In the current study, we sought to evaluate the efficacy of a PRT program primarily on glycemic control and muscle strength. The results have shown that PRT appears to provide better glycemic control and substantial improvements in muscle strength in untrained middle-aged type 2 diabetics with an Asian Indian ethnicity, compared to a control program. Among secondary outcomes, WC showed a greater reduction with PRT and this was associated with an increase in HDL-C levels. However, other secondary outcomes did not show any G × T interaction and were maintained over the intervention period.
It is suggested that several factors may contribute to the improvement in glycemic control seen with resistance exercise training programs: (1) Increases in muscle mass, which provide a larger reservoir for glucose disposal; (2) Direct effects on skeletal muscle that increase glucose transport activity; and (3) Improvements secondary to a loss of adipose tissue (in particular, visceral adipose tissue, which is known to be a contributor to insulin resistance).
The observed reductions in HbA1c levels in the PRT group in this study may have resulted from mechanisms involving increasing levels of muscle GLUT4, insulin receptors, protein kinase B, glycogen synthase and glycogen synthase total activity following PRT. Previous research has found that strength training for 30 minutes three times per week increases insulin action in skeletal muscle by the said mechanisms.[21
] Upper body and lower body muscular strength gains in the PRT group may have led to increased insulin action in skeletal muscle, an effect that may be attributed to qualitative muscular adaptations to resistance training.
The lack of any improvements observed in most of the secondary outcomes such as TC, TG, LDL-C, blood pressure and body composition in PRT group can be explained by the reduced duration of active muscle contraction in PRT sessions. PRT involves isolated brief activity of single muscle groups and the anaerobic metabolism involved necessitates rest periods between sets, hence, reducing the overall duration of active muscle contraction. This limitation could be overcome with aerobic training, where continuous activity of many large muscle groups is involved, thereby increasing the duration of active exercise. This could be the reason of improvements in the said outcomes observed in previous aerobic training trials. Our findings on the reduced HbA1c levels suggest that reduced duration of active exercise was not a factor determining glycemic control. This is supported by the findings from previous research[22
] where effects of aerobic, resistance and combination of both were evaluated on glycemic control. They observed similar changes in glycemic control in both resistance and aerobic training groups, irrespective of the duration of training.
Previous studies investigating the effects of PRT in T2D differ from the current study because most studies have been conducted on sedentary older adults predominantly from developed countries. The current study sample was untrained middle-aged T2D patients who are Asian Indians. It is known that Asian Indians manifest insulin resistance and T2D at a younger age and at a higher magnitude than any other ethnic groups[23
] Second, we sought to evaluate the effects of PRT since it has been debated that resistance exercise will be metabolically more beneficial in Asian Indians.[25
Scarcity in the literature on PRT effects in T2D, specifically in Asian Indians makes comparison of the observed results difficult. A pre-experimental research study conducted by Misra et al
] in Asian Indians have reported improved insulin sensitivity and reduced HbA1c and fasting blood glucose levels with PRT. HbA1c reductions of 0.54 ± 0.4% reported were comparable to 0.6 ± 0.5% in the current study. Another RCT conducted in Singaporeans reported 0.4 ± 0.6% reductions in HbA1c following PRT and 0.1% (95% CI: -0.5 to 0.3) difference when compared to aerobic training.[13
] We observed 0.6% difference between experimental and control groups. These data suggest that when compared to non-exercising control group, PRT leads to clinically significant reductions in HbA1c, but not when compared to aerobic training. The study in Singaporeans used similar duration of PRT as in the present study. Muscle strength results also showed a significant increase in the PRT group when compared to control group. These results are consistent with previous studies reporting large and significant increase in muscle strength following PRT when compared with non-exercising controls.[26
] Clinical significance of the observed increase in muscular strength is difficult to interpret, because we could not find any research on the minimal clinically important difference in muscular strength for people with T2D. Evidence suggests that loss of muscular strength is associated with loss of physical function in men with T2D.[28
] The increased muscle strength found in the present study may be clinically significant, because this may lead to improved physical function.
The results also suggested that PRT resulted in a greater reduction in WC than non-exercising control group. A previous study also demonstrated a significant reduction in WC,[13
] but the average reduction was 1.6 ± 2.6 cm compared to 1.84 ± 2.7 cm observed in the present study. WC has been considered as a robust predictor of abdominal visceral fat,[29
] hence, it could be hypothesized that reduction in WC also lead to the changes observed in HbA1c in response to PRT. But, this greater reduction in WC was not associated with any clinically meaningful changes in cardiovascular risk factors measured in this study, except the small improvements in HDL-C levels. These findings are contradictory to the study findings of Misra et al
] in a similar population where there was a significant reduction in TC and TG levels in response to PRT. This may be attributed to the 12-week duration of PRT compared to the 8-week duration in our study. Furthermore, it has been reported that a reduction of 3 cm or more in WC is required for any additional benefit in metabolic profile,[30
] whereas PRT group in this study only showed an average WC reduction of 1.84 cm.
The strengths of our study include the supervised and monitored exercise regimens, the good compliance to exercise programs and the absence of significant changes in medications during the protocol. The chief limitation of our study was an unequal number of men and women in the study sample. Future studies with equal gender distribution would yield more generalizable results. We measured HbA1c after 8 weeks of training; hence, the observed changes in HbA1c may not adequately reflect the effect of interventions. Future studies measuring HbA1c in response to longer exercise protocols may be considered. Since the study was of smaller duration, we have not measured the caloric intake and expenditure, hence giving lack of data on possible extraneous factors that may affect the study results. Nevertheless, clinically relevant results were obtained on the less-studied Asian Indian T2D population, giving directions for future clinical practice and research.