Introduction: Low cardiorespiratory fitness (fitness) and high levels of adiposity are independently associated with higher levels of blood pressure in adolescents. However, it remains uncertain whether the associations between fitness and blood pressure are due to fitness itself or results from lower levels of adiposity. Moreover, there are no studies that have determined the extent to which adiposity, including central adiposity, moderates the association between fitness and 24-hour ambulatory blood pressure (ABP).
Hypotheses: 1. Higher levels of fitness will be associated with lower levels of ambulatory systolic (SBP) and diastolic (DBP) blood pressure after adjusting for adiposity and covariates. 2. With adjustments for covariates, adiposity (body mass index [BMI], waist circumference [WC]) will modify the association between fitness and 24-hour SBP and DBP.
Methods: A cross-sectional study was conducted in Houston, TX with a sample of 370 adolescents aged 11-16 years. Demographically, the sample was 54% female, 37% African American, 31% Hispanic, 29% non-Hispanic white, and 3% other ethnic/racial groups. Fitness was assessed by a height-adjusted step test and estimated by heart rate recovery, defined as the difference between peak heart rate during exercise and heart rate two minutes post-exercise. Adiposity was measured using dichotomized values for percentiles of BMI (≥ 85th) and WC (≥ 50th). Ambulatory SBP and DBP (Spacelabs model 90207) were measured every 30-60 minutes over 24 hours on a school day. Mixed-effects regression analysis was used to test the hypotheses with the following covariates: activity, location, and position at the time of each ABP measurement, height, age, sex, ethnicity, sexual maturation level, and mother’s education level.
Results: Hypothesis 1: Each unit increase in fitness was associated with a decrease of SBP (-0.058 mmHg, p = 0.001) and DBP (-0.043 mmHg, p < 0.0001) after adjustment for WC and covariates. Each unit increase in fitness was associated with a decrease in SBP (-0.058 mmHg, p = 0.001) and DBP (-0.045 mmHg, p < 0.0001) after adjustment for BMI and covariates. Hypothesis 2: Fitness and BMI ≥ 85th percentile (or WC ≥ 50th percentile) interactions were not significantly associated with ambulatory SBP or DBP after adjustment for covariates.
Conclusions: Our findings indicate a small but statistically significant inverse effect of fitness on 24-hour ABP in adolescents, and no evidence of a modifying effect of adiposity on this association. Further research is needed to better understand the protective role of fitness on cardiovascular health in adolescents.