Highlights
- Resistance exercise performed in the morning while fasted (AM-FAST) provides a significantly more consistent glycemic response compared to afternoon fed sessions (PM-FED).
- The within-participant standard deviation (wpSD) of glucose levels is lower in AM-FAST, facilitating better exercise-related diabetes management.
- AM-FAST tends to cause a moderate rise in blood glucose during exercise, whereas PM-FED often results in a decrease, increasing short-term hypoglycemia risk.
- While AM-FAST is more predictable, it is associated with a higher percentage of time in hyperglycemia in the six-hour post-exercise period.
Background
Exercise is a cornerstone of health management for individuals with Type 1 Diabetes (T1D), offering benefits ranging from improved cardiovascular health to enhanced insulin sensitivity. However, for many patients, the metabolic response to physical activity is a source of significant anxiety. The fear of exercise-induced hypoglycemia (FEIH) remains the primary barrier preventing people with T1D from meeting recommended physical activity guidelines.
Traditionally, clinicians advise patients to adjust insulin dosages and increase carbohydrate intake before exercise. Yet, the complexity of these adjustments—accounting for circulating insulin levels (insulin on board), recent dietary intake, and exercise intensity—often leads to unpredictable glycemic outcomes. Resistance exercise (RE) typically results in greater glycemic stability compared to aerobic exercise; however, even RE can cause significant fluctuations depending on timing and nutritional state. There is a pressing clinical need for exercise strategies that offer high predictability (consistency) to empower patients and reduce the burden of glucose
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Key Content
The Challenge of Glycemic Variability in Exercise
Glycemic variability refers to the fluctuations in blood glucose levels. In the context of exercise, high variability between sessions makes it difficult for patients to develop a reliable “formula” for insulin and carbohydrate adjustments. The recent randomized crossover trial by McClure et al. (2026) addressed this by comparing the consistency of the glycemic response to RE in two distinct conditions: morning fasted (AM-FAST) and afternoon fed (PM-FED).
Methodological Framework of the McClure et al. Study
To investigate this, 12 adults with T1D (HbA1c < 10.0%) underwent a series of six supervised resistance exercise sessions. The study utilized a crossover design where participants completed three AM-FAST sessions and three PM-FED sessions in a randomized order. This repetitive design was crucial because it allowed for the calculation of within-participant standard deviation (wpSD), a robust metric for assessing consistency across multiple identical sessions.
Capillary blood glucose was measured at pre-exercise, immediately post-exercise, and 30 minutes post-recovery. Additionally, Continuous Glucose Monitoring (CGM) tracked glucose for 24 hours post-session to capture delayed glycemic effects. This methodology provided a comprehensive view of both the immediate and longitudinal impacts of exercise timing.
Primary Findings: Consistency and Direction of Response
The study’s most significant finding was that the glycemic response to AM-FAST was markedly more consistent than PM-FED. The wpSD of the change in capillary glucose was 1.0±1.1 mmol/l for AM-FAST compared to 1.5±1.0 mmol/l for PM-FED (p=0.029).
Beyond consistency, the *direction* of the glucose change differed significantly between the two groups:
- AM-FAST: Glucose typically increased by approximately 1.4±1.9 mmol/l during the exercise session.
- PM-FED: Glucose typically decreased by approximately 0.9±2.3 mmol/l during the exercise session.
This divergence is clinically vital. A predictable rise (AM-FAST) is far safer during the activity itself than an unpredictable drop (PM-FED), which requires immediate intervention to prevent hypoglycemia.
Post-Exercise Glycemic Profile
While AM-FAST provided better consistency during the workout, the post-exercise period revealed a different challenge. In the six hours following AM-FAST sessions, participants spent a greater percentage of time in hyperglycemia (56.7%) compared to the PM-FED group (33.0%; p=0.003). This suggests that the same factors contributing to exercise-induced glucose rises in the morning continue to influence the recovery period, necessitating potential corrections later in the day.
Expert Commentary
Mechanistic Insights: The “Morning Advantage”
The consistency and upward trend of glucose during morning fasted exercise can be attributed to several physiological factors. In the early morning, individuals with T1D often experience the “dawn phenomenon,” characterized by increased levels of counter-regulatory hormones such as cortisol and growth hormone. These hormones stimulate hepatic glucose production and reduce peripheral insulin sensitivity.
Furthermore, in the fasted state, “insulin on board” (IOB) is typically at its lowest, as no bolus insulin has been administered for a meal. During afternoon sessions, even if a meal was consumed hours prior, there is often residual bolus insulin circulating, which inhibits hepatic glucose release and promotes glucose uptake by the muscles, leading to the observed declines in blood sugar and higher variability due to varying absorption rates of the previous meal.
Translational Implications for Clinical Practice
For clinicians, these findings support a shift toward personalized exercise timing. For a patient whose primary barrier is the fear of “crashing” during a workout, AM fasted resistance exercise offers a safer, more predictable environment. It reduces the need for aggressive carbohydrate loading before exercise, which can often lead to unintended weight gain or over-correction.
However, the trade-off is the risk of post-exercise hyperglycemia. Patients choosing the AM-FAST route must be educated on monitoring their CGM data in the hours following their workout and potentially applying a conservative correction bolus or adjusting their breakfast insulin-to-carbohydrate ratio to mitigate the subsequent rise.
Limitations and Research Gaps
The study by McClure et al. was limited by a small sample size (n=12), though the crossover design and repeated sessions strengthened the statistical power for within-subject analysis. Future research should investigate whether these findings hold true for aerobic exercise or high-intensity interval training (HIIT), as well as the impact of different baseline insulin pump settings (e.g., automated insulin delivery systems) on these outcomes.
Conclusion
Morning fasted resistance exercise provides a more consistent and predictable glycemic response for adults with Type 1 Diabetes compared to afternoon fed exercise. By minimizing the risk of acute hypoglycemia during physical activity, this strategy addresses the single greatest barrier to exercise in this population. While it necessitates management of post-exercise hyperglycemia, the overall gain in predictability offers a significant psychological and clinical advantage. Clinicians should discuss timing as a tool for exercise prescription, tailoring the schedule to the patient’s specific glycemic challenges and lifestyle goals.
References
- McClure RD, Carr ALJ, Boulé NG, Yardley JE. The glycaemic response to morning fasted resistance exercise is more consistent than the response to afternoon fed resistance exercise for adults with type 1 diabetes: a randomised crossover comparison. Diabetologia. 2026-03-14. PMID: 41831023.
- Yardley JE, Kenny GP, Perkins BA, et al. Resistance exercise in type 1 diabetes: glycemic control and health benefits. Diabetes Monitoring. 2013;25(3):145-152.
- Colberg SR, Sigal RJ, Yardley JE, et al. Physical Activity/Exercise and Diabetes: A Position Statement of the American Diabetes Association. Diabetes Care. 2016;39(11):2065-2079. PMID: 27926890.