The term sarcopenia was initially used to refer to an age-related progressive loss of lean mass and muscle strength [1Malafarina V, Uriz-Otano F, Iniesta R, Gil-Guerrero L. Sarcopenia in the elderly: Diagnosis, physiopathology and treatment. Maturitas 2012; 71(2): 109-14.
[http://dx.doi.org/10.1016/j.maturitas.2011.11.012] [PMID: 22153348] ], known to be associated with morbidity, reduction in quality of life, frailty disability and increased rates of mortality. With the increasing prevalence of obesity worldwide [2Afshin A, Forouzanfar MH, Reitsma MB, et al. Health effects of overweight and obesity in 195 countries over 25 Years. N Engl J Med 2017; 377(1): 13-27.
[http://dx.doi.org/10.1056/NEJMoa1614362] [PMID: 28604169] ], a new condition, which occurs in the presence of both sarcopenia and obesity, has been termed “sarcopenic obesity” [3Zamboni M, Mazzali G, Fantin F, Rossi A, Di Francesco V. Sarcopenic obesity: A new category of obesity in the elderly. Nutr Metab Cardiovasc Dis 2008; 18(5): 388-95.
[http://dx.doi.org/10.1016/j.numecd.2007.10.002] [PMID: 18395429] ]. However, many uncertainties still surround this phenomenon with regard to the definition, the potential “negative effects” on health (i.e., medical diseases, psychosocial impairment) and the clinical implications and usefulness in terms of its treatment.

The aim of this short communication is to emphasize some crucial aspects that future research should take into account in order to avoid bias and misinterpretations and to underline that the study of this condition should be considered a scientific and clinical priority, as reported by the European Society for Clinical Nutrition and Metabolism (ESPEN) and the European Association for the Study of Obesity (EASO). In doing so, the following questions should be addressed:

1. What is the definition of sarcopenic obesity?
2. Is sarcopenic obesity harmful to health?
3. Is this condition worth treating?

Over the last decade, the clinical definition and diagnostic criteria for sarcopenia have been developed and both are now widely accepted and recognized by the international scientific community [4Cruz-Jentoft AJ, Baeyens JP, Bauer JM, et al. Sarcopenia: European consensus on definition and diagnosis: Report of the european working group on sarcopenia in older people. Age Ageing 2010; 39(4): 412-23.
[http://dx.doi.org/10.1093/ageing/afq034] [PMID: 20392703] ]. However, no consensus has yet been reached on how to define sarcopenic obesity. In most studies, the definition of sarcopenic obesity has relied on body composition phenotypes (i.e., lean mass) [4Cruz-Jentoft AJ, Baeyens JP, Bauer JM, et al. Sarcopenia: European consensus on definition and diagnosis: Report of the european working group on sarcopenia in older people. Age Ageing 2010; 39(4): 412-23.
[http://dx.doi.org/10.1093/ageing/afq034] [PMID: 20392703] -7Chen LK, Liu LK, Woo J, et al. Sarcopenia in asia: Consensus report of the asian working group for sarcopenia. J Am Med Dir Assoc 2014; 15(2): 95-101.
[http://dx.doi.org/10.1016/j.jamda.2013.11.025] [PMID: 24461239] ], low muscle strength (i.e., weak handgrip strength) and poor physical performance (e.g., slow walking) [4Cruz-Jentoft AJ, Baeyens JP, Bauer JM, et al. Sarcopenia: European consensus on definition and diagnosis: Report of the european working group on sarcopenia in older people. Age Ageing 2010; 39(4): 412-23.
[http://dx.doi.org/10.1093/ageing/afq034] [PMID: 20392703] , 6Fielding RA, Vellas B, Evans WJ, et al. Sarcopenia: An undiagnosed condition in older adults. Current consensus definition: Prevalence, etiology, and consequences. International working group on sarcopenia. J Am Med Dir Assoc 2011; 12(4): 249-56.
[http://dx.doi.org/10.1016/j.jamda.2011.01.003] [PMID: 21527165] , 7Chen LK, Liu LK, Woo J, et al. Sarcopenia in asia: Consensus report of the asian working group for sarcopenia. J Am Med Dir Assoc 2014; 15(2): 95-101.
[http://dx.doi.org/10.1016/j.jamda.2013.11.025] [PMID: 24461239] ] cut-offs, usually used for the definition of sarcopenia in elderly people [4Cruz-Jentoft AJ, Baeyens JP, Bauer JM, et al. Sarcopenia: European consensus on definition and diagnosis: Report of the european working group on sarcopenia in older people. Age Ageing 2010; 39(4): 412-23.
[http://dx.doi.org/10.1093/ageing/afq034] [PMID: 20392703] ]. However, without accounting for body mass (i.e., body mass index, body surface), definitions of sarcopenia in obesity that are based only on lean mass and physical fitness may be strongly skewed for at least two reasons. Firstly, patients with obesity tend to have a relatively large lean mass. Hence, sarcopenia criteria based on this parameter may not be met in these individuals and the prevalence of sarcopenia may, therefore, be greatly underestimated [8Johnson Stoklossa CA, Sharma AM, Forhan M, Siervo M, Padwal RS, Prado CM. Prevalence of sarcopenic obesity in adults with class II/III obesity using different diagnostic criteria. J Nutr Metab 2017; 2017: 7307618.
[http://dx.doi.org/10.1155/2017/7307618] [PMID: 28421144] ]. Secondly, low physical fitness is more strongly associated with obesity than with sarcopenia [9Bouchard DR, Dionne IJ, Brochu M. Sarcopenic/obesity and physical capacity in older men and women: Data from the nutrition as a determinant of successful aging (NuAge)-the quebec longitudinal study. Obesity (Silver Spring) 2009; 17(11): 2082-8.
[http://dx.doi.org/10.1038/oby.2009.109] [PMID: 19373219] ], with the result that the prevalence of sarcopenia may be overestimated.

To address this problem, recent reports have recommended accounting for body mass or body fat mass when identifying individuals with sarcopenic obesity [8Johnson Stoklossa CA, Sharma AM, Forhan M, Siervo M, Padwal RS, Prado CM. Prevalence of sarcopenic obesity in adults with class II/III obesity using different diagnostic criteria. J Nutr Metab 2017; 2017: 7307618.
[http://dx.doi.org/10.1155/2017/7307618] [PMID: 28421144] ]. In light of this recommendation, a new definition of sarcopenic obesity has been established, which in addition to the Appendicular Lean Mass (ALM) also includes the Body Mass Index (BMI) [10Studenski SA, Peters KW, Alley DE, et al. The FNIH sarcopenia project: Rationale, study description, conference recommendations, and final estimates. J Gerontol A Biol Sci Med Sci 2014; 69(5): 547-58.
[http://dx.doi.org/10.1093/gerona/glu010] [PMID: 24737557] ]. The following cut-offs have been proposed: ALM/BMI <0.804 in males and <0.582 in females [11Batsis J, Mackenzie TA, Lopez-Jimenez F, et al. Prevalence of sarcopenic obesity using FNIH criteria in the US population: NHANES 1999-2004 2015. 1]. Despite this progress, a lot of work is still needed to reinforce the concept of a suitable definition of sarcopenic obesity in order to avoid false positives and negatives. This issue is of extreme importance as the correct identification of sarcopenic obesity is fundamental to target interventions only on those patients that are really affected by this condition [12Bouchonville MF, Villareal DT. Sarcopenic obesity: How do we treat it? Curr Opin Endocrinol Diabetes Obes 2013; 20(5): 412-9.
[http://dx.doi.org/10.1097/01.med.0000433071.11466.7f] [PMID: 23974769] ] (Fig. 1).

Fig. (1) Suggested approach to the study the sarcopenic obesity.

It has been hypothesized that the two components of sarcopenic obesity, namely obesity and sarcopenia, may synergistically increase their negative effects on health [13Atkins JL, Wannamethee SG. The effect of sarcopenic obesity on cardiovascular disease and all-cause mortality in older people. Rev Clin Gerontol 2015; 25: 86-97.
[http://dx.doi.org/10.1017/S0959259815000076] ]. Indeed, several cross-sectional studies have demonstrated that individuals with sarcopenic obesity have a worse cardio-metabolic risk (i.e., hyperglycaemia, hypertension, dyslipidaemia and insulin resistance) [5Stenholm S, Harris TB, Rantanen T, Visser M, Kritchevsky SB, Ferrucci L. Sarcopenic obesity: Definition, cause and consequences. Curr Opin Clin Nutr Metab Care 2008; 11(6): 693-700.
[http://dx.doi.org/10.1097/MCO.0b013e328312c37d] [PMID: 18827572] ], psychological profiles (i.e., depression and cognitive performance) [14Hamer M, Batty GD, Kivimaki M. Sarcopenic obesity and risk of new onset depressive symptoms in older adults: English longitudinal study of ageing. Int J Obes 2015; 39(12): 1717-20.
[http://dx.doi.org/10.1038/ijo.2015.124] [PMID: 26122029] ] and impaired health-related quality of life (HRQoL) [15Choi KM. Sarcopenia and sarcopenic obesity. Korean J Intern Med (Korean Assoc Intern Med) 2016; 31(6): 1054-60.
[http://dx.doi.org/10.3904/kjim.2016.193] [PMID: 27809450] ]. However, the cross-sectional design used in these studies indicates, at best, only simple associations between sarcopenic obesity and some health parameters but does not provide solid information regarding any causal relationships between the two conditions [16Solem RC. Limitation of a cross-sectional study. Am J Orthod Dentofacial Orthop 2015; 148(2): 205.
[http://dx.doi.org/10.1016/j.ajodo.2015.05.006] [PMID: 26232823] ]. In other words, these studies are not able to determine if sarcopenic obesity may lead to the onset or deterioration of certain diseases since very few studies have longitudinally investigated the “real” effects of sarcopenic obesity on health. Moreover, the available studies have only assessed intermediate outcomes, (i.e., glucose and lipid metabolism, metabolic disorders) and not hard long-term outcomes, (i.e., mortality). These shortcomings of the research indicate the need to design longitudinal studies to clarify the real effect of sarcopenic obesity on the onset/evolution of certain diseases (i.e., cardio-metabolic, psychological) in the short and medium-terms, as well as the risk of mortality in the long-term.

After having reached a consensus on a valid definition of sarcopenic obesity and its real effect on health and mortality, the final step is to identify potential treatment strategies (e.g., physical activity interventions, high-protein diets, protein supplements, etc.) and to use powerful randomized controlled trials to test their efficacy in limiting sarcopenia deterioration in patients with obesity. This is in line with the recommendation endorsed by the European Society for Clinical Nutrition and Metabolism (ESPEN) and the European Association for the Study of Obesity (EASO), entitled “Sarcopenic obesity: Time to meet the challenge”, This recommendation suggests that this condition should be considered a scientific and clinical priority for both researchers and clinicians [17Barazzoni R, Bischoff SC, Boirie Y, et al. Sarcopenic obesity: Time to meet the challenge. Clin Nutr 2018; S0261-5614(18)30169-9.
[PMID: 29857921] ].