How to fuel endurance training: the balanced approach

How to fuel endurance training, particularly Ironman and other ultra-endurance event training, is hotly debated in the athletic and scientific community. There are proponents of high-carbohydrate diets (overall macronutrient composition 60-65% energy from CHO), LCHF (low carb high fat) diets (overall macronutrient composition 75-80% from FAT) and then those who take a more balanced approached and periodise their carbohydrate intake so they can match their fuelling strategy to the demands of each session. 

A well-balanced training plan should incorporate a variety of sessions that target different systems and intensities. Athletes who want to get the most from their training, will rightly question what they should eat to optimise the benefits of their training so they can adapt in the most effective way. How an athlete should approach their fuelling strategy is a very good question, the same rules don’t apply to each session. I mentioned above that there are proponents of a balanced approach when it comes to fuelling. Unfortunately going down the middle isn’t very glamorous and the argument seems to sway back and forward between whether or not LCHF or just high carb is better. One camp seems to think carbohydrate is the devil and the other laughs at keto athletes saying that it’s all a nonsense and that they’ll keep eating all the pizza. 

What you’ll hopefully take from this article is that it’s very difficult to apply a blanket approach and that everyone will respond differently to individual fuelling strategies. I’m going to provide an insight into how I fuel my training and present the thinking behind it so you can make your own conclusions. After all, performance is in the hands of the performer. You need to know why you’re doing something and then work out how to apply it to your needs. 

In March 2016, a study conducted by Jeff S. Volek et.al, looked at the metabolic characteristics of keto-adapted ultra-endurance runners. The group was made up of twenty elite ultra-endurance marathoners and Ironman triathletes. Participants were split into two groups; one group habitually consumed a low carbohydrate diet and the other group consumed a high carbohydrate diet. Testing of the athletes involved all of them performing a maximal graded exercise test, followed by a 180 min submaximal run the day after, at 64% VO2max on a treadmill to determine metabolic responses. The graph illustrates the stark differences in energy metabolism during the 3hr run: 

The conclusion from the study was that highly trained endurance athletes who consume a low carb diet, had exceptionally high rates of fat oxidation in comparison to highly trained endurance trained athletes who consume a high carbohydrate diet. They also discovered that there was very little difference in muscle glycogen utilization and repletion patterns during and after 3 hours of exercise. However, they did not measure performance. This study does not conclude that well-fat adapted athletes perform better in endurance events than athletes who are less adapted. It does provide a very interesting insight into how what you eat can affect how you metabolise substrates.

This ability to manipulate what you metabolise as fuel, based on what you eat, acts as the basis for different arguments and approaches to fuelling endurance sport. 

To break down the theory as simply as possible; we have limited carbohydrate stores and it isn’t possible to fuel an endurance event, like Ironman, on exogenous carbohydrate intake. Therefore, to perform to our optimum, we need to be efficient at metabolising fat as a fuel source. Burning fat efficiently will spare muscle glycogen and make workloads more sustainable. A recent META article called out the importance of zone 2 training. This article didn’t go into detail about the variance of metabolic profiles you’ll see across athletes in this zone. Some athletes will metabolise very low amounts of fat in this zone whereas some athletes won’t reach their fat max until zone 4. If an athlete wants to race the bike section of Ironman at 75% FTP, but that workload is predominantly fuelled by aerobic glycolysis and fat oxidation is very low, they won’t have any chance of being able to achieve this and then run a successful marathon. Training intensity is one thing, but how training is fuelled will have a significant impact on what fuel is metabolised for energy, as demonstrated above, and in theory effect the ability to perform in endurance events. 

So, why doesn’t everyone just follow a low carb diet?

An individual will lower their ability to use carbohydrate as a fuel source when they follow a low carbohydrate diet. This could therefore have detrimental effect on high intensity exercise where carbohydrate is the primary fuel source. Fat cannot be metabolised anaerobically. As mentioned at the start of the article, we need to vary training intensities in our programme. High intensity training plays in an essential part in preparing for Ironman. So, following a low carb diet every day, could hinder your ability to perform harder sessions. 

In addition to this, consuming a low carbohydrate diet can result in downregulation of the transporters in the intestine. The impact of this is that carbohydrate can’t be absorbed as effectively. Athletes who avoid carbohydrate may be more likely to get gastro-intestinal problems in in long-distance events. Ironman is performed at an intensity where carbohydrate intake can play a critical role in how fast an athlete can go. Having a robust gut, that can tolerate higher levels of carbohydrate intake during Ironman, is an enormously important adaptation. I can consume 80-90g of CHO per hour in an Ironman with absolutely no ill effects. This took a lot of practice and work but our gut is just as trainable as our muscles. A lot of athletes have poor Ironman races due to gut issues and poor nutritional strategies. 

The main point I’m raising here is that we don’t need to choose one or the other, high carb or high fat. Optimising CHO intake and fat intake will help us develop our fat metabolism and carbohydrate metabolism. It is very rationale to come to the conclusion that placing an emphasis on different macronutrients on different days will have a benefit to Ironman performance. The caveat to this is that it’s much easier to do higher carbohydrate days. Western society diets are high in carbohydrate and eating a low carbohydrate diet can be challenging as takes careful planning and thought. If done incorrectly, it could have potentially negative effects on performance. 

What I’ll share now is how I approach low carb days and how I fuel my lower intensity sessions. The goal of my low carb training is to increase fat oxidation at a given intensity. With the theory being, as I improve my fat metabolism, I’ll be able to produce greater workloads from fat oxidation and spare muscle glycogen. The key intensity I want to improve my fat metabolism at is Ironman pace. My target bike power in an Ironman is roughly 240-260w’s and my target marathon run pace is 4.00min/km. If I can’t fuel those intensities with a high level of fat oxidation, then I’m reducing my ability to perform at them. My training plan is built around making those intensities as sustainable as possible. Training and nutritional strategies are governed by the demands of Ironman racing. Training isn’t always focused on making you go faster, it’s focused on making you not slow down and there’s a key difference.  

If carbohydrate availability is low, fat oxidation is increased. A common tool that athletes use to increase fat oxidation is training after an overnight fast. Basically, training in the morning with an empty stomach means low liver glycogen. I take this a step further though and fuel my sessions. For zone 2 sessions in the morning that are <90mins, I drink black coffee with MCT oil in it. MCT means medium-chain triglycerides and make up about two thirds of coconut oil. Having MCT oil before these sessions, primes by body to metabolise fat as a fuel source and helps improve fat oxidation. If I’m doing 2 lower intensity sessions in a day, I’ll limit carbohydrate intake over the course of the day to optimise fat oxidation. I don’t go crazy with counting numbers as I have a good feel for what I’m eating. However, I’ll try and limit carb intake to 100-150g’s. This means that’ll promote fat oxidation in both sessions. 

On longer sessions that are at Ironman intensity or just above, I’ll consume more fat. My breakfast will be made up of black coffee with MCT oil, a three-egg omelette (with some veg) and avocado. I’ll then continue to fuel the session with an MCT based energy drink. The key goal of these longer sessions is to get to the point where I’m training at Ironman race intensity without the need for high carb intake. Fuelling these sessions with fat is essential as it will help promote better fat oxidation and improve the ability to execute the right intensity. Doing them fasted places a lot of stress on the body and is a sub-optimal strategy. 

I will also perform some harder intensity training in a low carb state but the intensity will be closely monitored. A very effective training strategy to prepare for Ironman can involve doing training sessions that have higher intensity intervals, at the start, where you purposely try and deplete carbohydrate stores and then perform race intensity efforts after. This can be very demanding though and takes time to build up to. Another popular strategy is to do two sessions in a day. The first session is designed to reduce muscle glycogen. This is then followed by a low carb diet. The next session is performed with low muscle and low liver glycogen. This can promote fat metabolism adaptations but it’s very demanding and can require quite a lot of recovery time. 

My aim here is to show you how specific I am with my approach to fuelling in addition to the training itself and that there are lots of ways to execute your plan. Training will get you so far but how you apply yourself to the training will get you further. It’s not what you do, but how you do it! We don’t do the same training every day so why would you eat the same thing every day? A common theme across the META articles is that there’s no magic bullet. You can’t take a blanket approach to everything and you need to understand the decisions you’re making. The optimal way to approach your fuelling in training is to take a balanced approach and be as rational as possible with your thinking. The composition of your fuel should match the demands of the session and your goals. If you want to be good at Ironman, you need to think past how much of a difference some deep section wheels will make. You also need to take the time to think about what you’re doing and what goes into it so you can get the most from your effort. 

A final note on this subject is what works for me, might now work for you. I am very reluctant to tell people what they should eat as I don’t think it’s my place. I think it’s more appropriate to try and share some knowledge and point them in right direction so they can make the right choices through their own understanding. That’s ultimately how someone will change their behaviours and come to the right solution for them. Don’t let anyone tell you there’s one way to eat and you must do X, Y and Z because they simply don’t know that’s true. It is worth noting the potential health benefits of controlling carbohydrate intake though. What are optimal fuelling strategies for some elite athletes, may not be very healthy. You need to make a decision in your head, are you trying to optimise performance or health as they can be different things. There is no doubt that we have an over reliance on carbohydrate as a society and consuming excessive amounts on a consistent basis could be a cause of type 2 diabetes, cardiovascular disease and obesity. Thinking about carbohydrate intake could have benefits beyond optimising Ironman performance, it could help reduce the likelihood of developing serious health problems. My last sentiment is to think again why you’re doing something. At META, we’re trying to build better humans full stop, not just for sport. 

Takeaways:

·      Don’t take a blanket approach to your diet. Best practice is to marry up the demands of your goals/sports with your fuelling. 

·      There is no best diet and extremes very rarely work. There’s nothing wrong with the middle ground and what works for one person won’t necessarily work for you. 

·      Performance in endurance sport, most notably long-distance events, benefits from being able to metabolise fat and carbohydrate efficiently. You need to have good metabolic plasticity. 

·      What you eat before, during and after sessions will play a key role in what you metabolise as fuel.

·      Thinking about what the demands of the session and then fuelling appropriately will improve training adaptations. 

·      The ability to perform in high intensity sessions could be compromised if carbohydrate intake isn’t sufficient. 

·      Fuelling lower intensity sessions with low carb high fat nutrition can elicit greater levels of fat oxidation and therefore improve the ability to sustain that effort with a lesser reliance on carbohydrate. 

·      Take the time to understand the science and check your approach against what you want to achieve. 

·      Finally, you need to give things time to take effect and be consistent with your approach. Don’t expect to transform anything in a couple of weeks.