A Vicious Circle
The epidemic of diabetes in the United States is being fueled by multiple medical, social, and demographic forces. Among those forces is sleep apnea, which is now recognized as a major contributor to the development of diabetes. In sleep apnea, people stop breathing for periods of 10 seconds or more while they’re asleep, sometimes hundreds of times a night. These periods without breathing, known as apneas, both disrupt sleep and lower the level of oxygen in the blood. When breathing restarts after an apnea, it is generally with a loud gasp or snort.
People with sleep apnea are more than twice as likely to have diabetes as those who don’t. In addition, 50% of men with Type 2 diabetes have sleep apnea, compared to an estimated 4% of middle-aged men overall. Several recent studies have suggested that insulin sensitivity—the body’s ability to respond to insulin—decreases as sleep apnea severity increases.
A high body-mass index (BMI, a measure of body mass that takes both height and weight into consideration) is a risk factor for both sleep apnea and diabetes.
Link to diabetes
A number of mechanisms are thought to be involved in the interaction between sleep apnea and diabetes, including the following:
Stress response. Repeated arousals from sleep and interruptions in the delivery of oxygen to the body’s tissues caused by sleep apnea lead to the stress, or “fight or flight,” response. In the short term, the stress response causes increased heart rate and increased blood pressure. When it occurs repeatedly over time, it is a risk factor in the development of chronic high blood pressure, insulin resistance (one of the hallmarks of Type 2 diabetes), and cardiovascular disease.
Increased cortisol levels. Sleep deprivation or fragmentation may increase blood levels of cortisol (a stress hormone), which in turn raises both blood glucose levels and insulin secretion.
Inflammatory response. Sleep apnea is associated with both local inflammation of the upper airways and systemic inflammation, or inflammation of the endothelium (the lining of the blood vessels) and other organ systems. Similarly, obesity is associated with systemic inflammation, as well as dyslipidemia (unhealthy levels of cholesterol and triglycerides in the blood). Both systemic inflammation and dyslipidemia are associated with atherosclerosis and cardiovascular disease.
Lack of oxygen. Repeated episodes of oxygen deprivation may also cause the release of proinflammatory cytokines—proteins involved in the body’s immune response—that are associated with glucose intolerance (higher-than-normal blood glucose levels) and insulin resistance.
Link to metabolic syndrome
Both Type 2 diabetes and sleep apnea have also been linked to the metabolic syndrome, which is also sometimes called syndrome X. The metabolic syndrome is defined by a set of five medical conditions that together double the risk of atherosclerosis and confer a fivefold increase in the risk of diabetes. The five conditions are elevated fasting glucose levels, abdominal fat, high blood pressure, high triglycerides, and low high-density lipoprotein (HDL, or “good”) cholesterol. There is broad overlap between the suspected mechanisms of interaction between sleep apnea and diabetes and the features of the metabolic syndrome.
Elevated fasting glucose. Sleep apnea is independently associated with glucose intolerance and insulin resistance, whether or not a person is obese. Impaired glucose tolerance has also been linked with sleep restriction, insufficient sleep, and difficulty maintaining sleep, all of which typically occur with snoring and sleep apnea.
Visceral fat. Excess body fat, particularly in the abdominal area, is a good predictor of sleep apnea. Two-thirds of people who snore or have been found to have sleep apnea are obese. The severity of sleep apnea increases with increasing body-mass index.
High blood pressure. The risk of high blood pressure also increases with increasing severity of sleep apnea, and it has additionally been linked with insulin resistance.
Dyslipidemia. Elevated triglycerides and reduced HDL cholesterol are common among people with Type 2 diabetes, and they correlate with even mild sleep apnea, independent of body-mass index.
Sleep apnea is independently associated with each of the five metabolic syndrome conditions. The metabolic syndrome may be made worse by both untreated sleep apnea and untreated diabetes.
When people with sleep apnea and Type 2 diabetes are treated with continuous positive airway pressure (CPAP, a common treatment for sleep apnea), the resulting improvement in their diabetes control leaves little doubt that sleep apnea may play a role in the functional changes that accompany Type 2 diabetes.
A CPAP device consists of a small mask that fits over the nose, or in some instances the mouth, and is connected to a machine that creates a slight air pressure in the throat to keep the airway open. A number of studies have examined its use.
For example, a study published in the Journal of Clinical Endocrinology & Metabolism in 1994 used CPAP therapy in a group of men and women who had Type 2 diabetes and sleep apnea. After four months of CPAP therapy, their sensitivity to insulin improved significantly.
A study published in the journal Respiration in 2004 measured the effects of CPAP therapy on insulin sensitivity in nine obese people with Type 2 diabetes and sleep apnea. These people had achieved good blood glucose control by means of either medication and diet or diet alone. Nonetheless, after three months of CPAP therapy, their insulin sensitivity improved significantly.
In yet another study, this one published in the Archives of Internal Medicine in 2005, use of CPAP therapy for at least four hours a day over one to four months led to improvements in blood glucose levels after meals in 25 people with Type 2 diabetes and sleep apnea. In addition, it led to lower glycosylated hemoglobin (HbA1c) levels in 17 people whose starting HbA1c was higher than 7%. The HbA1c test is a measure of blood glucose control over the previous 2–3 months. A normal, nondiabetic HbA1c level is between 4% and 6%.
In people with both sleep apnea and diabetes, therefore, CPAP therapy is an effective element in a treatment regimen where the objectives are normalized sleep and improved blood glucose control.
Further potential benefits from the treatment of snoring and sleep apnea may include the following:
- Improved control of high blood pressure
- Reduction in inflammatory response
- Reduced risk of fatal and nonfatal cardiovascular events (such as heart attacks)
- Reduced utilization of health-care resources
Improved quality of life is a common but frequently underappreciated benefit of CPAP therapy. People who use their CPAP device regularly experience improved sleep quality and have less fatigue, more energy, and improved coping capability. Depression, a common accompaniment of sleep apnea, may be replaced by a more positive mood. All of these changes enable a person to engage more readily in daily activities. Weight loss and improved fitness then become real possibilities, with their added beneficial effects on glucose tolerance.
Many health-care providers are unaware of the association between snoring, sleep apnea, and diabetes, and without awareness, treatment opportunities are missed. Everyone with diabetes should be screened routinely for the symptoms of snoring and sleep apnea. When people with diabetes and their sleep partners are asked the following three questions, nearly half will respond positively and can be expected to benefit from a referral to a sleep specialist:
- Do you snore? (While not everyone who snores has sleep apnea, just about everyone with sleep apnea snores.)
- Do you wake up tired after a full night’s sleep?
- Do you have high blood pressure?
If you answered yes to any of these questions but your health-care provider has not talked about sleep apnea with you, bring it up at your next appointment. Treating sleep apnea can have remarkable benefits: Not only will you sleep better, but your level of insulin resistance may decrease significantly and you may reduce your risk of cardiovascular disease.