Abstract
Background: Leading sports nutrition experts recommend customised fluid and electrolyte replacement strategies for optimal rehydration. Inadequate replacement of sodium can impair performance and health, but replacement needs are unique to an individuals’ sweat sodium loss (SSL) - a product of sweat sodium concentration ([Na+]sweat) and sweat volume (or rate). Estimating [Na+]sweat requires laboratory analysis of an individuals’ sweat sample, and therefore the sports nutrition practitioner (SNP) can struggle to provide professional sports teams with customised sodium replacement advice in the field setting. Therefore research identifying groups prone to higher [Na+]sweat than others (‘saltier sweaters’) may help the SNP to do so.
Ethnicity may be a practical approach to identifying ‘saltier sweaters’ within sports teams. Small field studies in the US suggest that ‘White’ male athletes may incur higher SSL than their ‘Black’ counterparts. No such research has been conducted in New Zealand (NZ) which primarily consists of athletes identifying with the Maori, Pacific or NZ European (NZE) ethnic groups. The Maori and Pacific (MP) groups can be considered one group given their shared ancestral origins, and therefore the objective of this study was to investigate whether ‘saltier sweaters’ can be identified by the proxy of ethnic group in NZ. An overarching goal was to inform SNPs about the SSL of athletes identifying with the MP or NZE ethnic groups for their future practice in giving sodium replacement advice to either group of athlete.
Methods: Fifty-eight highly trained adult male team-sport athletes were recruited from four cohorts within NZ and one cohort in Tonga between June and September 2011. Participants’ body mass was measured before and after a 60 minute exercise protocol on a stationary bike. Sweat patches were placed on their right scapula to enable estimates of [Na+]sweat and ultimately SSL. Environmental conditions can differ between NZ and Tonga, and therefore a difference in heat acclimatisation status was suspected within the MP group of participants (MP-ALL). Consequently data from this group were further categorised as being collected in NZ (MP-NZ) or Tonga (MP-TGA). The study results were examined with and without the MP-TGA cohort to evaluate the potential confounding effect of a disparity in acclimatisation status.
Results: The mean [Na+]sweat of the MP-ALL group was significantly higher than the NZE group (37.6±19.7mmol.L-1 versus 34.9±17.6mmolL-1 respectively, p=0.003). There was no evidence of a difference in SSL between the MP-ALL and NZE ethnic groups (31.8±19.8mmol.h-1 versus 33.2±22.5mmol.h-1 respectively, p=0.871). In contrast, SSL was significantly higher among the MP-NZ compared to NZE group (42.2±16.3mmol.h-1 versus 33.2±22.5mmol.h-1 respectively, p=0.038). The mean [Na+]sweat was also significantly higher among the MP-NZ group compared to NZE (45.1±18.7mmol.L-1 versus 34.9±17.6mmol.L-1 respectively, p=0.013), although the MP-NZ group appeared to exercise at a marginally lower if not similar intensity than the NZE group (based on mean heart rate data). This may have attenuated the difference in mean [Na+]sweat as a consequence of a confounding effect on mean sweat rates which were similar if not marginally higher among the MP-NZ compared to NZE group (0.98±0.34L.h-1 and 0.89±0.33L.h-1 respectively).
Discussion: The main finding of this study is that sweat sodium replacement can be individualised among athletes on the basis of ethnicity in NZ. The MP-NZ ethnic group were ‘saltier sweaters’ and overall they incurred a higher SSL than the NZE group. This means sodium replacement needs were greater among the MP-NZ group and the difference may be more pronounced in reality when/if exercise intensity is held constant between the two groups. Overall this study is important for SNPs to be aware of in NZ before encouraging rapid rehydration between morning and afternoon training sessions – particularly in hot or humid environmental conditions.