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Dietary Zinc Intake and Male Reproductive Function in Young Men
Guilherme J. Wood, MD1, Joseph S. Gabrielsen, MD, PhD2, Audrey J. Gaskins, ScD3, Shanna H. Swan, PhD4, Jaime Mendiola, PhD5, Niels Jorgensen, MD, PhD6, Jorge E. Chavarro, MD, ScD3, Cigdem Tanrikut, MD2.
1University of Sao Paolo Medical School, Sao Paolo, Brazil, 2Massachusetts General Hospital, Boston, MA, USA, 3Harvard School of Public Health, Boston, MA, USA, 4Mount Sinai School of Medicine, New York, NY, USA, 5University of Murcia, Murcia, Spain, 6University of Copenhagen, Copenhagen, Denmark.
BACKGROUND: Zinc is an essential mineral obtained solely from dietary intake and is involved in the activity of over 200 enzymes in the human body. Zinc is particularly important for spermatogenesis: it facilitates DNA replication, stabilizes chromatin, and regulates steroid receptor expression. Zinc concentration is very high in prostatic secretions and seminal plasma, and alterations in these concentrations have been associated with changes in semen parameters in men. The effect of dietary zinc intake on global testicular function, however, has not been reported in humans. We, therefore, sought to examine whether zinc intake is associated with reproductive hormones and semen parameters in young men.
METHODS: Healthy male volunteers ages 18-22 (n=189) recruited in 2009-2010 completed a validated 131-item food frequency questionnaire and provided serum and semen samples. Nutrient intakes were estimated by summing the nutrient contribution of all food and supplement items reported in the questionnaire. Serum reproductive hormones were measured using standard assays. Semen analyses were performed according to WHO guidelines. Linear regression was used to analyze the relation between zinc intake (in quartiles) and reproductive hormones and semen quality parameters adjusting for total calorie intake, body mass index, smoking status, physical activity, meat and dairy intake, overall dietary patterns, abstinence time (for semen parameters only), and time of blood sample collection (for serum samples only).
RESULTS: Zinc intake was inversely associated with serum concentrations of sex hormone binding globulin (SHBG) (Table 1). On average, men in the highest quartile of zinc intake had SHBG levels that were 32% lower than men in the lowest quartile of intake (26.3 vs. 34.4 nmol/L, p for comparison=0.03). Men in the highest quartile of zinc intake also had significantly lower concentrations of inhibin B and total testosterone in calorie-adjusted analyses compared to men in the lowest quartile of intake; however after multivariate adjustment, these associations did not reach statistical significance (p for trend across quartiles=0.07 and 0.06, respectively). Zinc intake was not significantly associated with other reproductive hormones or semen parameters (Table 2).
CONCLUSIONS: Higher dietary zinc intake was associated with lower serum levels of SHBG and possibly lower levels of inhibin B and testosterone in healthy, young men. Zinc intake was not associated with any of the examined semen quality parameters. The testosterone findings should be interpreted with caution as we did not observe any association between zinc intake and calculated free testosterone, suggesting that differences in total testosterone could be solely attributable to the differences in SHBG. The inverse associations we observed between zinc intake and serum reproductive hormones may reflect altered function of zinc-containing enzymes involved in the regulation of steroid receptors and oxidative stress.
Table 1: Association between zinc consumption and hormone levels (energy adjusted value (95% CI)).
|Zinc Intake (mean mg/d(SD))||9.64 (2.34)||15.42 (1.11)||20.64 (2.15)||37.26 (16.21)|
|FSH (IU/L)||2.76 (2.26-3.27)||2.65 (2.23-3.08)||2.73 (2.30-3.16)||3.31 (2.82-3.81)||0.080|
|Inhibin B (pg/mL)||211.4 (190.1-232.7)||197.1 (179.0-215.2)||195.3 (177.3-213.4)||176.0 (155.1-196.9)||0.041|
|LH (IU/L)||3.92 (3.43-4.42)||3.51 (3.09-3.92)||3.67 (3.25-4.08)||3.73 (3.25-4.22)||0.937|
|Testosterone (nmol/L)||22.3 (19.9-24.7)||20.5 (18.5-22.5)||20.7 (18.7-22.7)||18.1 (15.7-20.4)||0.029|
|SHBG (nmol/L)||34.4 (30.3-38.5)||30.9 (27.4-34.3)||30.3 (26.8-33.7)||26.0 (22.0-30.0)||0.012|
|Free testosterone (nmol/L)||0.5 (0.4-0.5)||0.5 (0.4-0.5)||0.5 (0.4-0.5)||0.5 (0.4-0.5)||0.333|
|Estradiol (pmol/L)||94.7 (85.9-103.4)||95.3 (87.8-102.7)||91.2 (83.8-98.6)||83.9 (75.3-92.4)||0.069|
Table 2: Zinc intake and semen quality parameters (energy adjusted mean (95% CI))
|Zinc Intake (mean mg/d (SD))||9.64 (2.34)||15.42 (1.11)||20.64 (2.15)||37.26 (16.21)|
|Abstinence time (hr (SD))||102.23 (106.3)||90.42 (68.37)||82.28 (38.99)||92.77 (65.65)||0.834|
|Concentration (106/ml)||71.7 (51.6-91.7)||66.6 (49.6-83.6)||76.5 (59.5-93.4)||56.0 (36.4-75.6)||0.265|
|Total Motility (% motile A+B+C)||62.0 (57.3-66.8)||63.0 (59.0-67.1)||64.9 (60.8-68.9)||61.1 (56.4-65.7)||0.598|
|Progressive Motility (% motile A+B)||57.3 (52.3-62.4)||58.7 (54.4-63.0)||60.6 (56.3-64.9)||56.9 (51.9-61.9)||0.698|
|Normal Morphology (%)||8.2 (6.6-9.8)||8.5 (7.1-9.8)||9.0 (7.6-10.3)||8.8 (7.3-10.4)||0.666|
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