Is obesity a disease?

Question

A northern Indiana hospital that erected billboards with the message “Obesity is a Disease. Not a Decision” is facing a backlash from people offended by the signs’ suggestion that obesity isn’t a lifestyle choice.

I am sceptical of the notion that obesity is a disease in the classic sense. Is there any medical evidence to support the notion that it is?

Answer 1

No doubt it is a disease in medical sense of the word. It has a code on WHO's IDC-10 (The International Statistical Classification of Diseases and Related Health Problems, 10th Revision)

IDC-10 E66.0 Obesity due to excess calories

Common definitions of disease:

"A disease is an abnormal condition affecting the body of an organism." (Dorland's Medical Dictionary)

or

disease: a condition of the living animal or plant body or of one of its parts that impairs normal functioning and is typically manifested by distinguishing signs and symptoms (Merriam-Webster)

There is little doubt that obesity is a disease according to both of above definitions.

Now, complete misunderstanding comes from interpreting "obesity is a disease" as "obesity is caused by a disease". However, in reality diet and lack of exercise are the leading causes.

The cause of obesity is complex and multifactorial. Within the context of environmental, social and genetic factors, at the simplest level obesity results from long-term positive energy balance — the interaction of energy intake and energy expenditure. The rapid increase in the prevalence of obesity over the past 20 years is a result of environmental and cultural influences rather than genetic factors. With progressive improvements in the standard of living in developed and developing countries, overnutrition and sedentary lifestyle have supplanted physical labour and regular physical activity, which has resulted in positive energy balance and overweight. (2006 Canadian clinical practice guidelines on the management and prevention of obesity in adults and children)

There is absolutely no contradiction between obesity being a disease, and the fact that it's caused by peoples' bad habits. There are numerous diseases, which are caused by bad habits. For example alcohol abuse if considered disease (IDC-10 F10), so is cocaine addiction (IDC-10 F14). This is however no excuse to be obese, alcoholic or crack-head.

Answer 2

As stated by Vartec, obesity technically classes as a disease no matter what causes it. It CAN (doesn't have to) also be caused (thus be a symptom of) other diseases like Hypothyroidism (http://en.wikipedia.org/wiki/Hypothyroidism), possibly in combination with other factors.
Another one that can have obesity as a symptom is Cushing's (http://en.wikipedia.org/wiki/Cushing%27s_syndrome).
Certain medicines can also slow down the metabolism, or lead to excessive water retention, resulting in weight gain unless countered.
And that doesn't even go into the genetic disorders that can lead to obesity. Wikipedia lists http://en.wikipedia.org/wiki/Prader-Willi_syndrome, http://en.wikipedia.org/wiki/Bardet-Biedl_syndrome, and http://en.wikipedia.org/wiki/Cohen_syndrome as some examples in their obesity article.
Studies are currently being conducted into the potential of infections causing obesity (http://en.wikipedia.org/wiki/Infectobesity). States http://www.sciencedirect.com/science/article/pii/S1473309906705230 to say that "An association between viruses and obesity has been found in humans as well as a number of different animal species. "
so yes, obesity can be a symptom of various diseases, and doesn't have to be (in fact may well not be in a good port of the population) caused exclusively or mainly by lifestyle choices.