Major in the majors

Three variables decide almost everything

1 – Treat CGM accuracy during exercise as conditional

CGM is the most useful tool for learning how your body responds to exercise. It is also less accurate during exercise than at rest. Sensor lag, movement artefacts and rapid glucose change all stretch the gap between interstitial and capillary glucose.

The practical rule is to read the number alongside the trend arrow and the context, and to be slower to react to a single reading mid-session. Iterate over weeks, not within a session.

2 – Run the three majors checklist before every session

Before any planned exercise, run the same three-question checklist used through this guide.

• IOB – how much recent bolus insulin is still active? When was the last bolus given?
• Starting glucose – what is it now?
• Trend – which direction is it moving, and how fast?

If those three are accounted for, most of the rest of the planning is refinement rather than rescue.

3 – Sex differences are real and underappreciated

Yardley (2023), Heyman (2025) and the wider literature on sex differences in exercise response in T1D point in a consistent direction: glucose response to matched exercise differs across the menstrual cycle and between sexes more than the older literature suggested.

The practical implication is not a separate set of rules, but an explicit acknowledgement that the same session on different days of the cycle can behave quite differently – and that this is signal rather than noise. Tracking the pattern over a few cycles often reveals it.

4 – Fear of hypoglycaemia is a planning variable

Patton (2024) and the broader literature on fear of hypoglycaemia in T1D consistently identify it as one of the largest barriers to regular exercise. The fear is reasonable. The cost of avoiding exercise because of it is usually larger than the cost of the hypo it is trying to prevent.

Practical mitigation rather than dismissal is the useful framing. A rehearsed hypo response, fast-acting carbohydrate within reach, an exercise plan made in advance with the diabetes team, and CGM alarms set sensibly – all reduce the cognitive cost of starting the session.

5 – Barriers and facilitators matter as much as physiology

Garcia (2024) and Johansen (2024) characterise the barriers and facilitators to physical activity in T1D. Time, fear, lack of structured support, and the cognitive load of exercise planning all rank highly. Facilitators include peer support, structured programmes, and clear practical guidance from the diabetes team.

The implication for an individual is that the right exercise plan is one that fits the rest of life. The most evidence-based session you cannot actually do is worse than a less optimal one you can repeat for years.

6 – Altitude changes the rules

Altitude affects glucose metabolism, insulin sensitivity and CGM accuracy in ways the lowland literature does not fully cover.

The practical pattern many people find is that the first few days at altitude often need different settings from sea level, and that exercise responses can be amplified or blunted depending on intensity and acclimatisation. Plan altitude trips with the diabetes team rather than improvising on arrival.

7 – Muscle damage and creatine kinase

Dial (2021) examined the relationship between exercise-induced muscle damage and insulin sensitivity in T1D. Heavy eccentric work – long downhill runs, novel resistance training, certain team sports – can produce a multi-day pattern of altered insulin sensitivity that does not match the immediate post-exercise window most planning frameworks focus on.

The signal here is that the day of an unfamiliar or eccentric session is not the only day that matters. The two or three days that follow may need their own adjustments.

8 – T1DeXi shows the real-world pattern

The T1DeXi dataset and its follow-on analyses (Bergford 2023; T1DeXi adolescents 2024) provide one of the largest real-world exercise-and-T1D evidence bases now available. Two patterns stand out.

• Hypo risk on exercise days is meaningfully higher than on non-exercise days, and the risk extends into the following day after long sessions.
• Real-world exercise produces more variable glucose response than pivotal trials of the same protocols. The trial average is a starting point, not a forecast for any individual.

Calibrate expectations to the registry data, not to the clinical-trial average.

10 – When to ask your care team

This guide is education, not prescription. The point at which the levers in this guide stop being enough is the point to bring your diabetes team in. That includes – but is not limited to – recurrent unexplained exercise hypoglycaemia, post-exercise hyperglycaemia that does not resolve, new exercise types or intensities that change the pattern significantly, pregnancy, altitude trips, and any change to insulin regimen or AID system.

The role of this guide is to make the conversation with the care team a more structured one – to bring the patterns and the questions, rather than the raw chaos of a difficult week.