While obese people have a 50 percent chance of developing depression, doctors are unsure why. A study published last month shows there may be a chemical link between obesity and depression, but biological links to explain other depression risk factors remain unknown.

Psychiatrists have known obesity correlates with depression for years but haven’t known the biological reason, said David Bond, a University psychiatry associate professor and author on the study.

“[Many studies] have looked at whether being obese predicts later developing depression,” Bond said. “What they found is that people who are obese had about a 50 percent increased risk of becoming depressed.”

Other researchers have proposed psychological or social factors, such as low self-esteem and societal stigma, as causes for the effect, he said. But this study was aimed at finding a biological explanation for the gap in depression risk.

Appetite hormones, which help control the body’s metabolism and appetite, are produced by fat tissue, Bond said.

He said the hormones play a role in regulating mood. Experiments where animal subjects received the hormones showed they can act like an antidepressant.

In the study, when people had higher levels of two particular hormones, leptin and adiponectin, they were less likely to become depressed.

Leptin tells the brain to be less hungry when the body has enough fat, reducing appetite and acting somewhat like an antidepressant. In obese people, the chemical loses its potency.

“What does happen when people become obese is that they … produce a lot of leptin, but it seems to lose its effect,” Bond said. “People who are obese tend to develop … leptin resistance.”

Because of this, obese people don’t have as much of an antidepressant effect from leptin and are more likely to develop depression.

This resistance can also make it difficult to lose weight because leptin no longer controls appetite, Bond said.

Cooper Wilms, a University English junior who has struggled with depression, said he becomes lethargic and his eating habits become more extreme when he has a depressive relapse.

“When it hits hard, I’m usually locked in my room sleeping. People can’t be angry at a sleeping person,” Wilms said.

The study also showed that a different hormone, adiponectin, decreased risk of depression, Bond said. But as the body gains more fat tissue, it produces less adiponectin. This means the risk for depressive episodes can increase as people gain weight.

Wilms said when he’s feeling depressed, his weight tends to increase, and he loses energy.

“On my good days, I’m active,” he said. “On depressive days, it’s more lock myself in a room and sleep it off.”

Besides obesity, mental health professionals still don’t understand the biological reasons behind other depression risk factors, said Bonnie Klimes-Dougan, a University psychology associate professor.

One risk factor is whether someone’s family has a history of depression because those who grow up in families with a history of depression are more likely to have similar biological and experience comparable environmental factors, Klimes-Dougan said.

While psychologists know genetics play a role in many psychological disorders, they don’t know exactly what biological changes cause the increased risk, she said.

This includes other disorders with prominent connections to family history such as schizophrenia, Klimes-Dougan said.

“[For] any psychological disorder, even the ones that we know have the highest genetic contribution, we don’t have any specific [biological] mechanism,” she said.

It’s difficult for medical professors to nail down whether a factor affects risk for depression, said Gary Christenson, chief medical officer for Boynton Health Service.

“We have to realize a lot of our knowledge is based on correlations,” he said. “A certain variable may be associated with depression. The tricky question to figure out is, ‘is it causative?’”

One of the factors that research has shown correlates strongly with depression is childhood experiences such as physical abuse, conflict or substance abuse, he said.

But even with these factors, there’s no guarantee someone would develop depression, he said. There is likely a biological component that also affects how susceptible someone is.

“Each of us probably has a certain range of what is tolerable and not tolerable … there’s those who have no adverse events and still develop depression,” Christensen said.

People who develop depression without these negative experiences usually have a history of depression in their families, which could mean their family has a lower biological tolerance.

No single factor causes depression, he said.

“[It doesn’t happen] that 100 percent of people with [a] component develop depression,” Christenson said. “There haven’t been any of those clear markers. It’s a combination.”