Limitations
This study had numerous dietary restrictions that may not be fully representative of a person’s true diet outside of a clinical research setting. However, these restrictions also reduced potential confounding. This study had a relatively small sample size with all subjects being postmenopausal women, and the soy and casein were provided as protein powders that required the intake of large volumes of water on a daily basis and led to equal levels of withdrawals from each group. Nevertheless, our study was able to recruit enough participants to show statistically significant differences. Future studies should consider intervening with isoflavone-rich products that are less burdensome, such as traditional soy foods like tofu or isoflavone supplements. Future studies should also investigate how soy isoflavones in isolation may influence skin photoaging, whereas the SPII utilized in our study had a small proportion of added ingredients such as niacin and vitamin A, which may have contributed to dermatological effects of their own. This study was limited to Fitzpatrick skin types I, II, and III, and the results reported here cannot be generalized to all Fitzpatrick skin types. Similarly, because this study was limited to postmenopausal women, the results reported here cannot be extended to premenopausal or perimenopausal women. Because this study intervened with SPII with the active component of soy isoflavones, other isoflavone-rich soy foods such as tofu, soymilk, edamame, and tempeh may also exert similar benefits and should be the focus of future studies. 