High carb or low carb for endurance performance?

Such a controversial topic in the endurance world at the moment, with almost an even divide of athletes in each camp. Here is our debate. Fuel availability is the limiting factor for endurance performance. We can store around 4,000 calories of fat compared to 2000 calories of carbohydrate. Win for fat as fuel. But we can’t access our fat stores well and the type of diet we eat influences our bodies fuel choices during exercise. Our western diet favours carbohydrates. Win for carbohydrates as fuel. These are just my opening points. You can see how this debate is a conundrum. 

Let’s dive into the pro’s and con’s for choosing a higher or lower carbohydrate approach to training and racing for endurance events.

Why go low carb?

The key benefit of using fat as a fuel for energy in endurance sports is the non‐limited availability of fatty acids during exercise that negates liver glycogen depletion (1,2). There is evidence that long term keto adapted ultra‐endurance‐athletes can oxidise up to 1.5g fat/min during exercise. This would provide you with 810 calories of energy per hour. This is quite a lot more than athletes can consume through exogenous energy sources. However the evidence to translate this oxidation rate into endurance performance is currently only anecdotal. 

Despite this there are a number of endurance athletes who choose to take a low carb approach to training and some also in racing. The evolution of DeltaG Ketone ester has helped act as an additional energy source to supplement those wishing to limit carbohydrates as fuel. It is worth noting that DeltaG was created to complement proteins, carbohydrates and fats as a fuel source for athletes and not to be taken in solidarity. 

The major concern with high carbohydrate intake during endurance sports is the gut limiting factor. The means, how many carbs can one consume before they start getting gastrointestinal issues (GI)? Every athlete's gut limiting factor will be different. Some can consume 120g of carbs per hour without GI issues while others suffer at 70g per hour. The fat adaptation approach is more appealing to those with a lower gut limiting factor.

It is also very common for athletes to train low carbohydrates but race moderate to high carbohydrates. The notion behind this is to train your body to be able to metabolise fat as a fuel source then in racing you add on exogenous carbohydrates for energy and you have two fuel systems running. 

Why go moderate to high carb?

Carbohydrates can be stored in the muscles or the liver to be used as energy for exercise. Endogenous stores of glycogen can provide us with energy at moderate intensity for around 1.5-2 hours (3). Since most endurance events go longer than 2 hours, exogenous intake of carbohydrates is essential for performance.

There are multiple studies that show the relationship between the depletion of muscle glycogen and the onset of fatigue during endurance exercise (1). For many years the upper range of carbohydrate tolerability in endurance sports was 60g/hour for a single simple sugar source and 90g/hour for a combined simple sugar source. However, a 2020 paper on elite runners found that 120g of carbs per hour can be tolerated by some athletes over endurance events (4). Keep in mind that more carbs isn’t always better. We want to ingest enough to maintain blood glucose concentrations but not excessive amounts to cause gut issues. 

Low carb diets in and out of training can often lead to calorie restriction or low energy availability (LEA) which often disrupts hormone balance and the hypothalamic-pituitary-adrenal (HPA) axis for both genders (5,6,7,8). The HPA axis regulates our metabolism, stress, mood, digestion, immune system, sex drive and energy levels. With female athletes already prone to underconsuming calories, especially carbohydrates, going low carb during training is very likely to lead to LEA (7).

Females should also place more emphasis on greater carbohydrate intake and availability during the luteal phase of the menstrual cycle where there is greater sex hormone suppression on gluconeogenesis during exercise (9).

What I personally recommend

As a dietitian my recommendations are slightly different depending on the client I am seeing, however as a general rule I am in the camp of moderate carbohydrate intake for both training and racing. I believe that endurance athletes can overdo it on simple sugars with a ‘more is better’ approach. More is not better when you are stuck in a port-a-loo with diarrhoea. Listen to your body when you are training, do you have enough energy? Are you experiencing stomach cramps? If something feels off, look into the type of carbohydrates you are choosing to consume. Sometimes a simple swap to a different brand that offers a different simple carbohydrate can solve your problem.

Additionally, try and fuel for the work required. If you have a long training session with intervals coming up make sure you are fuelling well with more carbohydrates the day before and morning of the session. Alternatively, if you have an easy low intensity session you can do this fasted or on a lower carbohydrate, higher protein and fat nutrition source. 

Ask Ash! If you have any questions about fuelling for training or racing, feel free to send me an email at orders@aidstation.com.au 

Ash Miller
Dietitian and Nutritionist (Masters)
Bachelor of Physical and Health Education
Instagram: @ashthomo_nutrition 

References 

(1) Helge JW. A high carbohydrate diet remains the evidence based choice for elite athletes to optimise performance. J Physiol. 2017 May 1;595(9):2775. doi: 10.1113/JP273830. Epub 2017 Feb 8. PMID: 28044326; PMCID: PMC5407959.

(2) Bosch AN, Dennis SC, Noakes TD. Influence of carbohydrate ingestion on fuel substrate turnover and oxidation during prolonged exercise [Online]. J Appl Physiol 76: 2364–2372, 1994. 

(3) Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutr Rev. 2018 Apr 1;76(4):243-259. doi: 10.1093/nutrit/nuy001. PMID: 29444266; PMCID: PMC6019055.

(4) Viribay A, Arribalzaga S, Mielgo-Ayuso J, Castañeda-Babarro A, Seco-Calvo J, Urdampilleta A. Effects of 120 g/h of Carbohydrates Intake during a Mountain Marathon on Exercise-Induced Muscle Damage in Elite Runners. Nutrients. 2020 May 11;12(5):1367. doi: 10.3390/nu12051367. PMID: 32403259; PMCID: PMC7284742.

(5) McGrice M, Porter J. The Effect of Low Carbohydrate Diets on Fertility Hormones and Outcomes in Overweight and Obese Women: A Systematic Review. Nutrients. 2017 Feb 27;9(3):204. doi: 10.3390/nu9030204. PMID: 28264433; PMCID: PMC5372867.

(6) Miller WL. The Hypothalamic-Pituitary-Adrenal Axis: A Brief History. Horm Res Paediatr. 2018;89(4):212-223. doi: 10.1159/000487755. Epub 2018 May 2. PMID: 29719288.

(7) Wohlgemuth KJ, Arieta LR, Brewer GJ, Hoselton AL, Gould LM, Smith-Ryan AE. Sex differences and considerations for female specific nutritional strategies: a narrative review. J Int Soc Sports Nutr. 2021 Apr 1;18(1):27. doi: 10.1186/s12970-021-00422-8. PMID: 33794937; PMCID: PMC8015182.

(8) Cupka M, Sedliak M. Hungry runners - low energy availability in male endurance athletes and its impact on performance and testosterone: mini-review. Eur J Transl Myol. 2023 Apr 11;33(2):11104. doi: 10.4081/ejtm.2023.11104. PMID: 37052052; PMCID: PMC10388605.

(9) Sims ST, Kerksick CM, Smith-Ryan AE, Janse de Jonge XAK, Hirsch KR, Arent SM, Hewlings SJ, Kleiner SM, Bustillo E, Tartar JL, Starratt VG, Kreider RB, Greenwalt C, Rentería LI, Ormsbee MJ, VanDusseldorp TA, Campbell BI, Kalman DS, Antonio J. International society of sports nutrition position stand: nutritional concerns of the female athlete. J Int Soc Sports Nutr. 2023 Dec;20(1):2204066. doi: 10.1080/15502783.2023.2204066. PMID: 37221858; PMCID: PMC10210857.