We hypothesized that strength gains in the N+ST group, independent of changes in body composition, would result in significant improvements in insulin sensitivity, similar to what we saw in our pilot study (17
). Although we saw similar strength gains as in our pilot, ~30% (17
), these strength gains did not translate into improvement in insulin sensitivity. Other adult studies have shown that 10–30% increases in strength resulted in 10–50% increases in insulin sensitivity (15
), which suggests that although strength gains are associated with increases in insulin sensitivity, there is no clear dose-dependent effect of strength gains on insulin sensitivity. However, these adult studies did not include a dietary component and it is possible that the addition of the dietary component in our study diluted the effects.
We hypothesized that changes in carbohydrate quality in the N and N+ST group would result in reductions in adiposity and improvements in insulin secretion, similar to our ALAS pilot study (13
). Numerous cross-sectional studies have shown that fiber and/or sugar intake are inversely associated with glycemic control and poor insulin action (12
); however, few intervention studies have been conducted that focus on reducing added sugar and increasing dietary fiber. A study by Ebbeling et al.
) found that a low glycemic load diet improved insulin resistance compared to a conventional low-fat diet in 16 overweight adolescents (aged 13–21 years). In this study, while neither intervention group significantly decreased added sugar or increased fiber intake, there were significant reductions in overall carbohydrate intake. The reductions in carbohydrate could, in part, explain the significant decrease in the glucose IAUC. In addition, females in the intervention groups did not gain as much weight compared to the C group, which suggests that reduction in carbohydrate intake, regardless the quality, may have contributed to less weight gain in females but not males, similar to what we saw with the ALAS pilot. These findings also highlight the need to further explore the response to intervention as a function of dietary compliance.
It is important to note that there were several improvements in insulin indices across intervention group, although not significant. The N and N+ST group decreased 2-h insulin by 32 and 18% compared to a 24% increase in the C group. Insulin IAUC also improved, with the N and N+ST decreasing by 20 and 25%, while the C group increased by 9%, although not significant. With a larger sample size, these improvements in insulin indices could have reached significance.
There are several possible explanations for the null findings in insulin sensitivity and adiposity parameters of this study. Although we hypothesized that the different interventions would be additive, intervening on multiple behaviors may not be optimal. Research suggests that the number of behavior targets is inversely related to the magnitude of the intervention effects for obesity (28
). Specifically, an intensive nutrition and strength training program may have attempted to change too many health behaviors and therefore diluted the potential effects. Although little is known about the effects of energy compensation in response to exercise in children, research in adults has shown that when individuals exercise they often compensate with increased energy intake (30
) or reduced physical activity outside of the training session (31
) and this compensation is even more pronounced in females (32
). Thus, the participants in the N+ST group may have altered their intake in response to the exercise throughout the program. Subsequently, the diet records collected after the intervention would not really reflect this acute compensatory intake.
Alternatively, the dose and the duration of the strength training intervention, two times a week for 16 weeks, may not have been frequent or long enough to see improvements in health outcomes. The American Academy of Pediatrics, Council on Sports Medicine and Fitness, recommends a frequency of 2–3 days a week of strength training for at least an 8-week duration (34
). A 20-week intervention study conducted in obese, prepubertal girls showed that strength training three times per week resulted in improvements in intra-abdominal adipose tissue and insulin indices (35
). Although, strength training two times per week for 16 weeks in STEALTH program resulted in improvements in insulin sensitivity in males, strength training 3 days/week for 20 weeks may be more optimal for females.
In secondary analyses and subsequent papers, we intend to assess whether there are differences in compliance across intervention group. Although participants attended at least 12 of the 16 nutrition classes and at least 28 of the 32 exercise sessions, we may stratify the participants into lower (12–14 classes) vs. more (15–16 classes) in order to assess changes in health outcomes. We also intend to evaluate compliance in regards to dietary intake strength and/or physical activity levels in subsequent analyses. We are currently examining whether dietary compliers, those who reduced sugar intake and increased dietary fiber, had more improvements in insulin indices and adiposity parameters compared to dietary noncompliers. In future analyses, we will also assess whether participants who increased their strength and physical activity levels had greater improvements in adiposity and metabolic parameters compared to those who decreased their strength and physical activity levels. In addition, the motivation of the participants must also be considered. Motivational questionnaires that were administered pre- and postintervention will be useful in identifying the potential mediating role of motivational factors on health outcomes.
One potential limitation to consider relates to the small sample size. Although, the power calculation showed that we would be able to detect significant differences in change in insulin sensitivity across intervention groups, this calculation was based on pilot data from only boys and the intervention did not include a nutrition component. In addition, this study was not powered to detect differences in gender and a larger sample size would have allowed for better exploration of gender differences across groups.
In conclusion, this intensive, randomized control trial, designed specifically for overweight Latino teenagers based on previous successful pilot studies, did not result in the expected adiposity and metabolic improvements. These findings highlight the need for either different or stricter dietary (i.e., more aggressive modification of carbohydrate intake) and exercise (i.e., increased dose, duration, and intensity and different modality) approaches to elicit improvements in adiposity and metabolic parameters in this high-risk population. These findings also emphasize the need to further investigate the intervention response as a function of dietary, strength, and physical activity compliance, the potential mediating effect of motivation, and whether intervening on multiple health behaviors is optimal.