For years, we have learned (and teaching) that obesity arises from the excess intake of calories and underutilization of those calories. It is true that in the US, many people have poor diets and lack physical activity. This plays a huge role in the obesity epidemic.
As doctors, we get frustrated when patients tell us they are eating right and exercising, yet fail to lose weight. Sure many of them are not being straightforward. But, new research is showing us that obesity is a multi-factorial process.
There are many things found to be contributing to obesity other than just caloric intake and output.
What causes obesity?
1. Lack of energy balance
This is the most obvious cause and we are all familiar with it. Simply put, if you consume more calories than you expend, you gain weight. If you use more calories than you consume, you lose weight.
However, this does not take into account the types of calories patients ingest, and we are learning that this is very important in terms of the balance of carbs, proteins, fats, etc.
In the U.S., our environment also plays a role. Americans are exposed to the fast food culture, rich in fats and deficient in fruits and vegetables. Also, the American lifestyle is rich in inactivity as evidenced by the number of drive-thrus.
While environmental and behavioral factors play a major role in obesity, it is evident that there are other factors in play. It has been noted that obesity tends to occur more in some families and some people are more predisposed to developing this problem.
It has been proposed that certain genetic variants come into play with complex inheritable conditions such as obesity. It is still unclear what this genetic variant is at the present time. Studies of single gene disorders and animal studies suggest a wide variety of possibilities. Research is ongoing. 
3. Neuroendocrine factors
This is currently a big area of research and many companies are trying to use these findings to make a medication targeting these pathways to treat obesity.
Many neuroendocrine factors appear to be in play in obesity, especially certain types of obesity such as visceral obesity. This explains why it is more common in certain disease such as diabetes and Cushing’s disease. Many people suggest a central or hypothalamic origin of these factors. Many of these patients are also shown to have insulin resistance.
Experiments done in primates revealed a stress reaction is often followed by a syndrome nearly the same as seen in humans with visceral obesity, including visceral fat accumulation. These observations suggest a similar chain of events in humans.
Recently, studies of cerebral spinal fluid showed abnormal levels of catecholamine and neuropeptides in obese individuals. Serotonin levels have also been seen to be altered, which may provide some clue into “stress eating” and the emotional factors of obesity.
4. Lack of sleep
Many new interesting studies have come out linking lack of sleep with obesity. The largest study and longest study currently done on adults on this topic is the Nurses’ Health Study. It looked at 68,000 middle-age American women for up to 16 years.
Compared to women who slept seven hours a night, women who slept five hours or less were 15 percent more likely to become obese. There have been many studies done on 5 continents in children showing a similar correlation between decreased amounts of sleep and increased weight.
5. Other causes
Other possible contributors to obesity are certain disease, medications and emotional factors and shall be explored more in subsequent posts.
In the U.S., obesity has reached epidemic proportions. It is true that much of it is related to unhealthy eating habits and a sedentary lifestyle. But, there are clearly other factors that play a role into the development of obesity. This has been a big area of focus for researchers over the last few years and hopefully the new-found knowledge will help us further treat our patients who suffer from obesity, in addition to our dietary and exercise advice.
4) http://www.ncbi.nlm.nih.gov/pubmed/7532780 General Disclaimer: All information here is for educational purposes only and is not meant to cure, heal, diagnose nor treat. This information must not be used as a replacement for medical advice, nor can the writer take any responsibility for anyone using the information instead of consulting a healthcare professional. All serious disease needs a physician.