We hypothesized that the CAST approach would have synergistic improvements on both adiposity parameters and insulin resistance, specifically when compared with the N + ST group. Although the CAST approach did not result in significant improvements in insulin resistance, it did reduce all adiposity parameters when compared with the N + ST group and fasting glucose when compared with the N group. These results suggest that the CAST approach may be a good exercise modality for reducing adiposity and for improving some metabolic parameters in an overweight Latina adolescent population. To our knowledge, this is the first study that has used the combination of aerobic and strength training (CAST) in an overweight Latino adolescent population.
Numerous adult studies have shown aerobic exercise to improve insulin sensitivity or action (12
). Goodpaster et al. (12
) showed that a 16-wk aerobic intervention (4–6 d·wk−1
) in combination with energy restriction resulted in a 49% increase in insulin sensitivity in 25 obese adults. To date, there have only been a few studies that have assessed the impact of an aerobic only intervention on insulin sensitivity or action in children. In a 10-wk intervention conducted by Gutin et al. (16
), neither an aerobic training (n
= 12) nor a lifestyle education (n
= 10) improved fasting insulin levels in obese African American girls. In contrast, Ferguson et al. (11
) showed that an intensive aerobic training program (5 d·wk−1
for 4 months; 40-min training sessions) resulted in small, yet significant, improvements fasting insulin in obese boys and girls (n
= 79). Although these results suggest that aerobic-only interventions may be useful in improving insulin action in adults, the results are not quite as clear for children, and other forms of exercises should be explored to decrease type 2 diabetes risk for children.
Few intervention studies have used the CAST approach to reduce adiposity and metabolic-related parameters in adolescent populations. Byrne et al. (4
) randomly assigned 48 overweight adolescents to one of three 16-wk intervention groups: 1) aerobic training only, 2) strength training, and 3) combination of aerobic and strength training (CAST). Subjects who received the CAST achieved better cardiovascular gains (
) and lost more fat mass as measured by DEXA than the strength training group while maintaining similar strength gains (one repetition maximum), but no assessment of glucose/insulin indices was conducted. Another study by Watts et al. (38
) found that an 8-wk crossover CAST intervention (three times per week) resulted in significant decreases in abdominal and trunk fat as measured by DEXA in 19 obese adolescents but did not have an impact on total adiposity or fasting lipids or glucose. A recent study by Bell et al. (2
) showed that an 8-wk CAST program in 14 obese children resulted in a 22% increase in insulin sensitivity, as measured by euglycemic–hyperinsulinemic clamp, but no differences in body composition were found, and this study did not use a control group. In contrast, we found that our 16-wk CAST pilot intervention resulted in significant reductions in adiposity when compared with the N + ST group and significant reductions in fasting glucose when compared with the N group, but no changes in insulin sensitivity were found.
With our previous nutrition pilot study, we saw that significant reductions in added sugar intake were related to significant improvements in insulin secretion (8
). The significant reduction in total sugar, seen in the N + CAST group, may in part explain the significant improvements seen in fasting glucose. However, added sugar in this group did not decrease, which may explain the null findings for insulin secretion. In addition, the reductions in added sugar and carbohydrate intake seen in the N group also did not lead to significant improvements in insulin secretion. In subsequent analyses and articles, we intend to examine whether dietary compliers (i.e., those who decreased both total and added sugar and increased dietary fiber vs those who did not) and changes in food and beverage components affect changes in glucose/insulin indices. Assessing differences in dietary compliance, independent of intervention group, is needed to fully understand how changes in dietary intake may impact metabolic parameters.
We also expected that the strength gains seen in the N + ST and the N + CAST groups would have resulted in improvements in insulin sensitivity as seen in our strength training pilot study. However, there were no significant improvements in insulin sensitivity across intervention groups. It is important to note that the previous pilot study did not include a nutrition component and was conducted only with Latino boys. The current findings suggest that the improvements in strength, with or without changes in body composition, did not lead to similar improvements in insulin sensitivity in girls. However, in future analyses, we intend to explore whether participants who increased their strength and physical activity levels, regardless of intervention group, had greater improvements in adiposity and metabolic parameters compared with those who decreased their strength and physical activity levels.
Other investigators have shown that intense cardiovascular programs have resulted in similar adiposity improvements in obese children and adolescents (15
). Most notably, Gutin et al. (15
) found that a 16-wk (5 d·wk−1
for 40 min) cardiovascular exercise intervention resulted in a significant reduction of 0.7 kg of fat mass (or a 3.6% body fat reduction) in 80 obese adolescents. This reduction in fat mass is similar to what we saw with this CAST intervention; however, the frequency of this exercise program was only 2 d·wk−1
for 60 min. The less frequent CAST program may be more realistic and feasible for most individuals and more sustainable for long-term weight maintenance.
In addition, the CAST modality may be better suited for an overweight population because it includes only short bouts of cardiovascular components (2 min in length) coupled with strength training. Given that some of the participants in our study were extremely overweight, with six participants exceeding 250 lb (one in C, one in N, two in N + ST, and two in N + CAST), a 60-min aerobic intervention may not have been feasible, or as easily attainable, for these participants. The CAST approach allowed these obese teens to complete an accumulation of 30 min of cardiovascular exercise in an achievable fashion.
In addition, the motivation of the participants must also be considered. Numerous studies highlight the importance of assessing the potential mediating role of the motivational factors on health outcomes (28
). In subsequent articles and analyses, we intend to assess whether motivational factors, identified during MI sessions and in the various motivational questionnaires that were administered before and after intervention, affected dietary, strength and physical activity compliance, and subsequent health outcomes regardless of intervention group.
There are limitations of this study that should be considered. The first limitation is the relatively small sample size. The power calculation was based on change in insulin sensitivity between the strength training and the control groups from the strength training pilot study, and this calculation was conducted only in boys, which did not take into account the change in adiposity or the addition of the CAST design nor include a nutrition component. Nevertheless, this was a pilot, supplemental study, and we intend to test this combination exercise approach in larger intervention studies using both genders. The second limitation is the unequal sample sizes of each intervention group. As mentioned earlier, we oversampled in the CAST group to be comparable to the larger trial using both genders. Another limitation is that there was no comparison to an aerobic-only intervention group; however, we intend to do this type of comparison in future studies. There was also no functional assessment of physical activity (i.e., 6-min walk test) or cardiovascular fitness (i.e.,
) measured in this study, both of which would have potentially been useful in interpreting the improvements in adiposity seen in the CAST group.
In conclusion, a combined aerobic and strength training exercise intervention successfully reduced multiple adiposity measures by approximately 3% in a sample of overweight Latina adolescent girls compared with the N + ST group. However, neither the N + CAST group nor any other intervention group significantly improved metabolic parameters, with the exception of fasting glucose levels. These results suggest that even intense interventions that successfully reduce adiposity may not have synergistic improvements on metabolic parameters, particularly insulin indices. Although with time, the improvements in adiposity seen with the combined approach could reduce other type 2 diabetes risk factors. Further research investigating and identifying intervention approaches that improve both adiposity and metabolic parameters, particularly in high-risk population, is warranted.