Obese mice housed at human room temperature (22ᵒC) for only 5 days burn more fat and have less inflamed fat tissue compared to mice continued being housed at their thermoneutral temperature (29ᵒC); resulting in less obese and healthier animals. Furthermore, fat tissue seems to directly inform the brain about decreasing fat resources. These results were published by researchers from the Human and Animal Physiology Group from Wageningen University & Research in the scientific journal Frontiers in Physiology.
Diet-induced obese mice switched to room temperature (around 22ᵒC) reduced fat mass compared to the warmer housed control group housed at their thermoneutral temperature (29ᵒC). The mice housed at room temperature burned more energy and fats, were more active and lost weight within only five days. Also, less lipids were found in the blood of the colder housed mice, reflecting increased use of fat-derived energy sources. At 22ᵒC, the mice are thus at constant mild cold stress, and have to increase their internal heat production to maintain body temperature.
In contrast to fat depots under the skin, internal fat depots surrounding the organs are not known for having a big cold response; the study therefore focussed on those adipose tissues. In this study, lower gene expression levels of cytoskeleton formation - the structures that give the cell shape and stability – showed that fat cells are tending to decrease their size upon colder housing temperature. This is a logical result when fat mass is decreased and more energy is used to increase heat production, however it is surprising that these changes are established within 5 days with a very mild cold exposure. Also, adipose tissue inflammation was decreased in the colder housed mice, within 5 days of mildly lowering the housing temperature! This means that the fat tissue of mice switched to 22ᵒC is healthier than that of the mice housed at 29ᵒC, possibly reversing unhealthy obesity-related problems.
A link to the brain?
Interestingly, two genes that showed a massive decrease in expression in obese adipose tissue, cholecystokinin and tryptophan hydroxylase 2, are genes that are known to be expressed in intestines, but unknown in adipose tissue. In the intestines, these genes play a (direct and indirect) role in the digestion of food and informing the brain that the body is satisfied with food. It is hypothesized that the function of cholecystokinin and tryptophan hydroxylase 2 in adipose tissues involves neuroendocrine signalling; informing the brain of the energy status in the adipose tissues. More studies are needed to unravel the underlying molecular mechanisms.