Polystyrene nanoplastics have also been shown to disrupt luteinizing hormone levels (and LH is a driver of testosterone levels). Age-specific cutoffs for low testosterone levels were 409, 413, 359, 352, and 350 ng/dL, respectively." ref However, some studies show that testosterone levels do not decrease much after age 30 and are fairly steady into old age.ref For example, studies of the SHBG gene variants show an association with facial features such as jaw shape, which is linked to testosterone levels.ref
Each of the 12 genetic instruments described above was used as an exposure instrumental variable in our subsequent Mendelian Randomization analyses. For downstream analyses we produced genetic instruments using two approaches. Given the relatively small sizes of these replication studies, we used these data to validate genetic instruments in aggregate rather than as individual loci (Supplementary Table S28). Here, regression models were conducted on ventiles of the score, and were controlled for 10 genetic principal components, and additionally menopausal status in women (Figure ED2).
Testosterone administration has been shown to increase muscle mass and strength in a dose-dependent manner in young and older men (Bhasin et al. 2001; Bhasin et al. 2005) and in young women (Horwath et al. 2020). Other anabolic or anti-catabolic mechanisms have also been proposed (Dubois et al. 2012), all suggestive that testosterone plays an important role in muscle mass regulation. In skeletal muscle, testosterone and its metabolite, dihydrotestosterone, have a well-defined anabolic property, mainly through an increase in protein synthesis via the activation of the mammalian target of rapamycin (mTOR) pathway together with the androgen receptor (AR) signaling (Basualto-Alarcon et al. 2013; Zeng et al. 2017). Testosterone plays an integral role in the development and maintenance of male characteristics, including the development of primary and secondary sex characteristics and the maintenance of the reproductive system. Testosterone is an anabolic–androgenic steroid hormone produced mainly in Leydig cells of the testes in men and the ovary and the adrenal cortex in women.
At near confluence, cells were washed with PBS and cultured in serum-free SFM4CHO medium (Thermo Scientific HyClone, Logan, UT) for four days before the SHBG-containing medium was harvested. For the expression of SHBG protein, wild type (corresponding to the C genotype of rs6258) and rs6258 (corresponding to the T genotype of rs6258) SHBG cDNAs in the pRC/CMV expression vector were transfected into CHO cells, and G418 was used for selection of stably transfected cells. The steroid-binding properties of SHBG in diluted serum samples or tissue culture medium were determined by Scatchard analysis . In experiments evaluating SHBG binding capacity, serum SHBG concentrations were determined by two-site immunofluorometric assay (PerkinElmer Life Sciences, Turku, Finland) , or by a steroid-binding capacity assay . Detailed description of the free testosterone fraction measurements is provided in Text S1.
To further inform the role of SHBG, we additionally tested the 2 cis variants in SHBG as an instrument for SHBG. These genetic correlations were very similar to the observed phenotypic correlations (Table S15). In contrast to findings for other reproductive traits, we found no evidence for enrichment of gene expression in any brain cell type (Figure ED3). In both sexes, liver was the most enriched tissue (Figure ED3), consistent with its established role as the site of SHBG production. We additionally estimated bioavailable (free/unbound) testosterone in 382,988 individuals (Methods).
Controls were healthy unrelated Russians without any competitive sport experience. Athletes were Russian national team members who had never tested positive for doping. Athletic phenotypes (including muscle strength and power) are polygenic in nature, which implies that multiple polymorphisms influence these athletic phenotype (Guilherme and Lancha 2020; Moreland et al. 2020). Since fast-twitch fibers are required in high-energy movement tasks such as sprinting or weightlifting, their CSA is of vital importance for power athletes. Testosterone is a contributory trait in the complex nature of athletic phenotypes, and can influence athletic performance (e.g., increased neuronal activity, bone growth and hemoglobin levels) (Wood and Stanton 2012).

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