Another System Troubled by Age—Even Healthy Older Adults Can’t Handle the Heat

October 12, 2017

A new study published in Applied Physiology, Nutrition, and Metabolism by researchers from the Universities of Ottawa, Calgary, Sherbrooke, and Thessaly in Greece. Lead author Ryan McGinn explains how older adults are at higher risk for heat stress. 

By Ryan McGinn, MSc, MD (Candidate)

Increasing age is associated with a host of changes to health and physical function. While it has been thought for some time that older adults also have a reduced ability to regulate body temperature during physical activity and work, most of this information is based on small-scale studies. Given that the population is aging and an increasing proportion of older adults are performing physical activity and work, age-related effects on temperature regulation are becoming increasingly relevant. Taken together with climate change and the increasingly unpredictable outdoor temperatures, older adults are at high risk for heat-related illness and injury. 

Is age really just a number?

In our study, we tested 87 adults from 2070 years of age with a wide range of fitness and physical characteristics. They performed moderate-intensity cycling in a room regulated at 35°C to provide a sufficient level of heat stress that would increase core body temperature by ~1°C. Our study was the largest to date and used the world’s only whole-body direct calorimeter—a device that measures the total amount of heat lost by the body with extreme precision. 

Our main finding was that increasing age is the primary factor to explain differences in total heat loss during physical activity. In fact, we found that age explained ~40% of the difference in total heat loss among participants. This difference corresponded to ~5% decrease per decade in the functioning of our temperature regulation system. While fitness and body fat percentage have been shown in smaller studies to impact temperature control, they did not have a significant effect in our large cohort. 

 
Linear regression of total heat loss during physical activity in all 87 participants based upon their age in years

It is important to note that older age often presents with multiple comorbidities such as high blood pressure, diabetes, high cholesterol, and a number of other conditions. While many of these comorbidities will negatively impact total heat loss during physical activity, we ensured that all of our participants were free of any such conditions and were not taking any medications regularly. By doing so, we were able to isolate age from other confounding factors and conclude that increasing age is the biggest risk factor for lower heat loss in healthy adults. In other words, we are at progressively higher risk for dangerous heat stress as we get older with the most concerning part being that this is a non-modifiable risk factor.

How hot am I, anyway?

A logical solution to humans getting hotter more rapidly as we age would be to stop and rest when we start to feel the heat. In addition, we may interpret a higher body temperature as a feeling that we are working harder. Until now, there has been very little focus on the perception of heat and physical effort as it relates to total heat loss. 

As a critical supplement to our findings on total heat loss, we also asked participants how hot they felt and how hard they felt they were working throughout the experiment based on predetermined scales. The scale to rank how hot they felt ranged from 0 (quite cool) to 7 (excessively hot) while the rating for how hard they were working had a scale ranging from 6 (resting) to 20 (maximal effort). 

Curiously, we found that most participants had a similar perception of how hot they were. Across the ages from 20 to 70 years, despite often large differences in total heat loss, there was no difference in the thermal sensation. Put another way, the adults who were hotter did not feel any different! The rating of perceived exertion was slightly better in that some of the adults who were hotter felt like they were actually working harder. However, our data clearly show that the solution to preventing dangerous levels of heat stress as we age cannot be to rely on our perceptions for how hot we feel or how hard we feel we are working. 

Our study shows that increasing age is the biggest risk factor for dangerous levels of heat stress in otherwise healthy adults. Even healthy adults were shown to lose ~5% of their capacity for heat loss per decade. Unfortunately, the solution lies far from relying upon our own ability to sense when our body is too hot; adults in our study were unable to accurately perceive their level of heat stress. Rather, researchers and healthcare practitioners need to be proactive in establishing new guidelines that are adequate to identify and prevent heat-related illness and injury, even in healthy adults. 

The article entitled "Increasing age is a major risk factor for susceptibility to heat stress during physical activity" is available on the NRC Research Press website.

Filed Under: Applied Physiology Nutrition and Metabolism

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