Literature Retrieval Results and Basic Characteristics
Figure 1 shows the study selection flowchart. Through the literature search, we identified 719 citations. Using the EndNote X9 software for document management, we removed any duplicates and were left with 593 references. After excluding irrelevant reports by reviewing titles and abstracts, we then retrieved 51 full-text articles that were eligible. There were 17 articles with unrelated topics, 2 without full text, 9 conference abstracts, and 5 non-English/Chinese literatures excluded. Ultimately, 18 original articles were included, as shown in the study flowchart.
Study Characteristics and Quality Assessment
The study characteristics are presented in Table 1. We identified 18 eligible studies (N = 2538) [4, 5, 8,9,10, 13,14,15,16,17,18,19,20,21,22,23,24,25] that were published from 2011–2020. These studies included 1 RCT [8], 3 prospective observational studies [5, 10, 18], 8 retrospective observational studies [4, 9, 13, 15, 17, 19, 20, 25], and 5 cross-sectional studies [13, 14, 21,22,23]. The duration of follow-up ranges from 1 to 72 months. Four studies analyzed the influencing factors of hair loss after MBS [4, 5, 9, 10]. Two studies were published in Chinese [9, 23]; the others were available in English.
According to the Newcastle-Ottawa Scale and AHRQ, the quality of observational studies was Low or moderate. The RCT, with a score of 5 on the Jadad scale, was a high-quality study. The assessment of study quality is displayed in Table 1.
Meta-analysis of Incidence of Hair Loss After MBS
In this review, 18 articles [4, 5, 8,9,10, 13,14,15,16,17,18,19,20,21,22,23,24,25] including a total of 2538 patients were included. The overall incidence of hair loss ranged from 4.5% to 80%. Because of significant heterogeneity among these studies (p = 0.000, I2 = 98.4%) (Fig. 2), a random effects model was used to pool the results. The results showed that the pooled incidence of hair loss was 57% (95%CI, 42–71%). The Egger (p = 0.005) test showed that there was significant publication bias in the literature. This may be due to the low quality of the included studies.
Subgroup Analysis
We performed subgroup analyses by follow-up duration (≥ 12 months vs. < 12 months). When we looked at the subgroup based on follow-up duration, a significant pooled result (35%, 95% CI 33 to 37%, p<0.1) was observed for the articles with long-term follow-up (LT ≥ 12 months), but not for the studies with short-term follow-up (ST < 12 months) (58%, 95% CI 55 to 62%, p = 0.054). We found that the incidence of hair loss decreased with longer follow-up times, which decreased from58 (ST) to 35% (LT) (Fig. 3a).
In addition, we conducted a subgroup analysis based on the two most common procedures (SG vs RYGB) included in the study. When we looked at the subgroup of procedures, both the two subgroups had significant pooled results, but the incidence of hair loss was similar in both groups, with SG surgery (51%, 95% CI 48 to 54%, p< 0.1, I2 = 94.2%) and RYGB surgery (59%, 95% CI 55 to 63%, p<0.1, I2 = 92.0%) ( Fig. 3b).
Meta-analysis of Factors Influencing Hair Loss After MBS
Five studies analyzed the factors related to hair loss after MBS [4, 5, 9, 10, 23]. A total of 783 patients were included, including 419 hair loss patients and 364 controls. Because the number of included studies was less than 10, publication bias was not checked for this outcome. We did not perform subgroup analyses.
Zinc
Four studies were included in the meta-analysis of zinc [4, 5, 9, 10], with 701 patients overall (Table 2). Because of heterogeneity in included studies (p =0.05, I2 =95%) (Fig. 4), a random effects model was chosen to pool results. It showed that zinc concentration was lower in patients with hair loss after MBS (SMD, − 1.13; 95% CI, − 2.27 to 0.01, p = 0.05) with the follow-up time was 6.8 months to 15 months.
Table 2 Studies of zinc and hair loss Iron and Ferritin
For iron, a total of four studies were reviewed (Table 3). Three studies [5, 9, 10] analyzed serum iron levels. Analysis of extracted data showed that there were no significant differences in serum iron levels between patients with or without hair loss after MBS (SMD, − 1.10; 95% CI, − 2.93 to 0.73; p = 0.24). The heterogeneity was high I2 = 96% (Fig. 5a), random effect model was used.
Table 3 Studies of iron and ferritin and hair loss Two studies [4, 5] analyzed serum ferritin levels, and both showed lower levels in patients reporting hair loss after MBS (SMD, − 0.22; 95% CI, − 0.38 to − 0.05; p = 0.01). Significant heterogeneity was not found I2 = 0% (Fig. 5b), so a fixed effects model was used.
Folic Acid and Vitamin B12
Studies about the role of folic acid and vitamin B12 in hair loss after MBS are summarized in (Table 4). Two of the studies reported the outcome of folic acid levels. Because of heterogeneity among these studies (p < 0.0001, I2 = 48%), a fixed effects model was used to pool results. The result showed that folic acid levels were lower in patients with hair loss (SMD, − 0.88, 95% CI − 1.29 to − 0.46, p < 0.0001) (Fig. 6a). The overall analysis showed lower vitamin B12 levels in patients reporting hair loss (SMD, − 0.40; 95% CI, − 1.59 to 0.79; p = 0.51) (Fig. 6b). The heterogeneity was high I2 = 88%, a random effects model was used to pool results.
Table 4 Studies of folic acid and vitaminB12 and hair loss Gender and Age
Studies of gender and age in hair loss after MBS are summarized in (Table 5). Hair loss after MBS was more common in women (OR, 3.87; 95% CI, 0.59 to 17.59; p = 0.08; I2 = 85%) (Fig. 7a), and patients with hair loss were younger than controls (MD, − 2.45; 95% CI, − 4.26 to − 0.64; p = 0.008; I2 = 0) (Fig. 7b).
Table 5 Studies of age and gender and hair loss