An unfortunate truth: Many endurance athletes struggle through difficult-to-avoid injuries due to the nature of endurance training and high performance.

But some injuries and illnesses are avoidable. Specifically, those related to low energy availability (LEA), Relative Energy Deficiency in Sport (RED-S), and other associated injuries like bone stress injuries. Bone stress injuries can be connected to low carbohydrate availability, as reported by a recent paper from Trent Stellingwerff and colleagues: “Poor chronic carbohydrate availability, beyond or instead of LEA, is emerging as a potential mechanism also associated with some RED-S related outcomes.”

This review also suggests that: “under-recovery, specifically attributed to under-fueling, resulting in LEA and poor carbohydrate availability, is especially prevalent and often overlooked or misdiagnosed in overtraining syndrome situations”.

There are many reasons that athletes, particularly endurance athletes, may have a suboptimal energy intake and therefore can be exposed to a higher risk of RED-S and LEA. These include logistics, finances, time, knowledge, common myths (like “eating is cheating”) and unfortunately more pervasive issues around body image.

Is Under-fueling Really that Common?

Numerous studies have suggested that intake in athletes is inappropriately low. (Rothschild et al, 2021) The numbers in these studies vary slightly based on population, but numbers as high as 60% of athletes considered to be under-fueling are not uncommon. These include studies in elite athletes, non-elite athletes, and youth athletes in various sports including track and field, distance running (including ultra distance), triathlon (including long course) and winter endurance sports. Generally, female athletes are less likely to meet dietary recommendations than their male counterparts, exposing them to a higher risk of developing LEA. Overall, this means that such situations are very common across sports and levels.

What is Under-fueling?

Broadly, the term includes an insufficient total caloric intake as well as macronutrient intake. Studies in the area suggest that many athletes are not meeting recommended intakes for carbohydrates, though there is also a relatively small percentage of athletes in these studies not meeting protein recommendations. If either of these, or both, are not met, it’s likely that the athlete is not meeting caloric requirements either. Many studies have included this and do indeed support that this is prevalent in conjunction with under-consumption of certain macronutrients.

What is Adequate Fueling?

There are many recommendations around appropriate athlete nutrition, some of which are discipline dependent. Broadly though, most athletes, especially endurance athletes should ensure they are meeting total caloric, protein, and carbohydrate requirements. Micronutrients are crucial, but not discussed herein, as under-fueling in itself primarily refers to the focuses mentioned. The summary table below (Table 1) presents information on event fueling, daily, pre-exercise, during, and post exercise, and helps guide the readers in what adequate means.

Data Showing How Athletes Are Under-fueling
Table 1. From Casazza GA et al. 2018

What are the risks of under-fueling?

Under-fueling poses a myriad of risks for athletes and fall into the following categories: metabolic, psychological, endocrine, skeletal, and, of course, performance related.  Many of these are well publicized, for example: mood changes and performance decrements. Endocrine related changes involve predominantly reproductive system disorders, which again are relatively well-publicized. Skeletal system changes pertain mostly to the association of LEA with low bone mineral density, which predisposes athletes to bone stress injuries (stress reactions and stress fractures). The metabolic changes are less well known, but include changes to spare carbohydrate and glycogen stores, increasing free fatty acids circulating.

Additionally, there is more associative evidence suggesting LEA could be part of the anemia often seen in endurance athletes.

What will my glucose look like when I am under-fueling?

We are still learning a lot about this area. But we have seen some data from athletes that suggests glucose may be quite low during these periods. For example, the reports from Lisa Norden. There is interesting evidence that when LEA is present, for as few as five days, fasting glucose levels will decrease.

Low fasting levels might also have some other important consequences.

In a recent review, it appears that non-diabetic people without heart disease who had baseline fasting glucose levels below 72 mg/dL had a 56% increase in all-cause mortality compared to people with normal fasting blood glucose levels (or above 72mg/dL).

Such an effect seems to be related to the long-term consequences of high epinephrine levels that rise in response to low glucose levels. Or, more simply, low glucose levels may be a stressor to the body themselves.

Importantly, participants with fasting glucose levels between 83mg/dL and 72mg/dL did not have the same level of risk. Therefore, a low glucose level (below 72mg/dL) not only might signal a state of low energy availability but also an increased health risk.

Practical Tips to Avoid Under-fueling

  • Work with a coach or qualified nutrition professional to ensure you apply what is right for you depending on your fitness level and training aims.
  • Consider how any one training session fits into your week and understand that it is the weeks and months of training that drives success, not any one training session. So eat appropriately to fuel the training program.
  • When in heavy blocks of training, fasted training can make it a time challenge to eat enough and meet your daily needs: fueling during workouts may allow for a more appropriate total caloric intake.

Remember, fueling optimally allows for better training, better adaptations to training, and a better ability to perform on race day. The feedback that Supersapiens can provide is unbelievably helpful in ensuring appropriate fueling in athletes, while offering a potential early warning sign of low energy levels and dangerous energy deficiency.

The future of performance is well-fueled. Make sure you're optimally fueled with minute-by-minute 24/7 glucose tracking. Get your own biosensors here.


  1. Mountjoy M, Sundgot-Borgen JK, Burke LM, et al.IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine 2018;52:687-697.
  2. Stellingwerff, T., Heikura, I.A., Meeusen, R. et al. Overtraining Syndrome (OTS) and Relative Energy Deficiency in Sport (RED-S): Shared Pathways, Symptoms and Complexities. Sports Med (2021).
  3. Kettunen O, Heikkilä M, Linnamo V, Ihalainen JK. Nutrition Knowledge Is Associated with Energy Availability and Carbohydrate Intake in Young Female Cross-Country Skiers. Nutrients. 2021 May 22;13(6):1769. doi: 10.3390/nu13061769. PMID: 34067303; PMCID: PMC8224650.
  4. Wasserfurth, P., Palmowski, J., Hahn, A. et al. Reasons for and Consequences of Low Energy Availability in Female and Male Athletes: Social Environment, Adaptations, and Prevention. Sports Med - Open 6, 44 (2020).
  5. Jeffrey A. Rothschild, Andrew E. Kilding & Daniel J. Plews (2021) Pre-Exercise Nutrition Habits and Beliefs of Endurance Athletes Vary by Sex, Competitive Level, and Diet, Journal of the American College of Nutrition, 40:6, 517-528, DOI: 10.1080/07315724.2020.1795950
  6. Wardenaar F, Brinkmans N, Ceelen I, Van Rooij B, Mensink M, Witkamp R, De Vries J. Macronutrient Intakes in 553 Dutch Elite and Sub-Elite Endurance, Team, and Strength Athletes: Does Intake Differ between Sport Disciplines? Nutrients. 2017 Feb 10;9(2):119. doi: 10.3390/nu9020119. PMID: 28208581; PMCID: PMC5331550.
  7. Viner RT, Harris M, Berning JR, Meyer NL. Energy Availability and Dietary Patterns of Adult Male and Female Competitive Cyclists With Lower Than Expected Bone Mineral Density. Int J Sport Nutr Exerc Metab. 2015 Dec;25(6):594-602. doi: 10.1123/ijsnem.2015-0073. Epub 2015 Jul 1. PMID: 26131616.
  8. Beermann BL, Lee DG, Almstedt HC, McCormack WP. Nutritional Intake and Energy Availability of Collegiate Distance Runners. J Am Coll Nutr. 2020 Nov-Dec;39(8):747-755. doi: 10.1080/07315724.2020.1735570. Epub 2020 Mar 19. PMID: 32191158.
  9. Jesus F, Castela I, Silva AM, Branco PA, Sousa M. Risk of Low Energy Availability among Female and Male Elite Runners Competing at the 26th European Cross-Country Championships. Nutrients. 2021; 13(3):873.
  10. Masson G, Lamarche B. Many non-elite multisport endurance athletes do not meet sports nutrition recommendations for carbohydrates. Appl Physiol Nutr Metab. 2016 Jul;41(7):728-34. doi: 10.1139/apnm-2015-0599. Epub 2016 Mar 1. PMID: 27176786.
  11. Burke LM, Cox GR, Culmmings NK, Desbrow B. Guidelines for daily carbohydrate intake: do athletes achieve them? Sports Med. 2001;31(4):267-99. doi: 10.2165/00007256-200131040-00003. PMID: 11310548.
  12. Casazza GA, Tovar AP, Richardson CE, Cortez AN, Davis BA. Energy Availability, Macronutrient Intake, and Nutritional Supplementation for Improving Exercise Performance in Endurance Athletes. Curr Sports Med Rep. 2018 Jun;17(6):215-223. doi: 10.1249/JSR.0000000000000494. PMID: 29889151.
  13. Melin, A. K., Heikura, I. A., Tenforde, A., & Mountjoy, M. (2019). Energy Availability in Athletics: Health, Performance, and Physique, International Journal of Sport Nutrition and Exercise Metabolism, 29(2), 152-164. Retrieved Sep 23, 2021, from
  14. Ackerman, K. E., Holtzman, B., Cooper, K. M., Flynn, E. F., Bruinvels, G., Tenforde, A. S., … Parziale, A. L. (2018). Low energy availability surrogates correlate with health and performance consequences of Relative Energy Deficiency in Sport. British Journal of Sports Medicine, bjsports–2017–098958. doi:10.1136/bjsports-2017-098958
  15. Holtzman, Bryan1; Parziale, Allyson L.1; Cooper, Katherine M.1; Flynn, Erin1; Tenforde, Adam S.2; Ackerman, Kathryn E. FACSM1 Low Energy Availability Associated With Lower BMD And Bone Stress Injury Site In Female Athletes, Medicine & Science in Sports & Exercise: May 2018 - Volume 50 - Issue 5S - p 742-743 doi: 10.1249/01.mss.0000538447.25598.4b
  16. McKay AKA, Pyne DB, Burke LM, Peeling P. Iron Metabolism: Interactions with Energy and Carbohydrate Availability. Nutrients. 2020 Nov 30;12(12):3692. doi: 10.3390/nu12123692. PMID: 33265953; PMCID: PMC7761418.
  17. Liao H, Saver J, Yeh H, et alLow fasting glucose and future risks of major adverse outcomes in people without baseline diabetes or cardiovascular disease: a systematic review and meta-analysisB. MJ Open 2019;9:e026010. doi: 10.1136/bmjopen-2018-026010