Researchers at City of Hope in Los Angeles, USA, have identified a biological reason why we become more sensitive to cold as we age. The findings were published in the scientific journal Nature.

Unlike white fat, which primarily stores energy, thermogenic fat burns calories to produce heat and helps keep us warm. As we age, thermogenic fat becomes less active, which may explain why older people feel the cold more acutely.

“Understanding how the body controls this process may offer insights into obesity, type 2 diabetes and other metabolic disorders,” says study author Qiong “Annabelle” Wang, an associate professor at City of Hope’s Arthur Riggs Diabetes & Metabolism Research Institute.

Her work shows how signals from nerves and immune cells regulate the amount of heat thermogenic fat produces by shifting the balance between highly active and less active fat cells. Experiments in mice suggest that this system becomes less flexible with age, causing thermogenic fat to respond more slowly to falling temperatures.

City of Hope researchers previously found that thermogenic fat cells differ in how much heat they produce: some have high levels of UCP1, a heat-generating protein, while others have less of it and remain less active. In the new study, the scientists observed a similar mix in samples of human thermogenic fat, although most of the experiments were conducted in mice.

This mix of cells may give the body a built-in way to adapt to changes in temperature. When more heat is needed, signals from the nervous and immune systems prompt less active cells to increase heat production, and when that need passes, the system reduces activity. In older mice, however, this response became sluggish: their thermogenic fat could still increase heat production, but took longer to do so, making it harder for the animals to maintain their body temperature in the cold.

The researchers were able to partially restore heat production in older mice by blocking one step in the biological pathway responsible for this slowdown: the treated animals produced more heat and burned more energy. The drug used in the experiment, mifepristone, affects many parts of the body and is therefore not a treatment for age-related sensitivity to cold, but the findings show that the decline in thermogenic fat activity can be reversed.

“This possibility matters for more than just staying warm. Thermogenic fat also burns glucose and fat, and its activity declines with age. If scientists find a safe and precise way to restore this activity, this basic research could eventually deepen our understanding of age-related metabolic problems, including obesity and type 2 diabetes,” Wang added.