High-intensity interval exercise test stimulates growth hormone secretion in children
Introduction
Growth hormone (GH) is secreted in a pulsatile manner mainly during deep sleep, while during the day blood levels are very low or even undetectable [1]. As a consequence, GH deficiency is almost impossible to diagnose based on random GH blood sampling and in most cases, GH pharmacological stimulation test with serial blood GH levels is needed. The pharmacological GH stimulation tests have many limitations, such as patient risk (e.g. hypoglycemia), arbitrary cutoff levels, low reproducibility, and, most importantly, the interpretation of a “normal” GH response to pharmacological stimuli may not necessarily reflect physiological GH secretion [2]. These drawbacks highlight the need for a physiological stimulation test. Physiological stimulators for GH secretion include overnight sleep and exercise. Sleep has proved to be inadequate as a useful approach, consequently, exercise testing may serve as a promising solution for the diagnosis of GH deficiency [3].
GH responses to exercise testing in early studies showed good correlation with pharmacological stimulation tests, but were associated with large number of false negative results, probably due to exercise protocols standardization difficulties, including workload, intensity, duration, type of exercise, and timing of blood sampling [4,5]. When exercise intensity was controlled to 75–90% of the peak aerobic capacity and exercise duration lasted at least 10 min, the sensitivity and specificity of the test improved significantly [[6], [7], [8]]. This protocol is less practical for clinical use since it mandates at least two laboratory visits; the first to determine maximal aerobic capacity and anaerobic threshold, and the second for the exercise test itself. Moreover, 10 min of continuous exercise above the anaerobic threshold is very difficult to maintain in children and does not reflect the pattern of physical activity that children usually perform (characterized by short bursts of high intensity-activity on top of light to moderate aerobic-type activity) [9,10]. Therefore, in order to better represent physical activity of younger population, we previously evaluated the GH response to the Wingate Anaerobic Test (WAnT). While in adults the GH response was very good [11], this 30-s ‘all out’ cycling against resistance determined by sex and weight failed to stimulate GH response in children [12]. Currently, the reason for this difference between adults and children is unknown. It is possible that the reduced anaerobic capacity of children characterized by lower exercise-induced lactate levels and faster recovery was not sufficient to induce GH secretion. In addition, although closer to children's activity patterns, 30 s of ‘all out’ continuous anaerobic exercise does not reflect the much shorter exercise bursts that children perform regularly.
Therefore, the aim of the present study was to examine the GH secretion to high intensity interval exercise composed of repeated very short anaerobic exercises with active rest between them. This type of exercise, although very demanding physiologically and metabolically, better represents exercise patters of children, requires only a single laboratory visit, and is shorter than the currently commonly used GH stimulation tests.
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Participants
Seventeen children participated in the study (14 males and 3 females, age range 5.0–14.2 years, body weight 35.3 ± 12.6 kg, height 136.5 ± 15.2 cm, body mass index (BMI) 18.5 ± 3.3 kg/m2, BMI percentile 43.9% ± 29.8%). Three participants were overweight. Pubertal stage was determined using Tanner stage for pubic hair for both sexes as well as breast for females or testicular volume for males. Eight participants were pre-pubertal, seven were at Tanner stage 2, one at Tanner stage 3, and one at
Results
Seventeen participants completed the High-Intensity Interval Test for GH secretion. Their anthropometric measures, exercise and pharmacological provocation tests data, growth velocity and diagnosis are presented in Table 1. The average peak power of the ten 15 s intervals was 7.2 ± 2.5 W/kg, mean power was 4.7 ± 1.4 W/kg, and fatigue index was 56.2% ± 15.5%. Average peak power per body weight, fatigue index and pedaling rate at each sprint are presented in Fig. 1.
There was a significant
Discussion
Although exercise is a well-known stimulus for GH secretion and currently is the only physiological provocation test for GH secretion, it is not used routinely in clinical practice due to lack of standardized protocol. This study presents a new feasible exercise test protocol that better reflects the activity pattern that children usually perform and may be used as physiological stimuli for the diagnosis of GH deficiency in children. The exercise test protocol included 10 bouts of 15-s
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Funding
This research was supported by a grant from the “Time for Action” Foundation and NIH grant 5U01TR002004-02.
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