Adiposopathy (or sick fat) is defined as pathologic adipocyte and adipose tissue anatomic & functional disturbances, promoted by positive caloric balance, in genetically and environmentally susceptible individuals. The ensuing pathogenic endocrine and immune responses may directly promote cardiovascular disease, and may also cause or worsen among the most common metabolic disease encountered in developed countries. Because many of these metabolic diseases are major cardiovascular disease risk factors (e.g., type 2 diabetes mellitus, hypertension, and dyslipidemia), adiposopathy also indirectly increases CVD risk, and is an important contributor to the metabolic syndrome.
Most medical organizations recognize obesity as a disease. According to the Obesity Medicine Association Obesity Algorithm: "Obesity is defined as a chronic, relapsing, multi-factorial, neurobehavioral disease, wherein an increase in body fat promotes adipose tissue dysfunction and abnormal fat mass physical forces, resulting in adverse metabolic, biomechanical, and psychosocial health consequences." Consistent with this functional definition, patients with the disease of obesity can generally be categorized into patients with "fat mass disease" and "sick fat disease."  "Fat mass disease" represents pathologies related to abnormal physical forces, such as stress on weight-bearing joints, immobility, tissue compression, and tissue friction. "Sick fat disease" (adiposopathy) results from deranged endocrine and immune responses that contribute to the onset or worsening of elevated blood pressure, blood glucose, blood lipids, and other metabolic diseases. Clinically, a patient is diagnosed as having adiposopathy if an increase in body fat is the most likely cause of the onset or worsening of characteristic metabolic abnormalities.
In some respects, the diagnosis of adiposopathy is a diagnosis of exclusion. Type 2 diabetes mellitus has a number of potential causes largely unrelated to excess body fat, such as hemochromatosis, chronic pancreatitis, hypercortisolism, excessive growth hormone, genetic syndromes of insulin resistance, and decreased pancreatic beta cell function due to genetic syndromes or surgical excision. High blood pressure can be due to pheochromocytoma, primary hyperaldosteronism, hypercortisolism, hyperthyroidism, renal artery stenosis, various kidney diseases, and familial or genetic syndromes. Dyslipidemia can be due to hypothyroidism, poorly controlled diabetes mellitus, certain types of liver or kidney disease, and genetic dyslipidemias. However, in the absence of other secondary causes, and in the presence of new onset or worsening of adiposopathic metabolic diseases with an increase in body fat, then by applying the principles of Ockham's Razor (pluralitas non est ponenda sine necessitate), wherein among competing theories with similar predictions, the simpler one is better, then adiposopathy is the simplest explanation why increased body fat leads to metabolic diseases.
Diseased fat tissue surrounding various organs can cause illness, such as fat surrounding the heart, muscle, vessels, eyes, and bone. Some have suggested that diseased fat tissue surrounding the heart and vessels can contribute to inflammation and plaque rupture. Although not as well recognized, even the so-called "protective" subcutaneous fat tissue has the potential to be "sick" and contribute to metabolic disease. A prime example would be subcutaneous fat tissue found in the abdominal region. Accumulation of fat tissue in this region may have hormonal and immune activity, and thus the potential to cause metabolic disease, between that of visceral fat tissue and other areas of subcutaneous fat tissue.
However, other subcutaneous fat tissues also might contribute to metabolic disease, if the fat cells become too enlarged and "sick." Admittedly, subcutaneous fat cells typically are larger, and capable of storing more fat when needed. However, subcutaneous fat tissue represents the largest proportion of fat tissue in the body, and is the major source of leptin.
One potentially unfavorable effect of leptin is to increase blood pressure, as observed in animals. In humans, the observation of leptin-induced hypertension is not as yet conclusive. But to the extent that leptin may increase blood pressure, then the increase in leptin with subcutaneous fat cells (particularly when they become enlarged) could hardly be characterized as "protective".
Other potentially detrimental effects of enlarged subcutaneous fat tissue relate to free fatty acids. During fasting, the body can obtain energy through the release of free fatty acids from the triglycerides in fat cells. The fatty acids thus become available for release into the blood. If too high a concentration of certain fatty acids accumulates in the blood because of sick fat tissue, and the body is unable to recruit more healthy fat cells, then existing healthy fat cells become engorged (and thus also sick). The result is that blood fatty acid concentrations increase to levels toxic to tissues such as liver, muscle, and pancreas, and lead to a range of pathological metabolic conditions.
There are grounds for suspicion that sick abdominal fat tissue may produce factors that cause subcutaneous fat tissue to also become "sick" and further contribute to metabolic diseases.
In summary, although abdominal or visceral fat tissue is best described to contribute to metabolic disease, abdominal fat is by no means the only fat tissue depot that has the potential to become "sick" and capable of contributing to metabolic ill health.
Fat tissue is an active body organ involved in many processes critical to human health,8 including: (1) promotion of blood vessel formation (angiogenesis); (2) fat cell recruitment and development adipogenesis; (3) dissolving and reforming the structures around fat tissue (extracellular matrix); (4) generation, storage and release of fat; (5) growth factor production; (6) glucose metabolism; (6) production of factors that affect blood pressure (such as those associated with the renin–angiotensin system); (7) fat and cholesterol metabolism; (8) enzyme production; (9) hormone production; (10) steroid metabolism; (11) blood clotting (hemostasis); (12) element binding; (13) and immune response (described below). When fat cells and fat tissue remain healthy during fat weight gain, patients may avoid metabolic ill health. However, if enlargement of fat cells and fat tissue causes them to become "sick", then important fat tissue functions are disrupted, and deranged responses contribute to metabolic disease. When excessive body weight leads to adiposopathy, this represents a hormone or endocrine disease. Additionally, fat cells and fat tissue also produce many different types of immune factors. Inflammation is a contributing cause to metabolic disease, and the ultimate contribution of fat tissue to inflammation is determined by the production of both inflammatory and anti-inflammatory factors. From a pro-inflammatory standpoint, fat tissue (which includes fat cells and other cells, such as immune cells) produces factors including: (1) adipokines with cytokine activity such as leptin, interleukins, and tumor necrosis factor alpha; (2) acute phase proteins / reactants such as C-reactive protein; (3) adipokines of the alternative complement system; (4) chemotactic/chemoattractant adipokines; and prostaglandins (eicosanoids). From an anti-inflammatory standpoint, fat tissue produces various anti-inflammatory factors  with the most commonly described being adiponectin. If fat cells or fat tissue becomes "sick", then the release of too many pro-inflammatory factors and a decrease in too many anti-inflammatory factors often result in a net pro-inflammatory response can contribute to metabolic disease.
Because no accepted diagnostic criteria for adiposopathy exist, then no drugs have received specific treatment indications by regulatory agencies to treat "sick fat". However, weight loss therapies in overweight patients not only improve, or sometimes normalize various fat tissue factors that may cause or contribute to metabolic disease, but also improve and sometimes "cure" metabolic diseases such as type 2 diabetes mellitus, hypertension, and dyslipidemia.  By doing so, these therapies (which includes appropriate diet and physical exercise) effectively treat adiposopathy or "sick fat." Additionally, drugs that increase the recruitment of new, healthy fat cells may also help treat metabolic diseases. For example, peroxisome proliferator-activated receptor (PPAR) gamma agents are commonly used drugs to treat type 2 diabetes mellitus. Pioglitazone is an example of a PPAR gamma agonist drug that lowers blood sugar and improves lipid levels. As part of PPAR gamma drugs' mechanism of action is increasing the amount of functional or healthy fat tissue. As a result, many patients treated with these types of drugs increase body fat. Initially, it may seem odd and almost paradoxical to use a drug that increases fat tissue to treat metabolic diseases that are caused by too much fat tissue. However, when explained through the concept adiposopathy, no such paradox exists. Because PPAR gamma agents work by increasing the amount of healthy, functional fat, decreasing the proportion of sick abdominal fat tissue, and decrease "fatty liver". All of these effects upon fat tissue are effective in treating sick fat and improving metabolic disease. Thus, it is within the framework of "sick" versus "healthy" fat, that the rationale behind the use of these drugs is easier to understand.
It has been known since the 1970s that when fat cells become too big, they may become bloated and dysfunctional, or "sick". It has also been known since the 1940s that if fat gain occurs in the belly or abdominal (visceral) region, that this is another example of sick fat that promotes metabolic diseases. Finally, if fat growth exceeds its blood vessel supply, then the lack of oxygen delivery by the blood may also result in pathologic responses from fat tissue. In summary, it has been known for decades that adverse changes in fat cell and fat tissue anatomy result in sick fat which causes metabolic disease. More recently, an additional event that has prompted the concept and term of "adiposopathy" is the evolving recognition of the profound hormone and immune importance of fat tissue. In the past, fat cells and fat tissue were considered by many as being inert, or hormonally and immunologically inactive. However, this has been proven incorrect, and it is now generally accepted that fat tissue is an active hormone, and immune organ. Yet another historical event that moved medical science towards recognizing fat tissue as an underlying cause of metabolic disease has been the problematic issues that have arisen with the "metabolic syndrome". The metabolic syndrome is a commonly used medical term to describe atherosclerotic coronary heart disease risk factors that tend to cluster together. Over the years, there have been at least 15 other similar terms used to generally describe the same clustering. According to one common definition, a patient is said to have "metabolic syndrome" if he or she has 3 or more the 5 following criteria: (1) abdominal obesity, (2) elevated triglycerides, (3) reduced high density lipoprotein cholesterol levels, (4) high blood pressure, and (5) high blood sugar. However, different scientific and medical organizations have different definitions for the "metabolic syndrome". Also, the term "metabolic syndrome" does not describe, nor is it intended to describe a unifying cause of any disease. Since "metabolic syndrome" is not a disease, regulatory agencies (such as the Food and Drug Administration) do not approve drugs to treat metabolic syndrome, as a specific indicated use. Finally, the "metabolic syndrome" may be no better at predicting atherosclerotic coronary heart disease than an assessment of its individual components. As such, major scientific and medical organizations have questioned the usefulness of the metabolic syndrome, sometimes in open conflict with other major scientific and medical organizations.
Society and culture
The scientific and medical organizations are not alone in grappling with if and/or when too much body fat is actually a "disease". Many patients and clinicians have the same uncertainties. This is reflected by the attitude of many of the public as well. Many are often resistant to the idea that gaining too much fat is a disease, for a number of reasons. One illustrative example in pop culture would be the Greg Giraldo stand-up comedy bit sampled at length in the Lazyboy song " Underwear Goes Inside the Pants " for the album Lazyboy TV (2004). An excerpt of the publicly disclosed lyrics includes:
Americans, let's face it: We've been a spoiled country for a long time.
Do you know what the number one health risk in America is?
They say we're in the middle of an obesity epidemic.
An epidemic like its polio.
Like we'll be telling our grandkids about it one day.
The Great Obesity Epidemic of 2004.
"How'd you get through it grandpa??"
"Oh, it was horrible Johnny, but there was cheesecake and pork chops everywhere."
Nobody knows why we're getting fatter? Look at our lifestyles.
I'll sit at a drive through.
I'll sit there behind fifteen other cars instead of getting up and making the eight foot walk over to the totally empty counter.
Everything is mega meal, supersize. You want biggy fries with that, you want a jumbo fry, you wanna go large.
You wanna biggie fry,
You want thirty burgers for a nickel you fat mother f****r. There's room in the bag. Take it!
You want a 55 gallon drum of coke with that? It's only three more cents.
Given all the problems with medical terms such as overweight, obesity, and metabolic syndrome in describing when excessive body fat is a disease, adiposopathy has emerged as a term that reflects the fact that adipose (fat) tissue is no less capable of being diseased than any other body organ. The addition of the suffix "pathy" to an organ is not only accepted, but an historic universal identifier as to when a body tissue is diseased. Cardiomyopathy describes heart (cardiac) disease; myopathy describes muscle disease; encephalopathy describes brain disease; ophthalmopathy describes eye disease; retinopathy describes eye disease; enteropathy describes intestine disease; nephropathy describes kidney disease; neuropathy describes nerve disease; and dermopathy describes skin disease. Adiposopathy describes the disease of fat cell and fat tissue that frequently occurs with excessive fat weight gain.
Research and future directions
Regarding drug therapies, it has been stated that: "An emerging concept is that the development of anti-obesity agents must not only reduce fat mass (adiposity) but must also correct fat dysfunction (adiposopathy)." This is in recognition that the use of weight loss therapies and drugs in overweight patients must not only improve the weight of patients, but must also improve the health of patients. Unfortunately, not everyone understands the central role that sick fat plays in the development of metabolic disease. This is partly because in the past, most of the attention of what causes metabolic disease was given to other body organs. In the past, the importance of fat tissue was often ignored, even as the obesity epidemic was contributing to the epidemic of metabolic diseases such as type 2 diabetes mellitus, hypertension, and dyslipidemia. This has placed many clinical scientists at odds with basic researchers, clinicians and patients. The animal data supporting adiposopathy as a contributing cause to metabolic disease is overwhelming. Clinicians and patients well-known through "real life" experiences that gaining body fat often causes patients to develop or worsen high blood sugar (diabetes mellitus), high blood pressure (hypertension), and dyslipidemia (abnormal cholesterol or fats in the blood). In fact, clinicians often recommend, and patients often expect that weight reduction efforts will improve, if not cure these metabolic diseases. However, many clinical scientists and even medical organizations continue to resist what is obvious to others, and that is that fat cells and fat tissue are hormonally and immunologically active. Many refuse to acknowledge the scientific and clinical evidence that too much body fat can cause or worsen metabolic disease, if the fat becomes sick. In order to give greater recognition to the "adipocentric" (fat tissue as a central cause) paradigm of metabolic disease, a "Adiposopathy Working Group" was assembled to develop a consensus regarding whether adipospathy was truly an endocrine disease. In 2008, this group of experts in both the scientific and clinical field of Endocrinology reported their opinions in the International Journal of Clinical Practice. In this consensus paper encompassing approximately 2 years of research, the authors addressed the many philosophical misconceptions, and unfounded/uninformed facts regarding the relationship of adiposopathy to metabolic disease. Their final conclusion was that "adiposopathy is an endocrine disease".
This article incorporates material from the Citizendium article "Adiposopathy", which is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License but not under the GFDL.
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