edge science to maximise performance - after all, it’s the foundation we’ve built our products on. In today’s blog we’re bringing you insights into how you can make dramatic improvements to your endurance through nutritional periodisation strategies used by professional athletes. These methods are based on a concept known asfuel for the work required”. 


  • Chronic consumption of a low-carbohydrate diet can impair endurance performance. Instead, to maximise the ability to use both fat and carbohydrate during training, carbohydrate periodisation is recommended.
  • Even ultra-endurance athletes are likely to benefit from keeping carbohydrates in their diets, particularly to support performance during tougher sections of races.
  • You can use a combination of carbohydrate periodisation strategies such as twice-per-day training, fasted training, “sleep low”, and "train low".
  • We recommend you pick 1 to 3 of these strategies per training block and taper them out in the weeks before race day.
  • During most training blocks, completing about a third of all sessions in a state of low carbohydrate availability is generally a good way to go.

 Researchers at Liverpool John Moore’s University recently coined the term “fuel for the work required”, a concept that combines different carbohydrate periodisation strategies to facilitate improved sports performance. This paradigm can be boiled down to periodically and tactically altering carbohydrate intake around your training in accordance with training loads to produce specific training adaptations. Although low muscle carbohydrate stores (glycogen availability) is something you want to avoid in the lead up to and on race day, manipulating your carbohydrate intake around training sessions can maximise your ability to burn (oxidise) fat while retaining your capacity to burn carbohydrate effectively, ultimately improving race performance.

Many believe that we burn fat at low exercise intensities and carbs at higher intensities. While it’s true that at rest you primarily oxidise fat (both body fat and the fat you eat) but during max effort exercise you primarily oxidise carbohydrates (muscle glycogen and blood sugar), it’s often not quite as black and white as that

In endurance athletes, black-and-white thinking has often led those looking to maximise performance to follow a low-carbohydrate or ketogenic diet. While this would be a logical approach if you only used fat for fuel in endurance exercise, the reality is you don’t. Once you reach about 85% of VO2 max, you’ll be almost entirely using carbohydrates for energy. For context, top marathon runners race above 80% of VO2max, while slower runners might run at 50 to 60%. So while we often associate fat with endurance exercise, if you’re training to be an elite endurance athlete, carbohydrates are very important.

Research has shown that even when exercising at 65% VO2 max, people get roughly half their energy from carbohydrates, and this scales up or down depending on intensity. So while during an ultra event most of your energy will come from fat, carbohydrates play a significant role in energy production too. Since the maximum rate you can produce energy (ATP) from carbohydrates is about twice as fast as the maximum rate you can make ATP from fat, it makes sense that the faster you run/swim/cycle/row, the more carbohydrates you will use, and of course most of us want to improve our speed!

Why not just focus on eating carbohydrate and forget about fat oxidation?

The simple answer is that we don’t have enough carbohydrate stores in our body to keep exercising at a high intensity for very long, as most people store about 500 g of carbohydrates as muscle glycogen and this alone can only fuel about 1 to 2 h of exercise. When athletes run out of glycogen and/or have not fueled sufficiently during training, they need to slow down and might suffer from the effects of low blood sugar. This is the phenomenon known as “hitting the wall”.

Evidence from marathon runners competing between 1967 and 1988 showed that competitors slowed towards the end of the race, and researchers believe that this was due to runners using up all their glycogen stores early in the race by running too fast relative to ability. So for most, the question is not how fast can you go, but rather, how fast without cranking through all of your glycogen stores.

Now you might think that you don’t need carbohydrates because you don’t perform at marathon intensity during ultra endurance events. However, think about when you’re climbing a hill on a bike, scaling a mountain during an adventure race, overtaking another competitor or sprinting to the finish line... all of these activities require carbohydrate. It’s also not just the intensity of exercise that affects glycogen status: The longer the exercise bout, the more you’ll burn through your glycogen stores, which is very relevant to ultra athletes. So while you may not use most of your muscle glycogen in the first 2 h of a low-intensity exercise bout (as a marathon runner would), it will decline over time.

So to preserve muscle glycogen stores for when you really need them, you want to be able to oxidise fat at the highest intensity possible. So how do you do that? Well, maximum fat oxidation generally occurs at 45 to 65% of VO2max, although this depends on biological sex, age, training status, VO2max and diet. Given that training status and diet are the two of these we can manipulate, let’s focus on how you can alter these with some simple strategies.

While a low-carbohydrate diet does increase your ability to oxidise fat, it also impairs your ability to oxidise carbohydrates. Within a few days of starting a low-carbohydrate diet, there is downregulation of an enzyme (pyruvate dehydrogenase) that affects your ability to generate maximal amounts of ATP from carbohydrates. In other words, always eating a low-carbohydrate diet makes it difficult to maximally use carbohydrates for energy. You therefore need to use strategies that allow you to maximise fat oxidation without impairing your ability to burn carbohydrate.

Fuel for the work required strategies

Before trying any of these strategies, make sure that you are fueling adequately for your sport or exercise so that you do not suffer from the effects of relative energy deficiency in sport (RED-S). (For more on RED-S, check out this blog.) Making sure that you are adequately fueling your energy demands day to day might have a much greater impact on your performance than the following strategies! However, if you are already eating enough, you can benefit from trying some of these.

Twice-per-day training

Research has shown that training a muscle group twice per day every other day rather than training once every day can improve muscular endurance, boost maximal muscle glycogen levels, and increase mitochondrial density (mitochondria are key to making ATP, among other things). So, doing a second session later in the day when you have little muscle glycogen (after the earlier session) can lead to these positive adaptations. Consider doing harder session in the morning and a lighter one 4 to 8 h later.

Fasted training

Many athletes who work a 9-to-5 job likely do this already. Waking up and training on an empty stomach can provide positive adaptations for endurance performance. We do recommend that you consume some protein to minimise muscle losses without impacting the adaptations you’re trying to achieve though.

Recover low

This is where you simply minimise carbohydrates intake post workout. The most practical way to follow this is to eat low-carbohydrate meals after training for the rest of the day. Using CalmCalm & Rebuild, or Rebuild Long Range Fuel after sessions is ideal in this scenario.

Sleep low train low

This combines going to bed with little muscle glycogen (after restricting carbohydrate intake after training) with training fasted the next morning. Essentially, this is fasted training boosted by pre-sleep glycogen depletion. One issue with fasted training is that you might still have high levels of muscle glycogen, depending on how much you ate the day before, and sleep low train low this helps mitigate this problem.

How can you combine these methods?

It’s probably best to pick 1 to 3 of these strategies and implement them around specific sessions where you’re focusing on maximising training adaptations, rather than performance in the exercise bout. We recommend you do up to a third of weekly sessions in a state of low carbohydrate availability. When you get closer to race day, start tapering these out of training blocks and focus on in-session performance. Additionally, you want to avoid using all strategies at once as you won’t know what worked and won’t have as many options in subsequent training blocks!

In the example below (adapted from this paper), an endurance cyclist trains four times per week at 10 am and eats four times per day. Meals follow a “red”, “amber”, “green” approach, which relates to carbohydrate intake for each meal. Red is high carbohydrate, amber is moderate carbohydrate, and green is low or no carbohydrate.

 Carbohydrate Intake Schedule

You’ll notice that there are no specific amounts of carbohydrates per meal, as this will depend on the person’s needs (bodyweight, goals, etc). The key is to make sure that your most important or intense training sessions are completed after at least onegreen” meal as low carbohydrate availability will impair performance during intense sessions.

An example for a 60-kg athlete who’s following the above and eating 400 g of carbs per day might be:

  • Red = 120 to 140 g carbohydrate
  • Amber = 60 to 80 g carbohydrate
  • Green = less than 30 g carbohydrate (ideally none for green meals before and during training)

Please remember that there is more to your diet than your carbohydrate intake, so having more-calorie-dense foods (away from the training bout) can ensure that your calories do not get too low. Long Range Fuel high protein nut butters are perfect for this.

Wrap up

If you’re trying to find that edge over your competitor and have not tried or heard of fuelling for the work required, we highly recommend that you give some of these strategies a go. Although simple, these are, practically speaking, easy to implement and could be the difference between you pulling out the race and hitting a new PB!