Please cite this article in press as: Skinner TL, et al. Factors influencing serum caffeine concentrations following caffeine ingestion. J Sci Med Sport (2013), http://dx.doi.org/10.1016/j.jsams.2013.07.006 ARTICLE IN PRESS G Model JSAMS-906; No. of Pages 5 Journal of Science and Medicine in Sport xxx (2013) xxx–xxx Contents lists available at ScienceDirect Journal of Science and Medicine in Sport j our na l ho me page: www.elsevier.com/locate/jsams Original research Factors influencing serum caffeine concentrations following caffeine ingestion Tina L. Skinner a,∗ , David G. Jenkins a , Michael D. Leveritt a , Alastair McGorm a , Kate A. Bolam a , Jeff S. Coombes a , Dennis R. Taaffe a,b,c a The University of Queensland, School of Human Movement Studies, Australia b The University of Newcastle, School of Environmental and Life Sciences, Australia c Edith Cowan University, Edith Cowan University Health and Wellness Institute, Australia a r t i c l e i n f o Article history: Received 23 February 2013 Received in revised form 20 May 2013 Accepted 10 July 2013 Available online xxx Keywords: Caffeine Ergogenic Exercise DXA Supplement Sport a b s t r a c t Objectives: To determine whether differences in training status, body composition and/or habitual caffeine intake influenced serum caffeine concentrations following caffeine ingestion. Design: Single-blind. Methods: Trained cyclists/triathletes (n = 14) and active (n = 14) males consumed 6 mg kg -1 anhydrous caffeine. Peak, total and time to peak serum caffeine concentrations were determined from venous blood samples at baseline and 6 time-points over 4 h following intake. Body composition was assessed by dual energy X-ray absorptiometry and habitual caffeine intake by a questionnaire. Results: Trained cyclists/triathletes had 16% lower peak caffeine concentrations following caffeine inges- tion compared to active individuals, although this was not statistically significant (p = 0.066). There was no significant difference between trained cyclists/triathletes and active males in total (p = 0.131) or time to peak (p = 0.249) serum caffeine concentrations. Fat mass was significantly associated with total (r = 0.427, p = 0.038) but not peak (r = 0.343, p = 0.101) or time to peak serum caffeine concentration (ˇ = 0.00008, p = 0.961). There were no associations between habitual caffeine intake and peak, total or time to peak serum caffeine concentrations. Conclusions: Following caffeine ingestion three findings from the study were evident: (1) endurance- trained athletes trended towards lower peak caffeine concentrations compared to active males; (2) higher fat mass was associated with higher concentrations of caffeine in the blood over 4 h, and (3) habitual caffeine intake does not appear to influence serum caffeine concentrations. Identification of the opti- mal conditions to ensure peak availability of caffeine within the blood and/or overcoming some of the variation in how individuals respond to caffeine requires consideration of the training status and body composition of the athlete. © 2013 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. 1. Introduction Most investigations examining the potential for caffeine to improve exercise performance have used a dose of 3–6 mg kg -1 caf- feine, that is administered 1 h pre-exercise based on the assumption that (i) peak circulating levels are achieved 1 h following ingestion, and (ii) performing exercise whilst at peak levels will provide the greatest ergogenic benefit. 1–3 Studies that have investigated larger doses of caffeine (≥9 mg kg -1 ) have reported evidence of potential side effects, including impaired reaction time, alertness, gastroin- testinal distress, dizziness, anxiety, irritability and an inability to concentrate, which may negate any stimulatory effect of caffeine on performance. 4–6 Moreover, consumption at such high doses may ∗ Corresponding author. E-mail address: tskinner@hms.uq.edu.au (T.L. Skinner). be difficult to achieve through dietary sources or energy drinks. Therefore, the identification of the smallest ergogenic dose for ath- letic populations is important, however this may vary between individuals if caffeine metabolism is influenced by many differ- ent factors. Balogh et al., 7 reported a distribution of variance in caffeine elimination of 21.4% for within-individual variations and 78.6% for between-individual variations. These reported within- and between-individual variations compare reasonably well with similar studies 8,9 . Thus, identification of factors with the poten- tial to influence caffeine absorption, its appearance and subsequent clearance in the blood may, in part at least, overcome some of the variation in how individuals respond to caffeine intake. There is some support within the literature for the existence of differing responses to caffeine with training status, including increased resting metabolic rate and epinephrine release 10 and swimming velocity 11 in trained versus untrained individuals. Col- lomp et al., 11 investigated the influence of specific training on 1440-2440/$ – see front matter © 2013 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsams.2013.07.006