A recently published study led by researchers at University of California, San Francisco (UCSF) revealed that mice that remain in their warm, thermal ‘comfort zone’ most of the time eating fatty foods increased the risk of developing cardiovascular disease by over 50% compared to mice housed in a cooler environment whilst on the same food diet; these conditions, however, did not seem to have an impact on insulin resistance (a warning sign for the development of diabetes).
The study, titled “Thermoneutral Housing Accelerates Metabolic Inflammation to Potentiate Atherosclerosis but Not Insulin Resistance” published in the journal Cell Metabolism on Nov. 5, 2015, found evidence suggesting that the standard practice of keeping laboratory mice at a relatively cool temperature could benefit their health in the long run by keeping their metabolism active. According to a UCSF’s press release, at the same time, this could represent an issue for researchers studying metabolic disease in humans, as humans spend most of their time under regulated “thermoneutrality,” the optimized temperature for our metabolisms not having to make extra efforts to keep our body warm.
“This has had a profound effect on how I look at results coming out of my own lab,” noted the study’s senior author Ajay Chawla, MD, PhD and Professor at UCSF’s Cardiovascular Research Institute and UCSF’s School of Medicine. “I don’t believe anything until we do it again at thermoneutrality.”
Dr. Chawla studies the impact and effects of cold on metabolism and became interested in digging deeper into mice’s risk of developing metabolic disease when they were housed at cooler temperatures. The current debate in the scholarly community continues investigating the optimal temperature for the comfort and health of laboratory mice. The National Research Council recommends housing mice at 20-26 degrees Celsius (68-79 degrees Fahrenheit), much below the majority of animals’ thermoneutral zone, closer to 30 degrees Celsius (86 Fahrenheit).
In 20 years of research, findings led to the conclusion that diets that are high in LDL (low-density lipoprotein) cholesterol lead to chronic vascular inflammation, accelerating the growth of atherosclerotic plaques and causing heart disease. Recently, scientists have discovered that a more sensitive immune activity could also onset insulin resistance, an early red flag of diabetes.
Dr. Chawla and his team discovered that mice housed at comfortable 30ºC and eating fatty foods developed inflammation and grew twice as many atherosclerotic plaques than mice housed at the traditional 22ºC. However, insulin resistance saw no worsening due to the heightened inflammation resulting from warmer conditions. This observation was significantly contrasting with previous results. “This was a big surprise,” said Dr. Chawla. “It could be that inflammation has less to do with diabetes than we thought.”
The research team now believes the chronic chill that laboratory mice typically experience may protect them against chronic inflammation by accelerating the metabolism to warm the body. The heart rate of mice living at 22ºC is double that of mice living at 30ºC, meaning they spend twice the energy even when resting and have a much higher blood pressure. “The magnitude of the response is huge,” concluded Dr. Chawla. “We do all these studies on mice hoping to apply what we learn about mouse physiology to humans, but it’s like we’ve been studying humans on speed.”