The continuous demand for insulin in the face of insulin resistance, coupled with the glucolipotoxic environment associated with the metabolic syndrome (MetS), adversely affects the ‘quality’ of insulin produced and secreted by the pancreatic beta cells. This is depicted by increased circulating intact proinsulin concentration (IPC) which is associated with increased MetS severity and risk of cardiovascular (CV) mortality. High-intensity interval training (HIIT) has been shown to reduce insulin resistance and other cardiovascular disease risk factors to a greater degree than moderate-intensity continuous training (MICT).
PURPOSE: We therefore aimed to investigate the impact of MICT and different volumes of HIIT on circulating IPC.
METHODS: Sixty-six individuals with the MetS were randomized into 16 weeks of either: i) MICT (n=21, 30mins at 60-70%HRpeak, 5x/week); ii) 4HIIT (n=22, 4x4min bouts at 85-95%HRpeak, interspersed with 3min of active recovery at 50-70%HRpeak, 3x/week); or iii) 1HIIT (n=23, 1x4min bout at 85-95%HRpeak, 3x/week). A sub-analysis investigated the differential impact of these training programs on IPC in MetS individuals with type 2 diabetes mellitus (T2DM) (MICT, n=6; 4HIIT, n=9; 1HIIT, n=12) and without the condition (MICT, n=15; 4HIIT, n=13; 1HIIT, n=11). Intact proinsulin, insulin, and C-peptide serum concentrations were measured in duplicate following a 12-hour fast, via enzyme linked immunosorbent assays, before and after the exercise program. Fasting IPC was also expressed relative to insulin and C-peptide concentrations.
RESULTS: Following the training programs, there were no significant (p>0.05) between or within group differences in fasting IPC indices (Pre vs Post proinsulin, proinsulin:insulin, proinsulin:C-peptide: MICT ↓19%, ↑6%, ↑4%,; 4HIIT ↓19%, ↓8%, ↓11%,; 1HIIT ↑34%, ↑49%, ↑36%). In participants that did not have T2DM, only 4HIIT significantly reduced fasting IPC indices from pre- to post-intervention (Pre vs Post proinsulin, proinsulin:insulin, proinsulin:C-peptide: 4HIIT ↓32%, ↓26%, ↓32%, p<0.05; 1HIIT, ↑14%, ↑32%, ↑16%, p>0.05; MICT ↓27, ↓17%, ↓11%, p>0.05). There were no significant (P>0.05) changes in IPC indices in participants with T2DM.
CONCLUSION: Higher volume HIIT (4HIIT) improved insulin quality in MetS participants without T2DM.