Robinson, Lindsay
(2020)
Exploring the determinants of brown adipose tissue function in early life using infrared thermography.
PhD thesis, University of Nottingham.
Abstract
There are significant gaps in our understanding of the developmental physiology of brown adipose tissue (BAT) between the neonatal period and adulthood. Studies of BAT in childhood are limited to the retrospective analysis of radiolabelled positron emission tomography (PET) imaging untaken for paediatric oncological diagnosis, treatment or monitoring. This thesis aimed to utilise supraclavicular skin temperature (TSCR), derived from infrared thermography (IRT), as a non-invasive, surrogate marker of BAT thermogenic function in children and, to use this methodology to identify determinants of BAT function in early life.
Study I was undertaken in primary school-age children (6 – 11 years) within a free-living school environment, it explored the impact of age, sex, ambient room temperature and body mass index (BMI) on TSCR, and also sought to quantify the thermogenic response (∆TSCR) to a localised mild cold stimulus (single-hand immersion in cool tap water c.20˚C). BMI and ambient room temperature were identified as significant determinants of TSCR and ∆TSCR. The magnitude of the thermogenic response to cold exposure was less than expected (c. 0.1˚C) and a second study was undertaken to confirm the findings of Study II. Study II also explored the impact of dietary intake, activity and eating behaviour on TSCR. In line with Study I, an inverse relationship was identified between BMI and TSCR, but no other persistent relationships were observed. In addition, TSCR and ∆TSCR were higher in boys than girls in this slightly older study group (aged 8.5 – 11 years). Similar findings were also identified retrospectively in the Study I cohort of children when limited to children aged 8.5 years and above. The whole group thermogenic response to cooling in Study II was still unexpectedly small, and Studies III – V were undertaken to explore why this may have been.
Study III identified that TSCR and ∆TSCR were higher in participants exposed to mild anticipatory psychological stress (confirmed by raised cortisol) than when relaxed. The study protocol also unintentionally resulted in passive cooling, indicated by a gradual reduction in overall mean skin temperature (TMSK). Conversely, TSCR did not fall. During the relaxation phase, a significant increase in TSCR was observed in parallel to a reduction in TMSK, indicating that BAT may have been activated in response to passive cooling in addition to anticipatory psychological stress.
Study IV evaluated the effect of ambient room temperature under controlled laboratory conditions. A differential effect of ambient temperature on supraclavicular heat production was observed. When acclimated to a warm room both TSCR and energy expenditure (EE) increased, when acclimated to a cool room, despite an ongoing decline in TMSK neither TSCR nor EE changed significantly after exposure to an additional localised cold stimulus. Suggesingt that BAT may respond to very small changes in room temperature, and may have been near maximally activated at 18˚C in light clothing.
Study V examined whether a more severe and prolonged localised cold stimulus would enhance the supraclavicular thermogenic response. Using a water-perfused blanket, the forearm was selectively cooled at 14˚C for 45 minutes and the time course and magnitude of changes in TSCR were investigated. In this group of participants, the maximal TSCR response of c.0.4˚C was observed after thirty minutes. Participants also underwent MRI, from which subcutaneous adipose tissue (SCAT) depth was measured on the dorsal aspect of the upper thorax. The inverse relationship between BMI and TSCR in Studies II and II were also observed in this group of young adults and persisted even after adjustment for the insulative effect of regional SCAT.
In conclusion, small changes in TSCR are detectable using infrared thermography in children and young adults. Sex, BMI, ambient temperature and psychological state all impact upon TSCR indicative of an effect on BAT thermogenesis.
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