When not to fast: Before and during a big athletic competition or a high-intensity training.

This may seem obvious. Elite endurance athletes never compete in a fasted state, as this likely compromises performance and impacts perceived exertion. In addition, research accumulated over the last half-century suggests that the most beneficial nutritional intervention is one that can augment and preserve glucose levels.

So why is fueling for key races and high intensity training sessions one of the cornerstones of performance adaptations?

Under fueling is setting yourself up to underperform, while feeling terrible in doing so, and potentially impairing key adaptations and fatigue tolerance. This makes sense when examining glucose oxidation rates at exercise intensities >80% VO2max, where glucose is oxidized at roughly 5g/min. This equates to 300 g/h, well beyond the higher end of recommendations for intake during activity (90 g/h). Exceeding the 90g/h is possible, with some recent research indicating higher rates may be beneficial but with increasing consumption is the increasing risk of gastrointestinal distress as well as impacting your body’s metabolism of carbohydrates. Thus maximizing intake of carbohydrates is a key factor in peak performance, via ensuring glucose availability is maintained.

The key to a well-fueled training session is ensuring your trailing average glucose is high enough and that you then fuel appropriately whilst training, as needed. Maintaining a high flux of glucose to the working muscle will ensure the availability of the most effective and convenient energy source during moderate-to-high intensity sessions, glucose.

This will help your performance on the day and also positively influences recovery for upcoming training. The goal of key high intensity training sessions is to perform them as well as possible, not finish them and feel horrible without intaking any fuel.  

Preparing to Exercise in a Well-Fueled State

We all know about the benefits of carbo-loading before endurance exercise. It’s a time-honored training tradition. And for good reason: It’s been shown to enhance performance. Consuming a carbohydrate-based meal or snack in the hours preceding an event is one of the key methods to influence on-hours performance and glucose utilization.

Ensure your Trailing Average Glucose is high enough in the hours leading up to an event by eating enough carbohydrates. Make sure to keep an eye on your average baseline for your own physiology depending on how many carbohydrates you eat daily and work around it to either increase or decrease the total intake of carbohydrates. Increase overall carbohydrate intake while avoiding big glucose spikes, which are usually a result of being too high in readily available carbohydrates.

Make sure you take enough nutrition with you on your training session too. Use it for your training session as planned or in case it is needed later.

If you haven’t already set up your personalized GPZ, it is probably best to do that ahead of this test. On the contrary, if you never tailored your GPZ to your own physiology, this can be a good experiment to start the process.

Keep in mind that studies in which carbohydrates are consumed 1–4 h prior to exercise often report glucose and insulin levels declining to near-basal levels prior to exercise, favorably promoting exercise capacity and adaptations. Alternatively, when subjects consume carbohydrates ≤60 min before exercise, insulin and blood glucose levels are reported to be elevated immediately prior to exercise while likely negatively impacting subsequent performance. So try to eat your last meal or snack before training outside of the hours prior to starting. You can also define what works for you in terms of timing to start exercising depending on what and on how much you normally eat before training.

What to Expect in a Glucose Curve

Depending on the various aspects of your training session, such as intensity and duration, and individual factors – glycogen stores and metabolic adaptations – your glucose data will have some personalization to account for. Apply the information below to your analysis and open discussion in our Facebook Athletes Group if you have any questions.

Given this session is intended to be well-fueled, it is likely your glycogen stores will be sufficiently full and you will perform at high intensity—so let’s get into your GPZ!

Rise in Glucose

THE WHAT: If your exercise is of an intensity sufficiently high, you will see your body liberating liver glycogen to aid in fueling. This will cause a rise in your glucose levels.

THE WHY: This happens due to:

1) The brain that is telling the liver to output some glucose.

2) Muscle glycogen metabolism that signals the liver to control for the muscle glucose burning rate at the level of the muscles.

3) Intake of carbohydrates immediately before exercising or right at the start that will influence glucose appearance kinetic.

These will all likely cause a rise in glucose. And the glucose rise may continue if your exercise is sufficiently high intensity. It will also likely rise any time you drop your intensity a little bit, at least initially. (The output of glucose is more than uptake in this situation, at least briefly).

THE ACTION: If glucose levels continue to rise, it is worth considering that if the intensity is sufficiently high, at some point, your glycogen stores will be exhausted, your RPE will go up, intensity will drop, and you will feel quite terrible.

Consider fueling in this phase only if your aim is to increase or maintain exercise intensity.

Plateaus in Glucose

THE WHAT: When exercising at a relatively consistent intensity, your blood glucose will likely be quite stable. Plateaus often occur at moderate intensities, but can occur at high intensities.

If this happens during a well-fueled, high intensity workout, you will likely see it after an initial spike, and see it maintained around your GPZ for a period of time, before more fuel is required to prevent a dip in glucose. Always take into account the need to pre-plan your feeding schedule during exercise in terms of fuel amount, timing, and type. Always base it on available scientific evidence and consult with your coach/nutritionist beforehand.

THE WHY: A plateau in your glucose levels is an indication that things are relatively stable physiologically. Keep exercising at the same intensity and you will likely exhaust your liver glycogen stores. How quickly depends on the intensity of the workout. Your body might be capable of maintaining a relatively stable glucose level for an hour of high intensity effort. Interestingly, the glucose curve might be used as a sign of metabolic adaptation with positive adaptation showing longer maintenance of a stable glucose curve and/or plateauing at a higher intensity.

THE ACTION: Use a standardized intake (such as 60g/h or whatever dose you have planned for), look at your Trailing Average Glucose, and stick with this rigorous intra-workout fueling strategy. Complete a training session of a steady output in what should be high zone 3 or above (see below for further details). Then repeat and try to adjust your fuel intake based on your average glucose level. If you can achieve the previous output with less fuel intake to maintain glucose on the second attempt, or you utilize an identical intake but perform better, this might be a good indication that you have improved your ability to oxidize fat and use it as a fuel, sparing glucose for higher intensity efforts.


Aim to keep 12h and 24h Trailing Average Glucose the same prior to your 2 sessions.

In session 1, complete a high intensity workout, above your aerobic threshold (high zone 3 in a 5 zone model) or higher than this. This may be something like a 60-70 minute time trial (running or cycling). Fuel appropriately during this workout to maintain your glucose levels and achieve your best performance. Use whatever dose that you can easily tolerate while meeting scientific guidelines.

In session 2, try the same workout with less fuel (or no fuel) during the workout. If you can maintain the same glucose levels, you might have worked on your ability to use fat as fuel which spares your glycogen stores. The alternative is to use the same fueling strategy and improve your time trial effort, with the same glucose levels. This too might indicate improved use of fat as fuel, sparing your glucose and driving performance.

Keep in mind that a growing body of evidence suggests that maintenance of a stable and sustainable glucose level during exercise has been linked to performance capacity and exercise tolerance.

Drops in Glucose Levels

THE WHAT: Dropping glucose levels can occur for a few reasons: Initial drops in glucose can be due to increased use or demand for glucose as a result of changing requirements (aka utilization). An initial drop is normalized when your metabolic rate catches up to energy requirements. (A good reason for warming up is to stabilize circulating glucose levels before the start of your training).

Avoiding this initial drop is not really possible, but a warm up will help limit it and make it occur during the warm up rather than during your activity.

A similar mechanism causes drops in glucose when you ramp up intensity of training. This usually normalizes quite quickly though, provided you have the glycogen stores available or are fueling during your training. Any imbalance between glucose provision (from the liver) and glucose utilization (in the muscle) will cause either a drop or a rise in your circulating glucose levels.

With sufficient duration for the intensity you are training or competing at, your glucose will eventually drop at the point of glycogen exhaustion. At this point you will notice the impact to your performance and glucose levels.

THE WHY: More concerning drops in glucose levels occur with longer session durations, where glycogen stores are depleted. These drops may follow sufficiently high intensity exercise to utilize stores of glycogen, the timeline for this is largely dependent on intensity and somewhat individual based on carbohydrate and fat oxidation rates and pre-training glycogen stores. This can happen quite swiftly if the exercise intensity is high enough.

During rapid drops in glucose levels you might experience fatigue, empty legs, dizziness and  ultimately, if glucose level hits the hypoglycemic target, you would have the familiar feeling of really poor.

THE ACTION: Once you start seeing significant drops in glucose levels, exercise intensity and quality is compromised. If you continue the session for an extended period of time, it is recommended to intake additional carbohydrates to try and maintain your GPZ. If the drop in glucose levels was unplanned and unexpected, you should consider making real-time adjustments to your fueling strategy, accounting for the goal for the session. Ideally this does not occur as you are intaking fuel preemptively during the session and you maintain your glucose levels within your GPZ.

Experiment: establish your GPZ during high-intensity training

Make sure you have a good understanding of your normal baseline Average Glucose. Everyone's will differ slightly. To determine your own, don't change your diet or exercise routine for 48 hours and note what your Average Glucose is. Use this to set you initial baseline Target Average.

Try to bump your Target Average up by 5-10 mg/dL in the 24 hours prior to the event to help fill your glycogen stores. Try to hit this target while minimizing steep glucose spikes.

From there, ensure you have an appropriate intra-workout fueling strategy in place and have the fuel sources on hand.

Experiments to observe using the 5 Zone Model:

  • Complete 30-60 minutes at high Zone 3 intensity
  • Complete 30-60 minutes at low Zone 3 with some intermittent 5 minute efforts at Zone 4 or 5
  • Adjust either the hour intake of carbohydrates (e.g. 45/60/90 g/h) and/or the timing of ingestion (e.g. every 5/15/30’) and see what dose/frequency combination gives you the best performance in terms of maintaining your GPZ.

For each of these note how you felt, performed and compare your output to your glucose graph. Note your time in GPZ.  

*Try keep your post workout meal the exact same as other sessions and see the difference the exercise makes to your glucose response.


  1. Ormsbee MJ, Bach CW, Baur DA. Pre-exercise nutrition: the role of macronutrients, modified starches and supplements on metabolism and endurance performance. Nutrients. 2014 Apr 29;6(5):1782-808. doi: 10.3390/nu6051782. PMID: 24787031; PMCID: PMC4042570.
  2. Coyle, E F et al. “Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate.” Journal of applied physiology (Bethesda, Md. : 1985) vol. 61,1 (1986): 165-72. doi:10.1152/jappl.1986.61.1.165
  3. Malone, James J et al. “Exogenous carbohydrate and regulation of muscle carbohydrate utilisation during exercise.” European journal of applied physiology vol. 121,5 (2021): 1255-1269. doi:10.1007/s00421-021-04609-4
  4. Mohebbi, Hamid et al. “Hyperinsulinaemia and hyperglycaemia promote glucose utilization and storage during low- and high-intensity exercise.” European journal of applied physiology vol. 120,1 (2020): 127-135. doi:10.1007/s00421-019-04257-9
  5. Fell, J Marc et al. “Carbohydrate improves exercise capacity but does not affect subcellular lipid droplet morphology, AMPK and p53 signalling in human skeletal muscle.” The Journal of physiology, 10.1113/JP281127. 26 Mar. 2021, doi:10.1113/JP281127
  6. Impey, Samuel G et al. “Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis.” Sports medicine (Auckland, N.Z.) vol. 48,5 (2018): 1031-1048. doi:10.1007/s40279-018-0867-7