Growth hormone responses to varying doses of oral arginine

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Abstract

Intravenous (IV) arginine invokes an increase in growth hormone (GH) concentrations, however, little is known about the impact of oral arginine ingestion on the GH response.

Objective

The purpose of this study was to determine the dose of oral arginine that elicits an optimal GH response and to determine the time course of the response.

Design

Eight healthy males (18–33 years – 24.8 ± 1.2 years) were studied on 4 separate occasions. Following an overnight fast at 0700 h, a catheter was placed in a forearm vein. Blood samples were taken every 10 min for 5 h. Thirty minutes after sampling was initiated, the subject ingested a dose of arginine (5, 9 or 13 g) or placebo (randomly assigned).

Results

Mean resting GH values for the placebo, 5, 9 and 13 g day were 0.76, 0.67, 2.0 and 0.79 μg/L (n = 6), respectively. Integrated area under the curve was not different with 13 g (197.8 ± 65.7 min μg/L), yet it increased with 5 and 9 g compared with the placebo (301.5 ± 74.6, 524.28 ± 82.9 and 186.04 ± 47.8 min μg/L, respectively, P < 0.05). Mean peak GH levels were 2.9 ± 0.69, 3.9 ± 0.85, 6.4 ± 1.3 and 4.73 ± 1.27 μg/L on each day for the placebo, 5, 9 and 13 g days.

Conclusion

In conclusion, 5 and 9 g of oral arginine caused a significant GH response. A 13 g dose of arginine resulted in considerable gastrointestinal distress in most subjects without augmentation in the GH response. The rise in GH concentration started ∼30 min after ingestion and peaked ∼60 min post ingestion.

Introduction

Arginine is a semi-essential amino acid that serves many purposes in the human body. Intravenous (IV) studies consistently show a large GH response when an arginine bolus is introduced [1], [2]. IV arginine administration is used clinically as a provocative test to establish GH deficiencies [2], [3], [4], and the doses used to stimulate GH release range from 2 to 40 g. Oral arginine, however, has been used primarily by the athletic population in efforts to enhance the exercise-induced GH response, yet little is known about the effects of oral arginine on GH release. Administration of oral arginine for 30 days has been shown to increase resting GH levels in postmenopausal women [5]. Acute administration of oral arginine has been reported to increase [6] or have no impact on GH secretion [7]. Previous research has administered doses ranging from 1.2 to 17 g/day [5], [6], [7], [8], and has frequently used only one sample to determine GH levels. However, no studies have examined the impact of various doses of oral arginine on the GH response. Potentially, studies administering too small of a dose of arginine may not observe a true GH response or the changes in GH levels may have been missed because of inadequate sampling. Thus, understanding the dose of oral arginine that elicits a GH response and the time course of the metabolic response after ingestion could lead to future studies investigating correct supplementation strategies and combining it with exercise. Therefore, the purpose of this study was to investigate the dose–response relationship of oral arginine on the resting GH response using frequent blood sampling, and to determine the time to the start of the GH response and the time to peak GH levels.

Section snippets

Methods

Eight healthy young males (age 18–33 years) participated in this study and signed a consent approved by the Syracuse University Institutional Review Board. The mean height, weight, age, and body mass index (BMI) were 179.2 ± 2.4 cm, 78 ± 3 kg, 24.8 ± 1.2 years and 20.2 ± 0.7 kg/m2, respectively. None of the subjects were taking any medications prior to and during the study, and all subjects were non-smokers.

Each subject reported to the Human Performance Laboratory on 4 occasions (placebo, 5, 9 and 13 g day).

Results

The purpose of this study was to establish a dose response curve and a timeline after the ingested arginine elicits a GH response. For adequate absorption of oral arginine, it must be mechanically transported from the gut into the blood for a GH response to occur. Because the GH iAUC responses comprised a bimodal distribution, we categorized the subjects into those who had a GH response across provocations (responders) and those who did not (nonresponders). Two non-responders experienced GI

Discussion

Exogenous arginine administration elicits a profound GH response particularly when administered as an IV bolus (1,2,4), but a paucity of research has examined the effects of oral arginine on GH concentrations, and no studies have been conducted to establish if there is a dose response relationship. Our data indicated that 5 and 9 g of oral arginine results in a significant GH response compared to placebo, while 13 g of arginine had a substantially lesser effect. The rise in GH occurred at ∼30 min

Acknowledgement

This study was funded in part by R03AG16370-01.

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