Aging Increases Enterocyte Damage during a 3-Hour Exposure to Very Hot and Dry Heat: A Preliminary Study

SIMPLE SUMMARY: Older age is associated with a greater risk of hospital visits and death during extreme heat events (heat waves). Previous research has identified that older people have reduced sweating and cardiovascular responses to heat compared with younger people, which could explain these increased health risks. However, prolonged heat exposure can also damage the gut lining, potentially resulting in microbes to ‘leak’ from the intestines into the blood circulation. If left untreated, this can cause widespread inflammation across the body which can be fatal. Although damage to the gut lining is well accepted to contribute to the heat stroke, no studies have determined whether older people could be more at risk of heat-related gastrointestinal lining damage. We exposed 16 young, and 16 older people to a 3-h heat exposure in a very hot and dry environment, and tested whether there was a group difference in blood markers of gut lining damage. We showed that gut lining damage occurred to a greater extent in older people, which in a real-world heat wave, has the potential to result in a greater release of microbes into the circulation. ABSTRACT: Profound heat stress can damage the gastrointestinal barrier, leading to microbial translocation from the gut and subsequent systemic inflammation. Despite the greater vulnerability of older people to heat wave-related morbidity and mortality, it is unknown if age modulates gastrointestinal barrier damage and inflammation during heat stress. Therefore, the aim of this study was to determine if aging impacted enterocyte damage and systemic inflammatory responses to a 3-h exposure to very hot and dry (47 °C, 15% humidity) heat with accompanying activities of daily living (intermittent activity at 3 METS). Data from 16 young (age 21 to 39 years) and 16 older (age 65 to 76 years) humans were used to address this aim. In each group, log-transformed plasma concentrations of intestinal fatty acid binding protein (I-FABP(log)), interleukin-8 (IL-8(log)), and tissue factor (TF(log)) were assessed as indices of enterocyte damage, systemic inflammation, and blood coagulation, respectively, before and after the 3-h heat exposure. In the younger cohort, I-FABP(log) concentration did not increase from pre to post heat exposure (p = 0.264, d = 0.20), although it was elevated in the older group (p = 0.014, d = 0.67). The magnitude of the increase in I-FABP(log) was greater in the older participants (p = 0.084, d = 0.55). Across all participants, there was no correlation between the change in core temperature and the change in IFABP(log). There was no change in IL-8(log) in the younger group (p = 0.193, d = 0.23) following heat exposure, but we observed a decrease in IL-8(log) in the older group (p = 0.047, d = 0.48). TF(log) decreased in the younger group (p = 0.071, d = 0.41), but did not change in the older group (p = 0.193, d = 0.15). Our data indicate that I-FABP(log) concentration (an index of enterocyte damage) is increased in older humans during a 3-h extreme heat exposure. Future studies should determine whether this marker reflects increased gastrointestinal barrier permeability in older individuals during heat exposure.