The effect of female breast surface area on heat-activated sweat gland density and output

Female development includes significant morphological changes across the breast. Yet, whether differences in breast surface area (BrSA) modify sweat gland density and output remains unclear. The present study investigated the relationship between BrSA and sweat gland density and output in 22 young to middle-aged women (28 $$ 10 years) of varying breast sizes (BrSA range: 147-561 cm²) during a submaximal run in a warm environment (32  $$ 0.6°C; 53  $$ 1.7% relative humidity). Local sweat gland density and local sweat rate (LSR) above and below the nipple and at the bra triangle were measured. Expired gases were monitored for the estimation of evaporative requirements for heat balance (E_req, in W/m²). Associations between BrSA and (i) sweat gland density; (ii) LSR; and (iii) sweat output per gland for the breast sites were determined via correlation and regression analyses. Our results indicated that breast sweat gland density decreased linearly as BrSA increased (r = -0.76, P < 0.001), whereas sweat output per gland remained constant irrespective of BrSA (r = 0.29, P = 0.28). This resulted in LSR decreasing linearly as BrSA increased (r = -0.62, P = 0.01). Compared to the bra triangle, the breast had a 64% lower sweat gland density (P < 0.001), 83% lower LSR (P < 0.001) and 53% lower output per gland (P < 0.001). BrSA (R² = 0.33, P = 0.015) explained a greater proportion of variance in LSR than E_req (in W/m²) (R² = 0.07, P = 0.538). These novel findings extend the known relationship between body morphology and sweat gland density and LSR, to the female breast. This knowledge could innovate user-centred design of sports bras by accommodating breast size-specific needs for sweat management, skin wetness perception and comfort.