Obesity-drug makers Arena Pharmaceuticals (NASDAQ:ARNA)VIVUS (NASDAQ:VVUS), and Orexigen Therapeutics (NASDAQ:OREX) are three of the mostly hotly debated stocks in the biotech space. The fact that obesity afflicts more than 35% of American adults today defines the massive market opportunity these three companies have, but investors may not completely understand the nuances of this complex disease. For instance, how do clinicians approach the treatment of obesity? Are medications an integral part of treatment, or are lifestyle changes more important? Can obesity be cured?

To shed light on these topics, Motley Fool health-care analyst Max Macaluso spoke with Dr. Domenica Rubino, a weight-management expert and representative of The Obesity Society. In the following video segment from their discussion, Dr. Rubino discusses what can cause obesity in addition to a poor diet. A transcript follows the video.

The relevant video segment can be found between 1:25 and 3:01.

Max Macaluso: What are some of the causes of obesity, besides poor diet?

Dr. Domenica Rubino: There's obviously poor diet, manifested in a lot of different ways in terms of how we eat, and there's different places that don't have healthy food, or people who don't have access to healthy food.

Sedentary -- we're all very, very sedentary. Unfortunately, we mostly drive to work, take the elevator, and sit at our computer, and go home and sit at our computer to relax. Exercise and just generalized movement and activity is a problem and contributes a lot to weight and the development of these diseases.

There are other things, though. Poor sleep. A lot of us -- because now we can be up all night, 24/7, on the computer -- we don't get enough rest, and the risk increases for less than five hours of sleep. People have increased risk for weight gain and development of diabetes, and if they have sleep apnea, that's an issue too.

The other thing that really can contribute to obesity that many people may not be aware of is drugs. Various medications -- either medications for depression, medications for heart disease, medications for allergies, medications for diabetes as well, which is an issue -- can all lead to increases in weight gain.

There's a myriad of factors. There can be psychological factors. Many people who are struggling from either depression or anxiety can have problems with weight. Alcoholism can be a contributor to weight, so there's various mood issues also.

Or people who are immobile -- if you get injured, you're in a wheelchair, you have severe osteoarthritis, that can aggravate weight as well, because then you're not able to move.

Quitting smoking -- there's a lot of different factors, Max, that contribute to obesity, which is why it's so complicated.