Obesity and the struggle to lose weight are among the most challenging issues I encounter in my medical practice.  At first glance, the problem and solution seem rather straightforward.  As New York mayor Michael Bloomberg recently put it, “If you want to lose weight, don’t eat.  This is not medicine, it’s thermodynamics.  If you take in more than you use, you store it.”

But as somebody who regularly works with people battling obesity, I can attest that Bloomberg’s words do not come close to capturing the complex realities overweight individuals face. For example, I sometimes see patients with severe autoimmune or psychiatric conditions who must take medications like corticosteroids and antipsychotics that stimulate  their appetite and cause significant weight gain.  Even more often I work with people whose life situations do not permit regular exercise or a consistently healthy diet.  Think of the single mother working long hours who must get her children out to school in the morning and cook and care for them at night.  For her to find the time to regularly exercise and make nutritious meals is a real challenge.  And this does not even take into account factors like how well she is sleeping or what is going on in her life.  As David Berreby points out,

“sleeplessness and stress have been linked to disturbances in the effects of leptin, the hormone that tells the brain that the body has had enough to eat.”

Berreby’s fascinating piece The Obesity Era considers the

“troublesome fact , reported in 2010 by the biostatistician David B Allison and his co-authors at the University of Alabama in Birmingham: over the past 20 years or more, as the American people were getting fatter, so were America’s marmosets. As were laboratory macaques, chimpanzees, vervet monkeys and mice, as well as domestic dogs, domestic cats, and domestic and feral rats from both rural and urban areas. In fact, the researchers examined records on those eight species and found that average weight for every one had increased. The marmosets gained an average of nine per cent per decade. Lab mice gained about 11 per cent per decade. Chimps, for some reason, are doing especially badly: their average body weight had risen 35 per cent per decade. Allison, who had been hearing about an unexplained rise in the average weight of lab animals, was nonetheless surprised by the consistency across so many species.

How do we explain the increase in obesity across species?  Berreby writes that

it isn’t hard to imagine that people who are eating more themselves are giving more to their spoiled pets, or leaving sweeter, fattier garbage for street cats and rodents. But such results don’t explain why the weight gain is also occurring in species that human beings don’t pamper, such as animals in labs, whose diets are strictly controlled. In fact, lab animals’ lives are so precisely watched and measured that the researchers can rule out accidental human influence: records show those creatures gained weight over decades without any significant change in their diet or activities. Obviously, if animals are getting heavier along with us, it can’t just be that they’re eating more Snickers bars and driving to work most days. On the contrary, the trend suggests some widely shared cause, beyond the control of individuals, which is contributing to obesity across many species.

Some proposed causes for the obesity epidemic include changes in the bacteria in our gut, viruses, industrial chemicals, heating and air conditioning, and light exposure.  Regarding our gut bacteria,

a study in humans by Ruchi Mathur and colleagues at the Cedars-Sinai Medical Center in Los Angeles, published in the Journal of Clinical Endocrinology and Metabolism earlier this year, found that those who were overweight were more likely than others to have elevated populations of a gut microorganisms called Methanobrevibacter smithii. The researchers speculated that these organisms might in fact be especially good at digesting food, yielding up more nutrients and thus contributing to weight gain.

Evidence for a possible viral culprit is suggested by the fact that

a virus called Ad-36, known for causing eye and respiratory infections in people, also has the curious property of causing weight gain in chickens, rats, mice and monkeys. Of course, it would be unethical to test for this effect on humans, but it is now known that antibodies to the virus are found in a much higher percentage of obese people than in people of normal weight. A research review by Tomohide Yamada and colleagues at the University of Tokyo in Japan, published last year in the journal PLoS One, found that people who had been infected with Ad-36 had significantly higher BMI than those who hadn’t.

I find the possible role of air conditioning and heaters in our obesity epidemic to be especially interesting. Berreby writes that

there is a ‘thermoneutral zone’ in which a human body can maintain its normal internal temperature without expending energy. Outside this zone, when it’s hot enough to make you sweat or cold enough to make you shiver, the body has to expend energy to maintain homeostasis. Temperatures above and below the neutral zone have been shown to cause both humans and animals to burn fat, and hotter conditions also have an indirect effect: they make people eat less. A restaurant on a warm day whose air conditioning breaks down will see a sharp decline in sales (yes, someone did a study). Perhaps we are getting fatter in part because our heaters and air conditioners are keeping us in the thermoneutral zone.

Our increased exposure to light is another interesting potential factor in why we’re getting bigger. Berreby points to a 2010 in the Proceedings of the National Academy of Sciences reporting that mice exposed to extra light put on nearly 50 per cent more weight than mice fed the same diet who lived on a normal night-day cycle of alternating light and dark.  Some scientists theorize that this effect occurs by light at night robbing mice (and humans) of natural cues about when they are supposed (and not supposed) to eat.  

I recommend Berreby’s entire piece.  It resonates with my experience that treating obesity is tremendously challenging and complex work.