Exploring interindividual differences in fasting and postprandial insulin sensitivity adaptations in response to sprint interval exercise training.

Previous studies have concluded that wide variance in changes in insulin sensitivity markers following exercise training demonstrates heterogeneity in individual trainability. However, these studies frequently don't account for technical, biological, and random within-subject measurement error. We used the standard deviation of individual responses (SD_IR) to determine whether interindividual variability in trainability exists for fasting and postprandial insulin sensitivity outcomes following low-volume sprint interval training (SIT). We pooled data from 63 untrained participants who completed 6 weeks of SIT (n = 49; VO₂max: 35 (7) mL⋅kg^-1⋅min^-1) or acted as no-intervention controls (n = 14; VO₂max: 34 (6) mL⋅kg^-1⋅min^-1). Fasting and oral glucose tolerance test (OGTT)-derived measures of insulin sensitivity were measured pre- and post-intervention. SD_IR values were positive and exceeded a small effect size threshold for changes in fasting glucose (SD_IR= 0.27 [95%CI 0.07,0.38] mmol⋅L^-1), 2-h OGTT glucose (SD_IR= 0.89 [0.22,1.23] mmol⋅L^-1), glucose area-under-the-curve (SD_IR= 66.4 [-81.5,124.3] mmol⋅L^-1⋅120min^-1) and The Cederholm Index (SD_IR= 7.2 [-16.0,19.0] mg⋅l²⋅mmol^-1⋅mU^-1⋅min^-1), suggesting meaningful individual responses to SIT, whilst SD_IR values were negative for fasting insulin, fasting insulin resistance and insulin AUC. For all variables, the 95% CIs were wide and/or crossed zero, highlighting uncertainty about the existence of true interindividual differences in exercise trainability. Only 2-22% of participants could be classified as responders or non-responders with more than 95% certainty. Our findings demonstrate it cannot be assumed that variation in changes in insulin sensitivity following SIT is attributable to inherent differences in trainability, and reiterate the importance of accounting for technical, biological, and random error when examining heterogeneity in health-related training adaptations.Highlights This study tested whether true interindividual variability exists for changes in insulin sensitivity and glyceamic control following 6-weeks of low volume sprint interval training (SIT).The high level of technical, biological, and random error associated with repeated measurements of insulin sensitivity and glycaemic control, means we can neither confidently conclude that there is evidence of true interindividual differences in the trainability of these outcomes following SIT, nor confidently identify responders or non-responders for such parameters.Researchers contrasting responders vs. non-responders for a given parameter, either to understand mechanisms of adaptation and/or develop physiological/genetic/epigenetic predictors of response, need to be aware that identification of responders and non-responders with sufficient certainty may not be achievable for parameters with a high level of technical, biological, and random error.