Hypothermia -- when the body's temperature drops significantly below normal -- is especially deadly for the elderly. Older people become hypothermic despite the fact that they are more likely to live inside a home than on the street, and nearly half who become hypothermic die.
By contrast, children rarely succumb to the disorder. Younger adults are also less susceptible than the elderly, whose impaired ability to maintain core temperature during cold stress is widely documented. These contrasts have led physiology researchers to investigate whether specific characteristics of the body are responsible for our ability to deflect the cold. In a recently published study researchers have found that certain characteristics, which change with age, affect younger and older persons differently.
The study was conducted by David W. DeGroot and W. Larry Kenney of the Intercollege Graduate Degree Program in Physiology and Noll Laboratory, Pennsylvania State University, University Park, PA; and George Havenith, Department of Human Sciences, Loughborough University, Loughborough, UK. Their study, entitled "Responses to Mild Cold Stress Are Predicted by Different Individual Characteristics in Young and Older Subjects," appears in the December edition of the Journal of Applied Physiology (http://jap.physiology.org/).
Ten Characteristics and Body Core Temperature
The study examined the relative influence of ten physical characteristics thought potentially to play a role in how the body's core temperature and tissue insulation react to cold. The characteristics they reviewed were age, sex, weight, body surface area, body surface area-to-mass ratio, sum of skin folds (an estimate of body fat), percent body fat, appendicular skeletal muscle mass (ASMM), and two thyroid hormone concentrations, T3 and T4.
Forty-two young (18-30 years; 21 men, 21 women) and 46 older (65-89 years; 24 men, 22 women) individuals participated. The volunteers were nonsmokers and took no medications that could alter their cardiovascular or thermoregulatory responses to cool temperatures. Participants underwent a standardized medical screening and physical exam, and researchers measured or calculated the ten physical characteristics noted above for each subject. Researchers then inserted a thermometer sealed in a pediatric feeding tube into each participant who then entered a controlled environmental chamber and was positioned in a reclining position. The room's baseline temperature remained stable for 20 minutes and was decreased thereafter at a rate of 0.2°C per minute for 20 minutes and 0.05°C per minute after that to approximate mild cold exposure. The participants were removed when visible, sustained shivering was observed by the investigators or reported by the volunteer.
Multiple-regression analyses were performed to determine the predictors of body temperature and tissue insulation, and standardized regression coefficients were analyzed to determine the relative influence of each of the ten candidate variables.
Findings and Conclusions
The researchers observed the following:
These results suggest that the well known changes in body composition characteristics with aging in turn influence how the body deals with the cold as we grow older. Characteristics that are important in young people become less important with aging, and previously-insignificant characteristics rise in importance.
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