Depression linked to rise in body temperature – while certain smells can trigger happy memories, studies find

People with depression have higher body temperatures than people without, a major study has found.

The researchers, led by a team from the University of California San Francisco (UCSF) analysed data from 20,880 individuals in 106 countries, collected over seven months. The findings, published in Scientific Reports, confirm earlier smaller-scale studies showing a link between depression and higher body temperature. We do not know, however, which way the causation works – or indeed if there even is a causal link.

Dr Ashley Mason, an associate professor of psychiatry at UCSF, said: “To our knowledge, this is the largest study to date to examine the association between body temperature – assessed using both self-report methods and wearable sensors – and depressive symptoms in a geographically broad sample.”

The researchers suggest there could be a number of reasons for the link. It might be that depression is tied to metabolic processes that generate extra heat, or tied to cooling biological functions that aren’t operating properly. Or there might be a common shared cause, such as mental stress of inflammation that impacts both body temperature and depressive symptoms separately.

The study data showed that as self-reported depression symptoms became more severe, body temperature averages got higher. There was also some association between higher depression scores and lower daily temperature fluctuations, but not to a statistically significant level.

Previous research has found that hot tubs and saunas can lessen the symptoms of depression, albeit in small sample groups. It’s possible that the self-cooling this triggers, through sweating, is having a mental effect too.

“Ironically, heating people up actually can lead to rebound body temperature lowering that lasts longer than simply cooling people down directly, as through an ice bath,” says Mason. “What if we can track the body temperature of people with depression to time heat-based treatments well?”

Scent can unlock memories

Another intriguing finding about depression comes from a much smaller study, which has found that scent therapy can help to unlock memories in people with depression.

People with depression often struggle to recall specific memories about their lives. The trial, published in the journal JAMA Network Open, recruited 32 adults with major depressive disorder and used familiar scents — such as coffee grounds, oranges and Vicks VapoRub — as prompts for the participants to recall specific memories. So, for example, if presented with coffee, a person might think of meeting up with their sibling for lattes on a specific spring afternoon.

Study participants were asked to recall memories — positive or negative — after sniffing 24 odour samples from glass jars or after hearing words that described those odours. The odours covered a broad range, including lavender, cumin, whiskey, cough syrup and shoe polish. Researchers found that the participants recalled more specific memories when prompted by smell.

People with depression have a bias towards grouping together individual events into broad categories and generalising the emotions tied to those events, rather than being able to single out individual positive memories. This keeps the person entrenched in negative thinking patterns that are difficult to break.

The trial sought to disrupt those patterns by helping people recall specific memories.

Kymberly Young, an associate professor of psychiatry at the University of Pittsburgh and senior author on the study, said: “It was surprising to me that nobody thought to look at memory recall in depressed individuals using scent cues before.” Scents are known to trigger strong emotional memories that is generally unmatched by other stimuli.

“If we improve memory, we can improve problem solving, emotion regulation and other functional problems that depressed individuals often experience,” Young said.

Michael Leon, a professor emeritus of neurobiology and behaviour at the University of California, Irvine, who was not involved with the study, said: “The olfactory system is the only sensory system that has a direct, superhighway access to the memory centres of the brain and the emotional centres of the brain. All the other senses have to take the side streets to get there.”