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In the two decades since obesity was declared a nationwide epidemic, researchers have been struggling to determine why 36 percent of American adults are now obese compared to fewer than 15 percent in 1990. With the growing list of health problems linked to obesity — including a study published today that found severely obese individuals may lose up to 14 years of life — the race to find answers has never been more urgent.

Unfortunately, it sometimes seems like these researchers are running in place.

One finding published online Tuesday in the Journal of Preventive Medicine found that while kids born to obese parents have a higher risk of becoming obese themselves, those who have an obese older sibling have an even greater risk of obesity — especially if their sibling is the same gender as themselves.


No big surprise there since many of us follow in the footsteps of siblings that we admire. (I took up track in high school thanks to my older brother’s running habit.)

But the research, based on surveys of nearly 2,000 parents who had one or two children, also uncovered some odd discrepancies. For example, younger siblings were more likely to be obese if their older siblings were very athletic, exercising once a day or more.

“We were kind of confused about that ourselves,” said Mark Pachucki, a senior scientist at the Mongan Institute for Health Policy at Massachusetts General Hospital. “We don’t have a great explanation for the finding.”

Another study grabbing headlines on Monday also caused some confusion. Stanford University researchers examined government surveys conducted in 1988 to 2010 and found that our increasing girth was likely due to a decline in physical activity rather than an increase in calorie intake. The researchers found that the percentage of Americans who reported that they never exercised increased from 19 percent to 52 percent among women and from 11 percent to 44 percent among men during that timespan — while our average daily calorie intake didn’t change.


“I was surprised to see their conclusions that changes in caloric intake weren’t significant since previous studies suggest that a small 5 to 10 percent increase in our calorie consumption through the years has contributed to increased obesity,” said obesity researcher Emmanuel Pothos, an associate professor of integrative physiology and pathobiology at the Tufts University School of Medicine, who was not involved with the study.

His own research has been focusing on the intractable nature of obesity — how difficult it is for some people to avoid and how tough it is for many to reverse.

“There is a certain brain mechanism that sabotages all the knowledge we have about good nutrition and exercise,” Pothos said. “It doesn’t take a lot to convince us that these are good things, but why do we still fail to follow the government recommended guidelines?”

His studies on rats prone to obesity suggest that the levels of the“pleasure” brain chemical dopamine are 50 percent lower than rats who are resistant to becoming obese. Research on obese humans found that they tend to have fewer dopamine receptors in their brain compared to those who aren’t overweight.

What this means, Pothos said, is that obesity might trigger overeating as people need to eat more of their favorite foods to get pleasurable sensations. Such a drive for pleasure, he added, is akin to an addiction.


He and his colleagues argued in a 2012 commentary published in the journal Physiology & Behavior that obesity should be considered an addictive disorder because a rich piece of fudge or juicy steak can set off the same pleasure pathways in the brain as a hit of cocaine and lead to the same compulsive behaviors to achieve a sort of “high” from the substance.

“The point we were trying to make is that people who treat obesity should have an education in addiction to understand the causes of relapse that can make people relax back into their bad diet or lifestyle,” Pothos said.

But Pothos also underscored the long list of unanswered questions that still need to be addressed to fully understand the current obesity epidemic. Are babies born to obese pregnant women destined to become obese themselves due to genetic programming in the womb? How much has our increased sugar consumption contributed to the epidemic? Are we doomed by the digital, automated world we’re living in that allows us to get by with little to no muscle power?

“We’ve been looking at the shared experiences of families and peers, as well as the role of genes and environmental factors and trying to disentangle the influence that each one has,” Pachucki said. “But we still have a huge way to go to find the answers.”

Deborah Kotz can be reached at dkotz@globe.com. Follow her on Twitter @debkotz2.