We should be able to be both fat and happy. Why are the two conditions so often linked?

PROBLEM: Studies have shown that obesity and depression function as reciprocal risk factors: People who are obese are more likely to be depressed, and vice versa. From a behavioral basis, it's possible that each can actually cause the other. Doctors go so far as to argue in favor of treating the two conditions together as interrelated illnesses -- a study funded by the National Institutes of Mental Health found that among overweight women with major depressive disorder, the only ones who were able to lose weight were those who improved their depression. 

And that's not to neglect the genetic basis of both -- we know one exists, even if it's not entirely clear which genes are involved. Since we are aware of a gene variant that's closely linked to obesity, researchers at McMaster University in Ontario hypothesized that the predisposition to depression might be found in the same place.

---

• How Reading Cosmo Affects Perceptions of Sex
• Even 'Moderate' Drinking Impairs Brain Cell Formation
• In Macular Degeneration, iPads Are Better Than Print

---

METHODOLOGY: The researches examined 17,200 DNA samples, which had been drawn from 21 different countries by the Population Health Research Institute. They sequenced the subjects' genomes and compared that to their diagnoses, based on the criteria for major depressive disorder outlined by the most recent edition of the Diagnostic and Statistical Manuel.

RESULTS: The link between this particular gene (FTO) and obesity was confirmed in this population. There was also a modest positive association between obesity and depression. But FTO turned out to be associated with a reduction in the risk of depression: With each additional copy of the allele, one's risk of being clinically depressed was reduced by 8 percent.

Doing a bit of a double-take, the researchers went back to three other large, international studies. In each, they found a similar association. Taken together, the four studies continued to show this eight percent reduction in risk of depression associated with the FTO gene.

BMI did not mediate the gene's effects on depression risk, suggesting that FTO has a direct impact on depression.

CONCLUSION: This is the first evidence, wrote the authors, that the gene variant associated with obesity is also associated with protection against major depression, independently of its effect on BMI.

IMPLICATIONS: The 8 percent reduction is small but significant -- enough for there to be reports that we've found the "obese but happy" gene. Perhaps the real problem is that we think of being obese and happy as counterintuitive. As the author of the 2010 study told Reuters, "Overweight and obesity, can induce low self-esteem and body dissatisfaction," Luppino explained, "especially in Western countries where thinness is often considered a beauty ideal. Both low self-esteem and body dissatisfaction are known to increase the risk of depression." These results suggest that that we might do well to rethink just how obvious the connection between being obese and being depressed really is.

The full study, "The protective effect of the obesity-associated rs9939609 A variant in fat mass- and obesity-associated gene on depression" is published in the journal Molecular Psychiatry.