The relationship between obstructive sleep apnea (OSA) and weight gain is complicated. On one hand symptoms of OSA tend to cause people to be more prone to weight gain. These symptoms include lack of sleep, feeling sleepy in the day, and feeling weak or fatigued. On the other hand, obesity leads to the development and worsening of sleep apnea in several ways such as airway narrowing, increasing the chance that an airway will collapse and decreasing lung volume.

Weight gain causing OSA

Excessive weight gain leads to airway collapse by narrowing the upper airway. Fat builds up around the airway in the pharynx (located behind the nose and mouth and around the airway). This narrows the airway caliber and adds additional weight that compresses the airway, especially when lying down. There may also be increased compliance (or flexibility) of the upper airway walls, from hormonal changes in individuals with excess weight gain.

Obesity and lung volume

In addition to narrowed airways, excessive weight in the stomach can reduce the amount of air the lungs can hold (lung volume). Lung volume can be decreased in overweight individuals because abdominal fat presses up against the lung limiting its expansion. In addition, lung volumes are reduced by greater concentrations of adipokines. Adipokines are chemicals produced by fat cells. Adipokines act in the brain and reduce the signals telling the chest wall to expand to increase volume. When lung volume is reduced airways become more collapsible. This is because when the lungs fill with air they pull on the airway making it more rigid. A lower lung volume pulls less, leading the airway to be more compressible. To get a picture of how this happens, imagine a string being held at both ends by your two hands. When you pull there is less slack in the string and it becomes tighter.

Fat distribution and body weight

Picture two individuals gaining weight. One develops sleep apnea, the other does not. Why should that be? It may have something to do with where the fat settles. When individuals gain weight fat may get stored in the stomach, behind the neck or around it. With weight gain, changes may occur to structures surrounding the upper airway and increase neck size. When fat deposits in the upper airway it makes it more likely to collapse. This is countered by muscles surrounding the airway. While one is awake these muscles are most active. But during sleep the muscles aren’t as strong. A large neck size is a good way to estimate the degree of fat in the neck. Fat distribution in the neck is believed to contribute more to the development of apnea than does weight in other areas of the body. Even within the neck, it is possible that fat deposition in one area or another may make it more likely to develop sleep apnea.

Imaging studies suggest areas on the side of the throat called the lateral pharyngeal fat pads may play an important role in sleep apnea. Fat deposits are also concentrated around the jaw, tongue, uvula, and soft palate. Studies have shown that the lateral fat pads show the greatest increase in size with weight gain and also decrease in size with weight loss. There is some debate as to whether the total amount of fat around the neck is more important than the location of the fat. Weight gain around the airway makes the passage smaller. Obese patients have higher fat content within these muscles making the airway passage smaller. As the size of the airway gets smaller, there is also greater risk of collapse.

Gaining weight also increases muscle

Where there are more fat deposits, there is also more muscle development. Probably, this is to support the additional weight load. It is estimated that approximately 25% of the weight gain in obese people is caused by increased muscle mass. As a result, there is a corresponding increase in the volume or the amount of space occupied by the uvula and tongue related to the muscle mass further worsening airway narrowing.

Weight loss may help cure apnea

Just as weight gain may contribute to sleep apnea development, weight loss may reverse it. Individuals who have sleep apnea and lose weight often have dramatic improvement in the severity of their condition. Since, weight gain is not always the cause of sleep apnea and other factors such as facial anatomy play a role, losing weight may not be as substantial in some cases. Furthermore, weight loss is a difficult prospect for more people. Sustaining weight loss is even harder. Therefore, weight loss is often suggested as an adjunct to other forms of treatment for sleep apnea, such as continuous positive airway pressure or CPAP.