The Complex Interactions Between Obesity, Metabolism and the Brain
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fnins-13-00513 May 22, 2019 Time: 17:2 # 1 View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CONICET Digital REVIEW published: 24 May 2019 doi: 10.3389/fnins.2019.00513 The Complex Interactions Between Obesity, Metabolism and the Brain Romina María Uranga1,2* and Jeffrey Neil Keller3* 1 Instituto de Investigaciones Bioquímicas de Bahía Blanca, Universidad Nacional del Sur-Consejo Nacional de Investigaciones Científicas y Técnicas, Bahía Blanca, Argentina, 2 Departamento de Biología, Bioquímica y Farmacia, Universidad Nacional del Sur, Bahía Blanca, Argentina, 3 Pennington Biomedical Research Center, Louisiana State University System, Baton Rouge, LA, United States Obesity is increasing at unprecedented levels globally, and the overall impact of obesity on the various organ systems of the body is only beginning to be fully appreciated. Because of the myriad of direct and indirect effects of obesity causing dysfunction of multiple tissues and organs, it is likely that there will be heterogeneity in the presentation of obesity effects in any given population. Taken together, these realities make it increasingly difficult to understand the complex interplay between Edited by: obesity effects on different organs, including the brain. The focus of this review is Mario Eduardo Guido, to provide a comprehensive view of metabolic disturbances present in obesity, their Center for Research in Biological Chemistry Córdoba (CIQUIBIC), direct and indirect effects on the different organ systems of the body, and to discuss Argentina the interaction of these effects in the context of brain aging and the development of Reviewed by: neurodegenerative diseases. Russell H. Swerdlow, The University of Kansas, Keywords: obesity, overweight, metabolic dysfunction, brain, weight loss United States Qingchun Tong, The University of Texas Health OBESITY Science Center at Houston, United States Obesity is often considered to result from excessive calorie consumption (food intake) and/or *Correspondence: insufficient or inadequate calorie expenditure (metabolic and physical activity) (Figure 1). Obesity Romina María Uranga is a complex and chronic non-communicable disease that affects more than a third of the world’s [email protected]; [email protected] population (Hruby and Hu, 2015). It has been shown that obesity in middle age is able to shorten Jeffrey Neil Keller life expectancy by 4–7 years (Peeters et al., 2003). A major problem with obesity is the diverse [email protected] set of health associated complications it promotes including hypertension, diabetes, increased cardiovascular risk, and cancer (Calle et al., 2003). The most commonly utilized tool used for Specialty section: measuring obesity today is the body mass index (BMI), defined as a person’s weight in kilograms This article was submitted to divided by his or her height in meters squared. By convention, a person with a BMI of less than Neuroendocrine Science, 25.0 is considered non-obese or “normal,” a person with a BMI between 25.0 and 29.9 is defined as a section of the journal overweight, and a person with a BMI of 30 or more is considered obese. A BMI of more than 40.0 Frontiers in Neuroscience deserves particular attention since it represents morbid obesity (also known as severe or extreme Received: 09 January 2019 obesity). This index provides a reasonable estimate of body fat, and it is more accurate than skinfold Accepted: 06 May 2019 measurements. However, the use of BMI has certain limitations, because it does not distinguish Published: 24 May 2019 between lean and fat mass, nor does it indicate anything about fat distribution. In this sense, Citation: computed tomography or magnetic resonance imaging are the most accurate methods to measure Uranga RM and Keller JN (2019) The Complex Interactions Between the amount of visceral fat. Unfortunately, these tests are expensive and require sophisticated Obesity, Metabolism and the Brain. equipment. Waist circumference, a more straightforward but more reliable method to measure Front. Neurosci. 13:513. abdominal adiposity, has become an increasingly important tool for classifying obesity (Hu, 2007). doi: 10.3389/fnins.2019.00513 Numerous studies have shown that many obesity-related risk factors depend mainly on fat body Frontiers in Neuroscience| www.frontiersin.org 1 May 2019| Volume 13| Article 513 fnins-13-00513 May 22, 2019 Time: 17:2 # 2 Uranga and Keller Obesity, Metabolism and the Brain distribution rather than excess weight per se. Hence, it is metabolic disease (Grauer et al., 1984; Snijder et al., 2003a,b). important to take into account how body fat is distributed in Indeed, vWAT has been demonstrated to expand majorly by an individual, for example, between subcutaneous versus visceral hypertrophy of preexisting adipocytes while sWAT expands (or intra-abdominal) fat. It is important to note that visceral by hyperplasia, i.e., the increase of the number of progenitor fat, but not subcutaneous fat, is more associated with metabolic cells and subsequent differentiation. It is important to highlight syndrome, which is further discussed below. that bigger hypertrophic adipocytes are usually associated with metabolic dysfunction. In line with this, very important in Adipose Deposition terms of metabolic disease is the fact that vWAT adipocytes Adipose tissue is a complex, specialized, multicellular organ are metabolically (i.e., lipolytically) more active than sWAT able to influence the function of almost all other organs. It adipocytes, thus releasing more free fatty acids to the bloodstream is a loose connective tissue composed mostly of adipocytes, (Wajchenberg, 2000; Hajer et al., 2008). vWAT is also associated but which also contains the stromal-vascular fraction of cells with the release of pro-inflammatory adipokines (Fontana et al., (vascular endothelial cells, preadipocytes, and fibroblasts) as 2007) and this would explain, at least partially, why central well as macrophages. Adipose tissue is very heterogeneous obesity is strongly linked with metabolic complications such as and, anatomically, consists of different fat depots with unique type-2 diabetes mellitus and cardiovascular disease, and many characteristics. This mentioned heterogeneity in adipose tissue others caused by elevated free fatty acids (Jensen, 2008). Indeed, is not only from an anatomical point of view but also from this characteristic of being metabolically less active than vWAT the characteristics of the tissue expansion, the regulation of makes of sWAT a very important source of free fatty acids during lipid metabolism and also the pattern of secreted factors food deprivation. It is important to highlight at this point that (collectively called “adipokines”) in a particular fat depot. All vWAT mass but not sWAT mass correlates with the development these characteristics bring with them metabolic consequences of insulin resistance (Chowdhury et al., 1994; Wajchenberg, 2000; that impact the whole body, including the brain. It is known Hoffstedt et al., 2018). A plethora of studies argue in favor of that each anatomical fat depot has a particular physiological women better distributing fat in the periphery (compared to men role, which implies having specific metabolic and hormonal having more central obesity) and thus having better metabolic characteristics. As aforementioned, there is strong evidence health (Kwok et al., 2016). showing that some fat depots are more robustly associated with Interesting experiments with transplantation of adipose tissue disease development and outcomes. have been performed in animals, and they have been very useful In mammals, adipose tissue forms in utero just before birth for assessing the different functions and metabolic properties and throughout life. Moreover, the continuous generation of of the different fat depots. For example, transplantation of new adipocytes has been shown in adult humans (Spalding subcutaneous fat from donor mice into visceral fat site of et al., 2008). Unfortunately, little is known about adipocyte recipient mice has shown to confer metabolic benefits in the development. However, the study of adipose stem cell biology latter, namely decrease in body weight and total fat, improvement results extremely important for understanding adipose tissue of insulin sensitivity, and lowering of both insulinemia and development, expansion, migration, and maintenance. Adipose glycemia (Tran et al., 2008). Unexpectedly, intraperitoneal tissue is classified as white adipose tissue (WAT) and brown transplantation of visceral fat from lean mice showed to improve adipose tissue (BAT). These two main classes are molecularly insulin sensitivity, suggesting that the metabolic performance of and functionally different. WAT serves majorly as an energy a certain fat depot is more important as a metabolic risk factor store whereas BAT dissipates energy generating heat. WAT is than the anatomical location or the amount of fat itself (Konrad spread throughout the body as subcutaneous and visceral fat. et al., 2007; Kwok et al., 2016). On the other hand, transplantation Subcutaneous WAT (sWAT) is a fat layer under the skin, with of visceral fat or subcutaneous fat into subcutaneous fat site has major depots in the upper and lower body. The upper body not shown to cause any alterations in body weight or metabolic subcutaneous fat consists of superficial and deep abdominal fat, profile, so it seems there are both donor and recipient site- extremity fat, and, in the case of