From the publication:
Eligibility criteria for the study selection were (i) observational case-control studies involving men with 25(OH)D <20 ng/mL (cases) and ≥20 ng/mL (controls) (ii) availability of mean ± standard deviation (SD) of total testosterone levels in both groups.
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Overall, at the pooled estimate of eighteen carefully selected case-control studies, vitamin D deficiency was associated with a slight, albeit just significant, decrease in circulating total testosterone levels but the result was burdened with a large between‑study heterogeneity. Noteworthy, when studies were grouped according to health-related characteristics of the populations under investigation, a significant positive association between 25(OH)D and testosterone levels, along with a noticeable decrease in heterogeneity, could only be demonstrated in studies of patients with frailty states, including men with advanced age and/or documented chronic comorbidities. This indicated that the significant association arising from the overall analysis reflected the inclusion of studies on frail series. In the sub-group of studies carried out in ‘non-frail’ populations, besides the loss of the statistical significance in the association between lower 25(OH)D and lower testosterone, a large between-study heterogeneity persisted. […] Taken together these data suggest that the claimed association between vitamin D deficiency and low testosterone could reflect nonbiological correlations due to similar distributions of shared risk factors and confounders.
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Further studies are warranted to elucidate a possible direct clinical contribution of Leydig cell dysfunction to vitamin D deficiency in men with frailty states related to chronic illnesses.