1 hour in a cold-water suit to induce shivering
In the current study, Dr. Sellers exposed adults (aged 40-75 years; 11 men and 4 postmenopausal women) with overweight/obesity (body mass index, 27-35 kg/m2) to 10 consecutive cold exposures of at least 1 hour of shivering per cold exposure.
“The shivering in this new research was more intense [than in prior studies] and was induced with a different cold exposure method – a 10° C water-perfused suit [compared with a prior study of 14-15° C, 6 hours/day]. This facilitated a shorter cold exposure duration, deemed feasible for the participants,” explained Dr. Sellers.
“At baseline, participants had glucose and A1c levels at the upper end of the normal criteria [5.5 mmol/l and 5.4%, respectively],” he said, referring to measurements that were suggestive of possible progression to type 2 diabetes.
He explained how the cold exposure was applied. “We induced the cold with a water-perfused suit worn by the participant, through which water flows at 10° C, and this cools the participant. Eventually, the participant starts to shiver, and does so for at least 1 hour every morning for 10 days.”
Participants’ shivering-induced heat production was measured via surface electromyography and visual observation to confirm the presence of shivering. Both before and after the 10-day course of shivering, physiological measurements were taken in the morning while participants were at rest in an overnight fasted state, and under thermoneutral conditions. Blood pressure and fasting blood glucose were measured.
A 2-hour oral glucose tolerance test (OGTT) was conducted twice for each participant: on the morning before the 10-day course of shivering and again on the morning after the final 10th day of shivering.
The primary endpoint was change from before to after the 10-day shivering intervention, as represented by the total area under the curve of glucose levels over time during the OGTT.
“This provides a measure of the glucose concentrations in the blood before and after the 10 shivering sessions over the 10 days.”
courtesy Sten van Beek
For the study, shivering was induced using a water-perfused suit.
Fasting glucose and blood lipids fall, glucose tolerance improves
After 10 shivering sessions, mean fasting plasma glucose decreased significantly in 13 out of the 15 participants, compared with before the first session (from 5.84 mmol/L to 5.67 mmol/L; P = .013).
Glucose tolerance during the OGTT improved by 6% (P = .041). “We can see that this was not due to a change in their insulin concentrations in the blood,” remarked Dr. Sellers, referring to the finding that plasma insulin concentrations at baseline and during the OGTT did not change.
Fasting plasma triglyceride and free-fatty acid concentrations also decreased significantly by 32% (P = .001) and 11% (P = .036), respectively.
“This is important because free-fatty acids are involved in the role of insulin resistance,” said Dr. Sellers. “In addition, the large reduction in serum triglycerides could have implications for atherosclerosis, which may also be beneficial.”
Dr. Sellers also found that systolic blood pressure decreased by 10 mm Hg or 7.4% (P < .001), while diastolic blood pressure decreased by 7 mm Hg or 8.1% (P < .001) on average. This lowering was seen in all participants.
“Again, quite strikingly, all participants showed” a reduction in blood pressure, said Dr. Sellers, which he noted relates to a decrease in resting heart rate (P = .062).