It may not surprise long time Supersapiens or readers of the blog that a continuous glucose monitor (CGM) may help in detection of overtraining. This is something we have previously noted, as well as noting the close ties between overtraining and underfueling

Despite this, having scientific studies to support the notion in a more controlled setting, rather than the anecdotal experience of athletes we are supporting is key. Thankfully as mentioned here, Supersapiens is supporting as much research as possible as a company and thankfully this is coming to fruition with one of our research partners based in Canada, who’s previous research we covered here, has just released another paper.

The researchers sought to answer the question of whether CGMs could be used in detection of overreaching, a notoriously difficult state to detect. Overreaching, involves a short period of excessive training with respect to recovery, which can be functional (classically, overloading for a short period of time as is often done in normal training) or non-functional overreaching (a prolonged period of overreaching). Overtraining, is a further step along the continuum, whereby the non-functional overreaching is prolonged, and excessive. Recovery times from the three follow a similar time course, that is, recovery from functional overreaching is swift, whereas non-functional overreaching is a little longer and overtraining syndrome recovery can be extended. 

What They Did

To induce overreaching, the researchers required experienced endurance athletes to train with their normal training load but then added two sprint interval sessions and an exercise test session a week for three weeks. The sprint interval sessions were initially four repetitions of a 30 seconds Wingate test (google “Wingate Test”, these things are torture) adding one repetition per week. This increased training load meant an increase to about 150% of normal training duration. 

They also did a number of tests during the experiment (see below figures for insight). Whilst also requiring the athletes to cut volume to 50% of normal in the week prior to (aka “PRE”) and post (aka “REC”) the block.

Figure 1: From Coates et al 2023, study design pictured graphically. OL=overload, RMR=resting metabolic rate, HRV=heart rate variability 

It should be noted that to avoid underfueling, the athletes were instructed to increase their intake in weeks 2-4 (the overreaching block).

The Testing

The researchers used a variety of measures, some to track metrics of overreaching, some for looking at metabolic changes and some to track potential underfueling. The below figure (figure 2) details the weekly exercise testing sessions, the resting tests occurred the following day from the exercise tests (these included things like RMR and HRV). In figure 1, above, the timing of resting tests is shown.

As can be seen from Figure 1, full testing protocols were undertaken in week 1 (PRE), 4 (“POST”) and 5 (“REC”), whilst weeks 2 and 3 saw the researchers use an exercise test and well being/mood assessment only.

Figure 2: From Coates et al 2023, weekly exercise testing session schedule shown graphically. 

Figure 2 illustrates protocol for the exercise testing sessions. Of note, wellbeing and baseline tests were performed pre-exercise. Following this 50g of glucose was ingested, 15 mins prior to exercise starting. The exercise is illustrated in the diagram. Rating of Perceived Exertion (RPE) was repeatedly taken during the test. 

What they Found

Comparing weeks 1 (PRE) and 4 (POST) the athletes displayed clear signs of overreaching, including suppressed mood/wellness, reduced power, HR and lactate during their time trial. Following a week of deload, week 5 (REC), the athletes responded positively. Their lactate and heart rate normalized whilst their power output improved above baseline testing levels. 

Figure 3: From Coates et al 2023

The most notable finding, and hypothesis of the study, was that there was altered substrate utilization during exercise when athletes were overreaching. The authors observed reduced CGM glucose levels and reduced carbohydrate oxidation during submaximal exercise and not at rest. This is illustrated in figure 3, above where submaximal exercise shows suppressed glucose. The 5km time trial showed a trend towards a reduction at POST compared to REC, however this trend is not statistically significant. 
There was also considerable individual variability in response.

Authors found no sign of low energy availability (LEA) in most participants, which is of particular importance given their common pathways. This included no changes in fasting glucose, which can be a sign of LEA (as discussed here). However, 3 participants did show signs of LEA, though removal of these participants did not alter the group results. 

Supersapiens’ Take

  • We are unashamed supporters and fans of field based and practically applicable research, so it should be no surprise that we loved this study and its real world applicability. 
  • The researchers induced a classic picture of functional overreaching, in which athletes exhibit signs of overreaching, but following a short period of deload, subsequently improve above baseline levels and no longer display signs of overreaching. They also tested regularly enough to show that the response was training related. 
  • The study cohort is representative of recreational athletes who are focussed on performance, in terms of both training hours and VO2Max. 
  • The researchers induced overreaching in a real-world type situation and whilst they cite the fact they suggested an increased intake but did not monitor this as a weakness, it makes the situation all the more real world in that this is akin to the actions of most coaches who do not have the resources to track intake for athletes they work with. 
  • The researchers suggest adding the use of CGM to heart rate at submaximal intensities where seeing suppression of both could be a potential signal that overreaching is occuring. Those who are overreaching, would note reduced CGM glucose levels for a given heart rate at submaximal intensity exercise. 
  • It should be noted that there are times when overreaching is expected. Overreaching, may be the goal, or a warning sign, as with many things it is “not NO but KNOW”. 


  1. Coates AM, Thompson KMA, Grigore MM, Baker RE, Pignanelli C, Robertson AA, Frangos SM, Cheung CP, Burr JF. Altered carbohydrate oxidation during exercise in overreached endurance athletes is applicable to training monitoring with continuous glucose monitors. Scand J Med Sci Sports. 2023 Dec 13. doi: 10.1111/sms.14551. Epub ahead of print. PMID: 38093477.
  2. Kreher JB, Schwartz JB. Overtraining syndrome: a practical guide. Sports Health. 2012 Mar;4(2):128-38. doi: 10.1177/1941738111434406. PMID: 23016079; PMCID: PMC3435910.
  3. 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).
  4. Flockhart, M., Larsen, F.J. Continuous Glucose Monitoring in Endurance Athletes: Interpretation and Relevance of Measurements for Improving Performance and Health. Sports Med (2023).