by the Society of Sarcopenia, Cachexia and Muscle wasting [25] and by the International Conference of Frailty and Sarcopenia Research [18]. Both also strongly recommended resistance exercise and the major treatment modality.
On the American side, the Sarcopenia Definition and Outcomes Consortium (SDOC) was funded by the National Institute on Aging (NIA) in 2015 with additional support of the FNIH. The SDOC aim is to develop evidence‐based diagnostic cut‐off points for lean mass and/or muscle strength that enable identification of people at risk or mobility disability as a target population of potential function‐promoting therapies [26]. As in the FNIH initiative, the SDOC is again using an epidemiological approach using several cohorts, mostly in the United States but also in Europe, in order to accumulate data from a large number of subjects and be able to calculate an algorithm predictive of sarcopenia outcomes. The project was completed in August 2019, and the final document with recommendations published in 2020 [27].
Sarcopenia is now extending well beyond older age, with recent initiatives trying to define sarcopenia within organ diseases [28] and even in pediatrics [29].
A global (European, Asian, American, and Australia/New Zealand) initiative is now in process to try to come to a consensus on an operational definition of sarcopenia that would finish this long trip.
NEW PLAYERS: BONE, FAT, AND MUSCLE
There are specific aspects not contemplated in current definitions of sarcopenia, as the role of fat, bone, or both which are still far to be settled [30]. They are discussed in detail in other chapters of this book. However, it seems pertinent to mention some aspects that are related to general definitions here.
Osteoporosis is a skeletal condition closely linked to sarcopenia, a condition that has been named osteosarcopenia. The coexistence of both conditions seems to increase the risk of falls and other outcomes associated with each condition alone [31]. There is still some discussion if osteosarcopenia should be defined by body composition (i.e. low muscle mass and low skeletal mass) or if muscle function should be part of the definition [32, 33].
Sarcopenia and obesity also coexist frequently in the so‐called sarcopenic obesity [34–37]. Increases in muscle fat and body weight have a strong influence in the accuracy and adjustment of most methods that estimate skeletal muscle mass. As in osteosarcopenia, a body composition approach (i.e. low muscle mass plus obesity defined as increased fat or by anthropometry) has coexisted in research with an approach that defines sarcopenia using functional measures, and efforts to refine the definition are under way.
THE FRONTIERS: FRAILTY, CACHEXIA, MALNUTRITION
Frailty, cachexia, and malnutrition are conditions that share some elements with sarcopenia: they are frequent in old age, predict adverse outcomes, and include in some way low muscle mass within their definitions, which may lead clinicians into problems when trying to sort out which condition predominates in a given patient [38, 39].
The Global Leadership Initiative on Malnutrition (GLIM) has proposed a definition of malnutrition that includes reduced muscle mass as one of the three phenotypic diagnostic criteria [40]. Thus, the finding of a low muscle mass with normal muscle function may suggest that malnutrition is present, although this may well be the start toward a malnutrition‐related sarcopenia.
Low muscle mass is also included in the most widely used definitions of cachexia, which also consider the role of low muscle strength [12, 41]. The border between disease‐related sarcopenia and cachexia (a time‐honored term used to describe severe weight loss and muscle wasting associated with severe inflammatory conditions) is quite blurred, usually depending on the degree of inflammation, the underlying pathophysiology, the triggering condition, and even the discipline the practitioner comes from [42].
The links between physical frailty and sarcopenia are addressed in a different chapter. However, it is relevant that the frailty phenotype includes unintentional weight loss (usually associated with muscle wasting), weakness (defined by a low grip strength), and reduced physical performance (slow walking speed) [43], all of them part of the definition of sarcopenia. Both conditions are closely linked, sarcopenia being a player in a relevant portion of cases with physical frailty [44, 45]. International and Asian definitions of physical frailty exist [46, 47].
THE RESEARCH ARENA
The definition of sarcopenia is rapidly evolving, as is true for many other common conditions and specialties [48–50].
Among the most relevant areas of research and debate that are needed to further improve the definition of sarcopenia some may worth mentioning, in no particular order [25, 45]:
Muscle mass measurements need to be improved from the present estimations to real measures, in order to decide how this parameter is best included in the definitions of sarcopenia [9].
The role of physical performance (as part of the definition, measure of severity, or upstream outcome) should be clarified [51].
Cut‐off points that are ethnically appropriate need to be developed.
Epidemiological studies enriched with complex populations (i.e. those living in nursing homes) are still needed to define the best cut‐off points for each parameter and technique used to define sarcopenia in their capacity to predict outcomes.
A practical way to separate cachexia, sarcopenia, and malnutrition in clinical practice, in order to improve clinical management, is needed, but may not be feasible in many cases.
The definition of sarcopenia when it comes as a comorbidity of other major diseases (i.e. liver disease, renal diseases, cancer, major surgery) is currently being addressed by many studies, but still many use the muscle mass paradigm not including function.
Agreement on which of the many adverse outcomes are more relevant to address sarcopenia both in clinical practice and research would increase the number of patients with the diagnosis and foster research of a wide range of therapies.
The need and role of simple screening tools [52] compared with muscle mass and function measures need to be established.
Finally, the concept of sarcopenia within a life course approach needs further refinement. Is sarcopenia an old‐age condition, or should the threshold be moved and extended to younger populations? If so, are the same definitions valid across the life span?
SUMMARY
Sarcopenia was originally defined as age‐related muscle mass. Recent definitions have extended this to include muscle function and muscle quality using different approaches. Current definitions have confirmed the concept that sarcopenia is relevant, frequent, and linked with adverse outcomes, but have not yet been able to extend the diagnosis and management to current clinical practice. The definition of sarcopenia is still work in progress.
REFERENCES
1 1. Rosenberg IH. Sarcopenia: origins and clinical relevance. J Nutr. 1997; 127:990–991.
2 2. Baumgartner RN, Koehler KM, Gallagher D, et al. Epidemiology of sarcopenia among the elderly in New Mexico. Am J Epidemiol. 1998; 147:755–763.
3 3. Delmonico MJ, Harris TB, Visser M, et al. Longitudinal study of muscle strength, quality, and adipose tissue infiltration. Am J Clin Nutr. 2009; 90:1579–1585.
