The Mediating Role of Brown Fat and Skeletal Muscle Measured by 18 F-Fluorodeoxyglucose in the Thermoregulatory System in Young Adults

Abstract

Objective: This study aimed to examine whether brown adipose tissue (BAT) or skeletal muscle activity mediates the relationship between personal level of environmental temperature (Personal-ET) and wrist skin temperature (WT). Moreover, we examined whether BAT and skeletal muscle have a mediating role between Personal-ET and WT (as a proxy of peripheral vasoconstriction/vasodilation).

Methods: The levels of BAT were quantified by cold-induced ¹⁸ F-fluorodeoxyglucose-positron emission tomography/computed tomography scan and measured the Personal-ET and WT by using iButtons (Maxim Integrated, Dallas, Texas) in 75 participants (74.6% women).

Results: The study found that BAT volume and metabolic activity played a positive and significant role (up to 25.4%) in the association between Personal-ET and WT. In addition, at the coldest temperatures, the participants with lower levels of WT (inducing higher peripheral vasoconstriction) had higher levels of BAT outcomes, whereas in warm temperatures, participants with higher levels of WT (inducing higher peripheral vasodilation) had lower levels of BAT outcomes. The study did not find any mediating role of skeletal muscle activity.

Conclusions: BAT volume and metabolic activity play a role in the relationship between Personal-ET and WT. Moreover, the data suggest that there are two distinct phenotypes: individuals who respond better to the cold, both through nonshivering thermogenesis and peripheral vasoconstriction, and individuals who respond better to the heat.

Figure 1

Mediation models of the relationship between Personal‐ET and WT with (A) BAT volume (milliliters), (B) SUV_mean, (C) SUV_peak, (D) metabolic activity (calculated as BAT volume  ×  BAT SUV_mean), and (E) skeletal muscle activity included as mediator variables. Paths a, b, c, and c’ are presented as unstandardized coefficients (SE). β = indirect effect (a × b paths) [lower‐limit CI; upper‐limit CI], lower and upper levels for bias‐corrected 95% CIs of the indirect effect based on 5,000 bootstraps. BAT, brown adipose tissue; CI, confidence interval; ns, nonsignificant; Personal‐ET, personal level of environmental temperature; SUV, standardized uptake value; WT, wrist skin temperature.

Figure 2

(A) WT mediation models of the relationship between the number of hours exposed to a certain Personal‐ET and BAT‐related outcomes in young adults. Path c shows the association between independent and dependent variables. Arrow a multiplied by arrow b shows the natural indirect effect (β) pathway, and the arrow for c’ shows the natural direct effect pathway. (B) Indirect effects (β) of the simple mediation analyses of WT on the association between the number of hours exposed to each degree of Personal‐ET (from 14°C to 28°C) and BAT volume. (C,D) Indirect effect for BAT SUV_peak and metabolic activity, respectively. (E) P_M of the simple mediation analyses of WT on the association between the number of hours exposed to each degree of Personal‐ET (from 14°C to 28°C) and BAT volume. (F,G) P_M for BAT SUV_peak and metabolic activity, respectively. Black dots indicate that zero was in the 95% confidence interval of the indirect effect, and therefore, the mediation was considered not statistically significant (P > 0.05). Red and blue dots mean that the mediation analysis was statistically significant but with a different direction. BAT, brown adipose tissue; Personal‐ET, personal level of environmental temperature; P_M, percentage of mediation; SUV, standardized uptake value; WT, wrist skin temperature. [Colour figure can be viewed at wileyonlinelibrary.com]