The initial search retrieved 606 papers. After the first round of screening, 366 papers were excluded for: (i) Languages other than English; (ii) Non-human studies; and (iii) Dealing with obesity without sarcopenia, or the latter without the former. The second round of screening excluded 229 articles due to: (i) Inappropriate paper type, not original research articles (i.e., clinical reviews, Letters to the Editor, chapters in a book and case reports); (ii) Descriptions of SO, but not health-related outcomes; and (iii) An unclear definition of SO or identification of individuals with this condition. Accordingly, following the screening process, 11 articles were included in the systematic review and underwent narrative synthesis and meta-analysis (Figure 1). The NOS checklist proved that the studies were of a high quality (n = 11) (mean score = 7.36 points) (Table 2).
Figure 1 The flowchart summarizing the study selection procedure.
In 2012, Sénéchal et al conducted a cross-sectional evaluation in which the authors assessed dynapenic obesity, defined as low leg muscle strength combined with abdominal obesity, in 1963 individuals with abdominal obesity. Of these patients, 566 had dynapenic obesity (data per gender is not available). Regardless of gender the mean age and mean BMI in the dynapenic obesity and non-dynapenic obesity groups were 65.4 ± 9.9 years and 29.9 ± 4.6 kg/m2 and 65.5 ± 9.6 years and 30.8 ± 4.5 kg/m2 respectively. Furthermore, 130 of the 566 individuals with dynapenic obesity had T2D compared to 196 of the 1397 individuals in the non-dynapenic obesity group.
One year later, Lu et al completed a cross sectional study in which they assessed SO defined as the coexistence of obesity (BMI ≥ 25 kg/m2) and sarcopenia based on the skeletal muscle index estimated by BIA. A sample of 180 individuals with obesity (60 males and 120 females) was recruited. Of the 60 males included in the sample 35 had SO compared to 80 of the 120 females. Regardless of gender the mean age and BMI in the SO group were 61.1 ± 9.9 years and 27.8 ± 2.6 kg/m2, and 69.9 ± 7.3 years and 26.8 ± 1.6 kg/m2 in the non-SO group. Moreover, 12 of the 65 patients in the SO group had T2D compared to 17 of the 115 patients in the non-SO group.
In early 2016, Poggiogalle et al conducted a cross sectional study in which the authors assessed SO using DXA, with SO defined as the coexistence of obesity (BMI ≥ 30 kg/m2) and sarcopenia (ASMM: height2 < 6.54 and < 4.82 kg/m2 for males and females respectively) or (ASMM: weight < 0.2827 and < 0.2347 for males and females respectively). This study enrolled a sample of 727 individuals with obesity (141 males and 586 females), with mean ages of 45.63 ± 13.53 and 45.76 ± 13.58 years, and mean BMIs of 37.56 ± 5.99 and 37.80 ± 5.77 kg/m2 respectively for each gender. Of the 141 male patients 68 had SO, while 350 of the 586 females had the condition. In addition, 155 of the 418 patients had pre-diabetes or T2D in the SO group compared to 70 of the 309 patients in the non-SO group.
In the same year, Ma et al performed a cross-sectional evaluation on SO defined by BMI and sex-specific 24-h urinary creatinine excretion, in 310 patients (166 females and 144 males) with obesity (BMI ≥ 30 kg/m2). Fifty-four of the 144 males and 52 of the 166 females had SO. The mean BMI and age of the SO group were 34.1 ± 4.0 kg/m2 and 71.8 ± 7.6 years, while they were 34.9 ± 4.4 kg/m2 and 67.8 ± 6.8 years in the non-SO group, respectively. Furthermore, 40 of the 106 patients had T2D in the SO group in comparison to 51 of the 204 patients in the non-SO group.
In 2017, Xiao et al performed a retrospective study on the prevalence of SO and its association with health outcomes in patients seeking weight loss treatment in a bariatric surgery setting. Body composition analysis was conducted by means of BIA and SO was defined by a fat mass:fat-free mass index (FMI: FFMI) ratio greater than the 95th percentile of sex, BMI and ethnicity-specific population-representative references. A sample of 144 adults with obesity (99 females and 45 males) were enrolled, with a mean age of 55.6 ± 11.5 years and a mean BMI of 46.6 ± 8.4 kg/m2. Of the 144 patients included in the sample 73 had SO (data per gender is not available). The mean age and BMI of the individuals with obesity only were 56.6 ± 12.7 years and 44.0 ± 7.6 kg/m2, compared to 54.6 ± 10.1 years and 49.1 ± 8.3 kg/m2 in those with SO. Furthermore, 34 of the 73 patients had T2D in the SO group in comparison to 36 of the 71 patients in the non-SO group.
In 2017, Kang et al conducted a large cross-sectional study to assess the association between SO and metabolic syndrome in postmenopausal women. SO was defined by the co-existence of sarcopenia (ASM/weight < 1 standard deviation below the mean of the reference group) and a BMI cut-off point for obesity which referred to a score of of 25 kg/m2 on the basis of the Asia-Pacific obesity criterion. The study included 1555 females with obesity, of whom 855 had SO, with a mean age of 62.91 ± 0.44 years and a mean BMI of 27.93 ± 0.11 kg/m2. On the other hand, 700 did not have SO and had a mean age of 61.05 ± 0.44 years and a mean BMI of 26.80 ± 0.07 kg/m2. In addition, 165 of the 855 patients had T2D in the SO group while 105 of the 700 patients in the non-SO group had T2D.
In the same year, a cross-sectional study by Aubertin-Leheudre et al aimed to examine the association between dynapenic obesity and metabolic risk factors in older adults (age ≥ 70 years). Dynapenic obesity was defined as low handgrip strength (u 19.9 in females; ≤ 31.9 in males) combined with a BMI of ≥ 30 kg/m2. The study included 670 participants with obesity (213 males and 457 females), of whom 256 had dynapenic obesity, with a mean age of 78.0 ± 4.6 years and a mean BMI of 34.9 ± 4.8 kg/m2, and 414 did not have dynapenic obesity, with a mean age of 76.3 ± 4.7 years and a mean BMI of 35.6 ± 4.8 kg/m2. Furthermore, 81 of the 256 individuals in the dynapenic obesity group had T2D while 133 of 414 individuals in the non-dynapenic obesity group had T2D.
In 2018, Park et al conducted a large cross sectional study in two sites, which included a total of 53818 adults with overweight and obesity of both genders (38820 males and 14998 females), of whom 6513 had SO defined as below two standard deviations of the mean of the skeletal muscle mass index for young adults assessed by BIA and a waist circumference of ≥ 90 cm for men and ≥ 85 cm for women. The mean age and BMI of the individuals with obesity only were 40.5 ± 9.2 years and 26.9 ± 2.2 kg/m2 compared to those with SO who had a mean age of 40.0 ± 11.3 years and a mean BMI of 30.7 ± 3.4 kg/m2. Moreover, 391 of the 6513 patients had T2D in the SO group compared to 2176 of the 47305 patients in the non-SO group.
In 2018, Kreidieh et al conducted a cross sectional controlled study in which body composition measurements were conducted by BIA using a definition that in addition to appendicular lean mass (ALM) also involved BMI, and patients were considered affected by SO if ALM:BMI < 0.512. The study included 154 females with overweight and obesity with a mean age of 33.26 ± 14.65 years and a mean BMI of 31.42 ± 4.94 kg/m2. Of the 154 female patients 31 had SO. Moreover, four of the 31 patients had T2D in the SO group compared to three of the 123 patients in the non-SO group.
In 2018, Khazem el al performed a cross-sectional controlled study on 72 adult males with overweight and obesity with a mean age of 32.79 ± 13.65 years and a mean BMI of 33.69 ± 5.84 kg/m2. In this study the authors used three different definitions proposed by Batsis et al, Levine and Crimmins, and Oh et al based on ALM:BMI and (ALM: weight) × 100% to define SO. Body composition was assessed by BIA. Based on each formula the prevalence of SO varied between 23.9% and 69.4%. However, based on the definition that was revealed to be more useful from the clinical perspective, 50 of the 72 patients had a reduced lean body mass with a prevalence of 69.4%. Moreover, three of the 50 patients had T2D in the SO group in comparison to one of the 22 patients in the non-SO group.
Finally, in 2018 Scott et al conducted a large sampled study that aimed to investigate the cross-sectional association between SO and components of metabolic syndrome in community-dwelling older men. SO was defined by the co-existence of sarcopenia as ALM/height < 7.26 kg/m2 combined with handgrip strength < 30 kg and/or low gait speed ≤ 0.8 m/s, while obesity was defined as a body fat percentage of o 30%. The study included 525 males with obesity, of whom 80 had SO, with a mean age of 80.3 ± 6.5 years and mean BMI of 27.2 ± 2.3 kg/m2 and 445 did not have SO, with a mean age of 75.9 ± 4.7 years and mean BMI of 30.7 ± 3.4 kg/m2. Furthermore, 29 of the 80 individuals in the SO group had T2D in comparison to 177 of the 445 individuals in the non-SO group.