Low zinc levels may contribute to gynocomastia in puberty

[Jinzang]

Low zinc levels are also associated with higher BMI and lower testosterone levels according to this small study:

This study aimed to determine whether there were any differences in trace element levels between adolescent boys with gynecomastia and control boys and to determine the correlations between the levels of trace elements and body mass index (BMI) and sex hormones. The pubertal gynecomastia group comprised of 41 patients. Control group comprised of 21 healthy male children. Analyses of trace element levels were performed atomic absorption spectrometry. The mean zinc level was 20% lower in gynecomastia patients, p = 0.0001. However, the mean copper and manganese levels of gynecomastia patients were not statistically different than the control group. There were significant positive correlations between plasma zinc and total testosterone levels in gynecomastia group. There was a significant negative correlation between plasma zinc levels and BMI. These results indicate that zinc deficiency might be one of the underlying factors of gynecomastia, the importance of which needs to be further elucidated.