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Shivering, exercise may aid brown fat function, thermogenesis
Shivering leads to the secretion of irisin, a hormone also triggered by exercise, which is integral to brown fat and thermogenesis, recent study results found. These findings suggest that this cold-activated endocrine mechanism could potentially be used as a strategy against obesity.
“This research may reveal why exercise increases secretion of a hormone that makes the body maintain its internal temperature," study researcher Francesco S. Celi, MD, of the National Institute of Diabetes and Digestive and Kidney Diseases, said in a press release. “Perhaps lowering the thermostat during the winter months could help both the budget and metabolism.”
The researchers evaluated secretion of irisin in 10 participants (four females, six males) with a BMI of 22 ± 2 kg/m2 and body fat of 24 ± 9%). The participants underwent progressive exposure to cold temperatures (27°C-12°C), followed by two exercise tests: exercise on cycloergometer at maximum capacity (VO2max) and submaximal exercise test at 40% (VO2max) for 1 hour. The results confirmed that irisin secretion occurs in proportion to intensity of exercise; serum irisin levels showed a greater increase (P=.07) after maximal exercise.
The effect of cold temperature on irisin fluctuation was then assessed, and the researchers found that arm-to-hand, skin-to-core and supravicular-to-chest temperature gradients increased by 13 ± 18% (P=.01), 45 ± 15% (P<.0001) and 4 ± 3% (P<.0001), indicating vasoconstrictive, insulative and thermogenic mechanisms, respectively. Cold-induced thermogenesis was evidenced by a 48 ± 37% (P<.01) increase in energy expenditure.
Of the 10 participants, seven claimed to have shivered, and the shivering activity was measured by surface electromyography (EMG). Cold-induced increases in EMG activity were greater in those who shivered, and circulating irisin levels also increased in the seven participants who shivered. Moreover, the researchers found that changes in irisin levels were positively associated with the extent of shivering activity (P<.001), and that the amount of shivering-induced irisin was comparable to that of exercise-induced irisin.
The irisin precursor FNDC5 also was found to have a thermogenic effect on human fat cells in a laboratory setting. When the fat cells were exposed to FNDC5, they burned more energy and generated more heat.
Based on these findings, the researchers theorized that exercise-triggered irisin secretion may have evolved from shivering-related muscle contractions, which serve to burn calories and generate heat.
Disclosure: Lee was supported by an Australian National Health Medical Research Council (NHMRC) Early Career Fellowship; the Royal Australasian College of Physicians (RACP) Foundations Diabetes Australia Fellowship and Bushell Travelling Fellowship; and the School of Medicine, University of Queensland, Australia. This study was supported by the Intramural Research Program of NIDDK, programs Z01-DK047057-02, Z01-DK071044, Z01-DK071013, and Z01-DK071014.