Adrenaline pushes glucose up at the start line. Sustained aerobic effort drives it down mid-race. The post-race window carries nocturnal hypo risk for hours. Race day is a completely different problem from training day.
Training prepares your body. Race day stresses your glucose management system in ways that training sessions rarely replicate. The combination of pre-race adrenaline, competitive intensity, unfamiliar food and timing, and the sustained effort of a race creates a glucose environment that requires a specific protocol rather than your standard training day approach.
This protocol is built from 15 years of racing with T1D across marathon distances. It is not theory. Every recommendation comes from real race experience, CGM data, and the understanding that the cost of getting it wrong on race day is higher than in training.
Keep your dinner the night before race day consistent with what you eat regularly before long training runs. This is not the time to experiment with carb loading strategies you have not practiced. If pasta works before your long runs, eat pasta. If rice and chicken is your pre-long-run meal, eat that.
Target a pre-sleep glucose of 7 to 9 mmol/L (126 to 162 mg/dL). If you are below 7, eat a small protein and slow-carb snack. Set a CGM alarm at 5.5 mmol/L. The last thing you want is a nocturnal low the night before a race.
Reduce your long-acting insulin by 10 to 15% if you are going to be racing at high intensity for more than 90 minutes. The race itself will increase sensitivity significantly and you do not want aggressive basal insulin overlapping with that effect.
Eat your standard pre-long-run breakfast 2 to 2.5 hours before the race start. Give a reduced bolus — typically 20 to 30% less than your normal breakfast bolus for the same meal. You want to be well-fuelled without having significant IOB at the start line.
Check your glucose 90 minutes before start. If you are below 7 mmol/L, eat 15 to 20 grams of fast carbs. If you are above 10 mmol/L with a rising trend, give a small correction, but be conservative — you will be moving soon and adrenaline will push glucose up anyway.
Your target glucose range at the start line is 6 to 10 mmol/L (108 to 180 mg/dL). Below 6, treat with 20 to 30 grams of fast carbs before you start. Above 12 with a rising trend and no IOB, consider a small correction, but be extremely cautious — once you start running, glucose will move.
Adrenaline is a glucose-raising hormone. Even if you are 8 mmol/L and flat at the start line, you may see your glucose climb in the first kilometre from the pre-race cortisol surge. This is expected and usually self-resolves once sustained aerobic effort begins.
Below 6 mmol/L: take 20 to 30 grams of fast carbs, delay start if possible, recheck in 15 minutes. 6 to 10 mmol/L: go. Above 10 mmol/L, flat or falling: go, the race will bring it down. Above 10 mmol/L, rising fast: if feeling well and ketone-free, go. Above 15 mmol/L: check for ketones, do not race if present.
For races lasting more than 45 to 60 minutes, you need to fuel actively. The standard recommendation is 30 to 60 grams of carbohydrate per hour for sustained aerobic effort. Start fuelling early — do not wait until you feel low.
If you have IOB from your pre-race meal, the lower end of this range (30 grams per hour) is more appropriate. If your last bolus was more than 3 hours ago and IOB is minimal, you may be able to sustain on 20 grams per hour with careful CGM monitoring.
Your fuel choices should be things you have practiced in training. Gels, dates, sports drinks, banana pieces — whatever you have used successfully in long runs. Race day is not the time to introduce new fuelling products.
Check your CGM at each fuel point. Do not just check the number — check the arrow. Flat or rising at 7 mmol/L at the halfway mark means you are on track. Falling at 7 mmol/L at the halfway mark means take your next gel now, not at the planned kilometre.
In a long race, glucose can drop faster than expected in the final 30 minutes as glycogen stores deplete. Keep a gel accessible for the final push. A glucose of 5 mmol/L with a flat arrow at 38 kilometres of a marathon is manageable. A double down arrow at 5 mmol/L with 4 kilometres to go requires you to stop and treat.
Do not bolus immediately after finishing. Your glucose will be in various states depending on how the race went, and your body is still in a high insulin-sensitivity state. Check your CGM first.
If you are above 10 mmol/L: give a conservative correction at 50% of your normal correction dose. Do not give a full correction. Insulin sensitivity is high and will remain high for hours.
If you are 7 to 10 mmol/L: eat a protein-focused recovery meal with minimal bolus. Your muscles will pull glucose efficiently for glycogen replenishment.
If you are below 7 mmol/L: eat immediately and delay any bolus until glucose is stable and above 8 mmol/L.
The most dangerous post-race window is overnight, particularly after afternoon or evening races. The combination of glycogen depletion, elevated insulin sensitivity, and fatigue creates a significant nocturnal hypoglycemia risk.
Set a CGM alarm at 5.5 mmol/L before bed. Eat a pre-sleep snack containing 20 grams of protein and 15 to 20 grams of slow-releasing carbs if your glucose is below 8 mmol/L at bedtime. Reduce your long-acting insulin by 15 to 20% on the night after a long race if you are MDI. Do not rely solely on alarm fatigue management — you will be tired and your response time to alarms will be slower than usual.
The protocol above is a framework. Your specific numbers — bolus reductions, fuelling rates, target ranges — will be refined through race experience and long run practice. Treat every long training run as a race simulation. Test your pre-race meal timing and reduction. Practice your fuelling strategy. Check your CGM at fuel points. Document what works.
By race day, your protocol should feel practiced rather than planned. The glucose management should run in the background of your racing, not occupy the foreground of your attention.
The Diabetic Wolf bolus calculator applies everything in this guide automatically. ICR by time of day, CGM trend adjustment, IOB correction, fat and protein split dosing, and exercise modifier.
Open the Bolus Calculator CGM Decision Tool