Quality assessment of included studies
| Reference | Selection of participants | Adequate description of study population | Validated method for ascertaining exposure | Validated method confirm outcome | Analysis and controls for confounders | Sample size calculation | Analytical methods appropriate |
| Burton et al33 | Low risk | High risk | Unclear | Low risk | Unclear | Low risk | Low risk |
| Ferrari et al34 | Low risk | Low risk | Low risk | Low risk | High risk | High risk | Low risk |
| Gili et al35 | High risk | High risk | Low risk | Low risk | High risk | Unclear | Low risk |
| Haas et al36 | Low risk | Low risk | Low risk | Low risk | Unclear | High risk | Low risk |
| Hansen et al37 | Low risk | Low risk | Low risk | Low risk | Low risk | Unclear | Low risk |
| Jacob et al38 | Low risk | High risk | Low risk | High risk | Unclear | Unclear | Low risk |
| Jyväsjärvi et al39 | Low risk | Low risk | Low risk | Low risk | Low risk | Unclear | Low risk |
| Jyväsjärvi et al40 | High risk | Low risk | High risk | Low risk | High risk | Unclear | Low risk |
| Karlsson et al41 | Low risk | Low risk | Low risk | Low risk | Unclear | High risk | Low risk |
| Katon et al42 | Low risk | Low risk | Low risk | Low risk | Unclear | High risk | Low risk |
| Little et al43 | Low risk | Low risk | Low risk | Low risk | Low risk | Low risk | Low risk |
| McGorm et al44 | Low risk | Low risk | Low risk | Low risk | High risk | Unclear | Low risk |
| Miranda et al45 | Low risk | High risk | Low risk | High risk | High risk | High risk | Low risk |
| Norton et al46 | Low risk | Low risk | Low risk | Low risk | Low risk | High risk | Low risk |
| Patel et al47 | Low risk | Unclear | Low risk | Low risk | High risk | High risk | Low risk |
| Portegijs et al48 | Low risk | Low risk | Low risk | Low risk | Unclear | Unclear | Low risk |
| Reid et al49 | Low risk | Low risk | Low risk | Low risk | Unclear | Unclear | Low risk |
| Schmitz and Kruse50 | Low risk | Low risk | Low risk | Low risk | Low risk | Unclear | Low risk |
| Schneider et al51 | Low risk | Unclear | Low risk | Low risk | Low risk | Unclear | Low risk |
| Smith et al52 | Low risk | Unclear | Low risk | Low risk | High risk | Unclear | Low risk |
| Smith et al53 | Low risk | Unclear | Low risk | Low risk | Unclear | High risk | Low risk |
| Smits et al54 | Low risk | Low risk | Low risk | Low risk | Unclear | Unclear | Low risk |
| Taylor et al55 | Low risk | Unclear | Unclear | Low risk | High risk | High risk | Low risk |
| van den Bussche56 | Low risk | Low risk | Low risk | Low risk | Low risk | High risk | Low risk |
| Williams et al57 | Low risk | Low risk | Low risk | Low risk | Unclear | High risk | Low risk |
‘Low risk’ indicates low risk of bias in that domain, ‘high risk’ indicates high risk of bias and ‘unclear’ indicates where risk of bias was unclear.