We studied healthy males challenged with a 900 Cal test beverage and correlated EE with the raw (ms2) and normalized units (nu) of total power (TP), low frequency/high frequency (LF/HF) and VLF spectral power of heart rate variability (HRV). The correlations were evaluated during 20？min of normal breathing (NB, control) and 20？min of paced breathing (PB) at 12 breaths min？1 (0.2？Hz). EE was not significantly correlated with any of the HRV variables before the metabolic challenge. After the challenge, EE was positively correlated with LF/HF and with VLF; VLF was also positively correlated with LF/HF during both NB and PB. These findings suggest that EE may be a correlate of LF/HF and of VLF spectral power of HRV in healthy adolescent/young adult males. The association of lower resting energy expenditure with lower amounts of VLF spectral power may occur in individuals with predilections for obese phenotypes. 1. Introduction Sympathetic regulatory mechanisms are at the nexus of nutrition, metabolism, and obesity. Heart rate variability spectral power (HRVSP) measurements of respiratory sinus arrhythmia are noninvasive indicators of the autonomic influences on heart rate regulation. Advances in knowledge of obese phenotypes have been impeded by the lack of noninvasive technologies for measuring the impact of body fat on regulatory mechanisms. However, this impediment has effectively been overcome by the advent of heart rate variability (HRV) analyses for elucidating autonomic mechanisms  which make it possible to differentiate a wide variety of conditions with common autonomic etiologies [2–6]. Previous studies have shown correlations between increments in vagal signaling and high-frequency (HF) HRVSP during controlled (paced) breathing [7–9], and paced breathing has often been used to limit the influence of HF HRVSP on the low-frequency/high-frequency spectral power ratio (LF/HF), a commonly used index of sympathetic influences. Although the percentage of body fat may be a determinant of HRVSP measured at rest [10, 11], the influence of body fat on HRV measurements was found to be nil when performed at rest and significant only during an autonomic challenge . We have demonstrated positive correlations of LF/HF with the respiratory quotient (RQ) before and after feeding  and negative correlations with the percentage of body fat in healthy young adult/adolescent African American males after overnight fasting and the latter only during 5？min periods of spontaneous, uncontrolled breathing and not during 5？min periods of paced breathing at 12
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