Parents may have another reason to avoid bisphenol A, or BPA, the estrogen-like chemical found in many plastic bottles and cans. BPA may be making our kids fat, new research suggests.

In a nationally representative study of nearly 3,000 children and teens, researchers found that kids with the highest levels of BPA in their urine were 2.6 times more likely to be obese compared to those with low levels of the chemical. The report was published in the latest issue of the Journal of the American Medical Association.

It’s the latest evidence that obesity might be affected by more than just diet and exercise, said Dr. Leonardo Trasande, a professor of pediatrics and environmental medicine at the New York University School of Medicine.

“Clearly poor diet and lack of physical activity contribute to increased fat mass, but the story doesn’t end there,” he said.

The link was statistically significant only for white children and adolescents, who made up 62 percent of the study participants, researchers said. Connections between the highest levels of BPA and obesity weren’t found in black or Hispanic youngsters. Researchers said that link would need more study. 

Perhaps one of the most striking findings was that the association between BPA and obesity extended even to children who were consuming the right amount of calories.  

“We found that BPA in a child’s urine was associated with the chance of being obese, whether they were eating too many calories for their age and gender, or not,” said Trasande. “Our hypothesis is that something happens to the kids’ metabolisms.”

No one knows exactly what that might be, but experimental studies have shown that BPA can make fat cells bigger, Trasande said. The chemical also has been shown to inhibit a hormone called adiponectin, which is involved in lowering heart disease risk. And, because BPA is actually a weak synthetic estrogen, the chemical may disrupt the balance of estrogen and testosterone, which may adversely affect caloric balance.  

BPA exposure in the U.S. is “nearly ubiquitous,” the researchers said. Nearly 93 percent of people aged 6 or older had detectable levels of BPA in their urine, according to a 2003-2004 National Health and Nutrition Examination Survey. About 99 percent of that exposure comes from dietary sources.

Currently most of the BPA exposure is from canned goods. The chemical is in the resins that manufacturers use to coat the insides of cans to block metals from leaching into foods as well as to prevent a metallic taste.

Many plastic bottle manufacturers voluntarily changed their formulations to exclude BPA after the chemical was linked in animal studies to a host of health ills, including possible developmental problems. 

While the Food and Drug Administration has barred the use of the chemical in baby bottles and children’s sippy cups, the agency isn’t yet convinced that BPA must be completely banned. The FDA called for more research because, officials said, it has “some concern about the potential effects of BPA on the brain, behavior and prostate glands of fetuses, infants, and children.”

Trasande hopes his study will help.

“The FDA decided to take a wait-and-see approach, specifically looking for more evidence regarding the potential health consequences of exposure,” he said. “We believe this study provides critical information that the FDA needs to consider as they evaluate the need and the risk involved with keeping BPA in food products.”

Experts not affiliated with the new research said the new study was carefully done.

“This is an important study, albeit just one study,” said Dr. Richard J. Jackson, professor and chair of environmental health science at the Fielding School of Public Health at the University of California, Los Angeles. “We must pay attention to environmental chemicals that meddle with metabolism.”

Dr. Elizabeth Proutparks, an expert in childhood obesity, agreed that the study was very well done, but cautioned that more research must be conducted before anyone can say for certain that BPA actually causes obesity.

Proutparks was also concerned that parents might see this study and skip canned fruits and vegetables altogether.

“I don’t think I would tell them not to eat canned foods and vegetables,” said Proutparks, a nutritionist and attending physician with the healthy weight program at the Children’s Hospital of Pennsylvania. “You’d be limiting these foods in low-income populations, who already have issues as it is.”

Proutparks did advise parents not to re-use water bottles that contained BPA, noting that obesity isn’t the most worrisome possible side effect from the chemical. The most concerning effects would be on the developing brains of fetuses and infants, she said.

“I think there are other studies and other reasons to look for bottles that don’t have BPA in them,” Proutparks said. “But we need to put it all in perspective.”

Related stories: 

• BPA levels soar after lunching on canned soup
• FDA: BPA banned in baby bottles
• Decay dilemma: Do kids need dental sealants?

 By Robert Bazell
Chief Science and Medical Correspondent
NBC News

How hard is shift work on a worker's body? 

Research out Wednesday from Brigham and Women’s Hospital in Boston demonstrates very precisely the way fighting the body's natural sleep patterns can increase the risk of type 2 diabetes, the most common form of the disease.

More than 21 million Americans are “shift workers,” according to U.S. Census figures. That is, they labor during the hours that most of us set aside for rest or sleep, either all or part of the time. That number is increasing 3 percent a year because of the nature of our service economy and the need for ever more people to take whatever work they can.

The sleep research team at Brigham and Women’s, under the direction of Dr. Charles Czeisler, has spent decades documenting how shift work can lead to increased obesity, heart disease, diabetes and many other health problems. In this latest research in their sleep lab they show how one mechanism creates the risk.

Twenty-one healthy volunteers were subjected to varying hours of sleeping and waking, light and dark, all designed to disrupt the body’s natural internal clock (the circadian rhythm.)

Within a few days, when the subjects ate a normal meal, their bodies did not respond in a normal way.

“Glucose levels went much higher and stayed that way for several hours,” said neuroscientist Orfeu Buxton, Ph. D., the study's lead author. “This was because of decreased insulin released from the pancreas. Together these reflect an increased risk of diabetes.”

The stress was so severe that during the three-week experiment three of the healthy volunteers became pre-diabetic. Fortunately, after nine days of normal sleep and waking, all symptoms disappeared.

Still, the experiment clearly demonstrates that shift work can make people diabetic. For people who already have diabetes or are pre-diabetic, it can make the conditions worse.

The advice from the scientists for those who perform shift work -- either out of necessity or choice:

• Try to make your daily clock as normal as possible.
• Get good sleep during the day -- finding, if you can, a quiet, very dark room. 
• Don’t eat big meals at a time when you feel your body clock is out of whack.

Sound advice, experts would agree.  But anyone who works odd hours knows how challenging such simple routines can be in the demands of a normal family and social life. This latest research is further evidence out-of-whack sleep’s harm to our health.

The research is published in Science Translational Medicine.  You can read an abstract here: 

http://stm.sciencemag.org/lookup/doi/10.1126/scitranslmed.3003200

 Robert Bazell is NBC's chief science and medical correspondent. Follow him on Facebook and on Twitter @RobertBazellNBC

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By Joyce Ho and Dr. Nancy Snyderman
NBC News

The nation’s ever-growing obesity epidemic may be far worse than originally thought. New research demonstrates that even people with a healthy Body Mass Index, a commonly used scale to measure body fat, could actually be obese and at risk for a host of complications.

A study published Monday in the journal PloS One found that using BMI as an indicator of obesity actually misclassifies 39 percent of Americans as “overweight” rather than “obese.” And because BMI doesn’t distinguish between fat and muscle, some people with normal BMIs may have dangerously high amounts of fat in their bodies.

Without an accurate measurement of body fat, the researchers say, millions of people don’t know they are at high risk for a number of obesity-related diseases.

“The fat is what causes heart disease, cancer, menstrual problems, depression, anxiety, and a host of medical problems,” said Dr. Eric Braverman, a professor at Weill Cornell Medical School and co-author of the study. “So if you want to save society from a lot of illnesses …  you have to identify how much fat they have.”

More than one in three adults in the U.S. are obese, as defined by a BMI of 30 or higher, according to the Centers for Disease Control. Obesity measurement, however, has been a controversial topic for years, and the widely-used BMI calculation has been called outdated by experts.

BMI is calculated through a simple formula: weight divided by height squared. The ease of calculation made this formula popular, even though it’s nearly 200 years old. In Braverman’s study, researchers compared the BMI with a different measurement, the Dual Energy X-ray Absorptiometry (DXA) scan. DXA scans, commonly done in women to check for osteoporosis, measure percentage of body fat, muscle mass, and bone density.

Of the 1,393 people studied, 26 percent were classified as obese when body fat was measured with BMI, whereas 64 percent of them were considered obese when measured with DXA. The misclassification was observed more often in women and increased with advancing age: 48 percent more women between the ages of 50 to 59 were classified as obese when measured with DXA instead of BMI, and among women ages 70 and above, 59 percent more were considered obese after getting a DXA scan. 

According to the authors, BMI is an inaccurate measure for obesity – but especially in this demographic, because as women age they lose more muscle to fat than men. BMI, which does not distinguish between muscle or fat, does not reflect this bodily change. 

“BMI doesn't tell you how much fat … you have,” said Braverman. “So without knowing how much fat you have, you can't really save people from illness. It is the number one predictor of who's going to live or die.”

Researchers also tracked blood levels of leptin, a hormone produced by fat cells that regulates hunger and energy expenditure. Increased blood leptin levels correlated well with DXA scan results, highlighting the potential for a simple leptin blood test to be a measurement for obesity.

Based on these findings, Braverman and his co-author Dr. Nirav Shah, the current New York state health commissioner, suggest lowering the BMI definition of obesity from 30 to 24 in women and down to 28 in males. Under these suggested guidelines, a woman who is 5’ 6” and 150 pounds would be considered obese. Under the current BMI standards, the same woman would be considered healthy.

“Fat is costing the country a fortune, by not measuring it,” said Braverman. “A dollar blood test and doing our bone density scans with body fat scans at the same time is going to save us an enormous medical cost in the end.”

NBC’s Stacey Naggiar, Chiara Sottile and Joo Lee contributed to this report.

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• America’s fattest cities
• Size matters for obese cancer patients’ drug dose
• U.S. advisers back obesity pill