During exercise: fuelling and checking
How much fast carbohydrate to take by session length, how often to check, why sensors run late, and when to stop for the day.
Once you're moving, two things keep a session on track: taking in enough fast carbohydrate for how long you're out, and checking often enough to catch a drop early. This page covers both, plus the one rule that ends a session.
Fuelling by how long you're out
These are published starting points for carbohydrate during activity. How much you need also depends on how much insulin is on board — more insulin means more carbohydrate.
Under 30 minutes. Usually nothing is needed. If you start under 5.0 mmol/L, take 10–20 g of glucose — more if you have a lot of insulin on board.
30 to 60 minutes. Around 10–15 g an hour for steady aerobic work. Up to 15–30 g every half hour if insulin levels are high.
1 to 2.5 hours. 30–60 g an hour, and up to 75 g an hour if insulin is high. This is where drinks start to work better than solid food.
Very high carbohydrate rates upset some stomachs. Endurance athletes train their gut to handle them; if you are new to longer sessions, build up gradually.
What to eat while you move
Fast carbohydrate works best during exercise: glucose tablets, sports drinks, gels. Chocolate and biscuits are too slow and tend to leave you high afterwards.
Carry fast glucose on your body, not in a bag on the sideline. A low doesn't wait for you to walk back to your kit.
Source: Riddell et al. 2017, Table 1.
While you're moving
Check often
Every 30 minutes if you use finger pricks, every 20 minutes if you use a sensor. A bench, a water break or the rest between sets is the moment to look.
Trust the symptoms
If you feel low and the sensor disagrees, do a finger prick. That habit is worth more during sport than at any other time.
Pressure can fake a low
Tight compression clothing, or pressure on the sensor, can give falsely low readings. This is worth knowing for cyclists and anyone in tight kit. (More on sensors and gear on the kit and conditions page.)
Below 3.0, you're done
Going again after a real low is how one difficult afternoon turns into a dangerous one. The session will still be there tomorrow.