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 ICD-10 (The International Statistical Classification of Diseases and Related Health Problems, 10th Revision)

ICD-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 (ICD-10 F10), so is cocaine addiction (ICD-10 F14). However this does not mean that you can just use "it's a disease" as excuse, they are results of one's decisions.

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In Global Burden of Disease metrics, high body-mass index ranks as the 6th highest death risk factor.

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, possibly in combination with other factors.
Another one that can have obesity as a symptom is Cushing's.
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 Prader-Willi syndrome, Bardet-Biedl syndrome, and Cohen syndrome as some examples in their obesity article.

Studies are currently being conducted into the potential of infections causing obesity.

The Wikipedia page on infectobesity states that:

An association between viruses and obesity has been found in humans as well as a number of different animal species.

quoting this review: Obesity and infection - Falagas and Kompoti - The Lancet, 2006

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.

Answer 3

The traditional understanding of a disease tend to blur our understanding of this word. There's really little we can do about it, other than observe the semantic evolution of this word as our societies move through the ages. In past times witchcraft was a disease and in modern times a relevant percentage of our society regards homosexuality as a disease.

Meanwhile the so-called official definitions, as noted for instance by WHO's classification, while useful in a professional sense, should probably only be considered as convenient scoped definitions with the purpose of facilitating the understanding and handling of a phenomena in a given context (on this case of a medical analysis). Scientific understanding tends it too to evolve; Epilepsy used to be a concern of priests and medical doctors used to try and find a cure for ugly laughter. But more to the point, not every professional agrees with WHO's classification of obesity. It in fact received some backlash from some professionals and medical organizations (like AMA).

Finally dictionary definitions tend to be the more dangerous, due to their tendency for generic abridged definitions. An important skill while looking at a Dictionary is remembering this much. Especially when trying to use it as a way to find answer to questions like yours. Point in case, the dictionary definitions given by other answers here would also mean someone without one hand is a person suffering from a disease.

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The only possible answer to your question is Obesity is a disease if you choose to think so. Some want to support the idea it is a symptom, not a disease. Others that it is a collection of diseases, others that it is not a disease, others yet (my personal camp) that we have yet little to hold on in order to properly classify it either way and should consider it a worrying social phenomena that requires more study.