Obesity can create sleep disorders, but research presented at a March NSF conference also shows that, for some individuals, the causation may flow the other way.
According to many experts, the United States, and much of the modern world, is experiencing an obesity epidemic. Globally, there are an estimated 1.7 billion overweight adults, and more than 300 million of them are obese.1 In the United States, data from the National Center for Health Statistics show that 30% of US adults 20 years of age and older—more than 60 million people—are now obese and the percentage of children and teens between 6 and 19 years in age that are overweight has tripled since 1980.2
The increase in obesity is fueling not only industries that specialize in weight loss and exercise, but also medical specialties related to treating obesity and obesity-related diseases. Bariatric surgeries, such as gastric banding and gastric bypass, experienced a nearly sixfold increase in the United States between 1998 and 2002, according to a study on hospital-discharge data presented this year in the American Journal of Public Health.3
One of the medical specialties that is benefiting from the increase in obesity is the field of sleep disordered breathing treatment. The link between obesity and disorders such as obstructive sleep apnea (OSA) is well known. According to Meir H. Kryger, MD, FRCPC, director of the Sleep Disorders Center at St Boniface Hospital Research Center, University of Manitoba, Winnipeg, Canada, 50% of people with a body mass index (BMI) of 40 or more (the general cutoff point for morbid obesity) have sleep apnea.
While no health care provider would ever wish ill health on people, it is an uncomfortable truth that the more obese people there are, the more people there are in need of sleep testing and OSA treatment. Financial experts have credited the obesity epidemic as part of the reason why sleep medicine is growing so rapidly, and why companies that produce products to treat sleep disordered breathing have reported record profits in these areas.
But while sleep disorders are partly a side effect of the obesity problem, treating them may also be part of the obesity problem’s solution. Paradoxically, along with exercise and a healthy diet, sleeping more may help reverse the obesity epidemic, said researchers who gathered at the National Sleep Foundation (NSF) conference on sleep and obesity in March.
Sleep physicians do come across patients that seem to have an easier time reducing their weight after their sleep disordered breathing is treated. The reduction in excessive daytime sleepiness that many people with severe OSA experience after treatment certainly helps motivate some to head to the gym instead of home to the couch. However, while anecdotal evidence may abound, scientific research supporting this finding has been sparse.
Still, obesity and sleep researchers who gathered for the NSF conference did point out many other connections between the two problems. It is important, several said, to not overlook the role lack of sleep could play in obesity and vice versa.
US obesity rates have been increasing for at least 100 years—long before the creation of fast food, cars, and television, said conference speaker David B. Allison, PhD, director of the National Institutes of Health-funded Clinical Nutrition Research Center. The two factors that people tend to focus on are food-marketing practices and sedentary lifestyles, he added. However, by directing so much attention at the two most obvious factors, could science be overlooking other components of the obesity problem?
Many parts of our environment began to change along with the industrial revolution of the 19th century, including, with the invention of the light bulb and greater use of electricity, how late after dark we could stay up. NSF polls from 2000 to 2006 have consistently shown how voluntary bedtime restriction is increasingly common in industrialized societies, said conference presenter Karine Spiegel, PhD, a research associate at the Université Libre de Bruxelles in Belgium. In fact, the two trends—less sleep and more obesity—have mirrored each other over the second half of the 20th century.
Light may influence the body’s circadian clock, and, in studies of mice whose circadian rhythm was disrupted through a mutation, researchers found that the mice’s food-intake rhythm was disrupted as well. In one study, the clock-mutant mice had a 10% increase in body weight compared to normal wild-type mice, said conference presenter Fred W. Turek, PhD, director of the Center for Sleep and Circadian Biology at Northwestern University in Chicago.
Several studies also have reported an association between the number of hours of sleep and BMI. The most recent, a 16-year longitudinal study of obesity and sleep in women released at the American Thoracic Society Conference in San Diego in May, found that women who sleep 5 hours or less per night weigh more on average than those who sleep 7 hours per night. According to the study’s results, women who slept for 5 hours per night were 32% more likely to experience major weight gain (defined as an increase of 33 pounds or more) and 15% more likely to become obese over the course of the 16-year study compared with women who slept 7 hours. Women who slept for 6 hours were 12% more likely to have major weight gain and 6% more likely to become obese compared with women who slept 7 hours per night. On average, women who slept 5 hours or less per night weighed 5.4 pounds more at the beginning of the study than those who slept 7 hours and gained an additional 1.6 pounds more over the next 10 years.
“That may not sound like much, but it is an average amount—some women gained much more than that, and even a small difference in weight can increase a person’s risk of health problems such as diabetes and hypertension,” said lead researcher Sanjay Patel, MD, assistant professor of medicine at Case Western Reserve University in Cleveland.
However, at the NSF conference, Emmanuel Mignot, MD, PhD, of Stanford University cautioned that these studies only provide evidence of a correlation between sleep restriction and obesity. Definitive proof of a causative link between decreased sleep and increased BMI may come only from interventional studies, and these studies pose multiple design and implementation challenges.
Despite these cautions, there is reason to be excited about the possibility of a connection between poor sleep and obesity. Some studies have found evidence that sleep loss can play at least a partial role in diet and behavior preferences that may contribute to obesity. When healthy young men curtailed their sleep for 2 to 6 days in laboratory studies, their bodies showed marked alterations in glucose metabolism. They became more sensitive to glucose, and the neuroendocrine regulation of their appetites was also affected. The hormone leptin, which helps tell the body when it has had enough to eat, decreased. In addition, the appetite-stimulating factor ghrelin increased. Not surprisingly, given what was going on in their bodies chemically after the sleep restriction, the young men showed increased hunger, particularly for calorie-dense foods with high carbohydrate content.4
More studies are needed to say if sleep loss being correlated with these changes in neuroendocrine regulation of appetite is significant, Mignot said. However, it is an interesting possibility.
Another interesting correlation between obesity and sleep disorders occurs in the body’s inflammatory response. Obesity is associated with increased levels of pro-inflammatory cytokines (proteins produced by white blood cells that act as chemical messengers between cells). When obesity is compounded by sleep apnea, these cytokines, IL-6 and TNFa, increase even more. If low-grade inflammation leads to insulin resistance, this could be a mechanism of association between sleep loss and obesity, reported Alexandro N. Vgontzas, MD, ScD, director of the Center for Sleep Disorders Medicine at the Department of Psychiatry, Pennsylvania State University’s Milton S. Hershey Medical Center in Hershey.
The picture grows even more complicated when one considers genetic factors that may also play a role. Although the mechanism behind the heritability of obesity is still poorly understood, the link between obesity and genetics has been a part of research theories for decades. Studies on twins have found a strong heritability (the proportion of the variance in a trait attributable to common familial factors) for obesity, and the situation is similar for sleep apnea. According to Patel, the heritability of the apnea-hypopnea index is close to 40%.5
Many of the susceptibility genes for obesity and OSA could be shared. However, it is also possible that, in some individuals, separate genetic predispositions are at work and the development of one disorder leads to the other. For example, a person who is genetically prone to developing OSA—such as someone with naturally lax connective tissue in the upper airway—could have such disturbed sleep that his or her metabolic function is changed to make obesity more likely.
If sleep does influence metabolism and appetite, could teaching healthy sleep habits and treating sleep disorders help obese individuals control their weight and help normal-weight people stay that way? More research is needed to say for sure, the experts cautioned. However, in the meantime, getting proper sleep certainly will not hurt and may help.
is editor of Sleep Review.
References
1. Blackburn G. Epidemiology of the worldwide epidemic of obesity: the future. Paper presented at: Sleep Loss and Obesity: Interacting Epidemics scientific workshop; March 27-28, 2006; Washington, DC.
2. Centers for Disease Control and Prevention. Overweight and obesity: rrequently asked questions (FAQs). Available at: www.cdc.gov/nccdphp/dnpa/obesity/faq.htm. Accessed June 11, 2006.
3. Smoot TM, Xu P, Hilsenrath P, Kuppersmith NC, Singh KP. Gastric bypass surgery in the United States, 1998-2002. Am J Public Health. May 30, 2006; [Epub ahead of print].
4. Spiegel K. Metabolism and appetite regulation. Paper presented at: Sleep Loss and Obesity: Interacting Epidemics scientific workshop; March 27-28, 2006; Washington, DC.
5. Patel S. The connection between obesity and sleep apnea: common and overlapping genetic links. Paper presented at: Sleep Loss and Obesity: Interacting Epidemics scientific workshop; March 27-28, 2006; Washington, DC.
