Weightlifting and Milk: Dairy and Soy Milk Show Similar Effects on Male Hormones

Estimated reading time: 6 minutes

By Tiffany Bruno, MS, RDN, Director of Education at Switch4Good 

 

Regardless of your gender, you probably have heard consuming soy may be harmful to your hormones. Men worry that it may cause them to develop breasts, known as gynecomastia. Women are worried about increasing their breast cancer risk, even though research shows soy can be protective against breast cancer. A newly published peer-reviewed study will help eliminate your fears about interfering with your body’s natural hormones, so you can feel more confident about drinking soy milk after a workout.

How Soy Got a Bad Reputation

The rumors about soy’s consequences have been circulating for decades. This comes in part from mice being injected with megadoses of soy isoflavones and growing tumors. However, the doses they were given were far greater than any human would consume. Additionally, the mice had their thymus glands removed, which normally plays a crucial role in immunity and fighting diseases.(1) Similarly, there was a man who developed gynecomastia after consuming 3 quarts of soy milk daily— that’s 12 servings!(2) Neither of these situations replicate a normal person consuming soy products, and therefore should not create fear for humans.

The buzzword when it comes to soy is isoflavones, which are phytoestrogens that can have estrogen-like effects in some tissues. However, there are biochemical differences between them and the estrogens naturally produced by humans. It is also important to note that negligible amounts of estrogens, progesterone, and testosterone have been detected in soy–less than both animal- and plant-based foods.(3)

In contrast, dairy products supply 60-80% of ingested female sex steroids due to dairy cows being pregnant during most of the time they are producing milk for consumers.(3) If people are concerned about their hormonal health, why aren’t they discussing dairy more?

Soy Has Been Debunked for Hormones

Before continuing, it’s important to highlight that many studies show soy does not negatively affect hormones. A meta-analysis of 41 clinical studies, including nearly 2,000 men, found no effects of soy or isoflavones on testosterone, free testosterone, estradiol, or estrone levels in men.(4) There was also no evidence of gynecomastia observed in men who consumed 15 grams of soy protein daily for 3 months.(5)

Similar results were observed in both pre- and postmenopausal women. A meta-analysis of 46 studies found no effects on estrogen levels or sex hormone binding globulin concentrations from consuming soy.(6) To learn more, check out our articles that focus on men’s health and women’s health.

If these aren’t enough to convince you, keep reading to learn about new evidence.

How Hormones Affect Body Composition

Sex hormones (testosterone, progesterone, and estrogen) can influence body composition by affecting anabolic pathways and adaptations to resistance training.

• Testosterone – Exercise-induced testosterone elevations have been shown to increase muscle hypertrophy and strength development, while suppression of endogenous testosterone production lowers the anabolic response to resistance training.(7,8)
• Progesterone – Elevated progesterone levels have been shown to lower serum testosterone levels and reduce thigh muscle cross-sectional area, showing a breakdown of muscle. Higher progesterone has also been shown to promote weight gain that is mostly or entirely fat in both males and females.(9,10)
• Estrogen – Estrogen has been shown to improve muscle mass, strength, and collagen content, yet, in tendons and ligaments, estrogen can decrease stiffness, affecting performance and increasing injury rates.(11)

Dairy milk contains sex hormones, and soy milk contains molecules that are structurally similar to sex hormones. Therefore, these two beverages have the potential to influence circulating sex hormones and how muscles respond to exercise.

Gaps in Current Research

Athletes have different nutritional needs from non-athletes.(12) They typically need more calories and carbohydrates to provide energy, as well as more protein to support muscle growth. When total daily protein requirements are unmet, consuming dietary protein surrounding exercise can enhance the ability to recover from, and adapt to training. Under these circumstances, people can promote recovery after resistance training by consuming protein shortly after completing their training session.(13) Both dairy and soy milk are convenient options that can help meet these protein needs.

This new study is the first that has compared the effects of dairy and soy milk consumption, as a recovery beverage following strength training, on acute circulating sex hormones.

Addressing the Gaps in Research

The new research study just published was titled “Acute Effects of Dairy or Soy Milk on Sex Hormones Following Resistance Exercise in Males: A Randomized, Crossover Pilot Trial”. It aimed to compare the acute effects of dairy milk and soy milk consumption on sex hormone concentrations following weight training.(14)

This study included men aged 18-28. Each underwent three strength training sessions: one to establish their working weights, one followed by consuming dairy milk as a recovery beverage, and one followed by consuming soy milk as a recovery beverage. By having each participant consume both beverages in different sessions, it allowed for direct comparison. Participants also eliminated all dairy and soy products from their diets two days prior to each trial to ensure any possible effects were washed out of their systems.

Their hormones were analyzed via blood sample at seven different timepoints and up to two hours after completing their strength training session.

Results of Research

The results of this study suggest that there is no difference in circulating serum sex hormones following consumption of soy versus dairy milk after weightlifting. Soy milk consumption did not decrease circulating testosterone or increase circulating total estrogen more than dairy milk. Both types of milk can provide viable options for promoting muscle protein synthesis and muscle hypertrophy in the context of post-exercise recovery.

In layman’s terms, soy does not interfere with hormones after exercise.

We asked one of our Switch4Good allies to weigh in on the study’s results. Cass Warbeck, MD says “as a plant-based physician, I’m excited to see more research dispelling the myths surrounding soy protein. In contrast to pervasive ‘bro science’, this study demonstrates that soy milk has comparable effects to dairy milk on sex hormones, like testosterone and estrogen, post-resistance training. It’s time we move past outdated beliefs and embrace the fact that plant-based proteins, like soy, can effectively support muscle health and performance.”

Conclusion: What To Do With This Information

One of the brilliant researchers and authors of this study, David Goldman, summed it up perfectly: “We’ve known for years that soy doesn’t feminize men or wreak havoc on their hormones. Our study extends this reality to guys who just finished exercising and might try soy milk as a recovery drink because they heard it’s high in protein, and maybe they’re lactose intolerant, want to avoid prostate cancer, or stave off heart disease. They might stick to dairy milk though, because they’ve heard that soy milk tanks testosterone, increases estrogen, and might counteract their hard-earned gains. I hope our study frees these guys up to give soy milk a test drive, guys who want to be strong and built now, but are looking out for their future too.”

We know there are multiple health consequences from consuming dairy products. If you have been hesitant to switch your post-workout recovery beverage to soy, this new study should help you feel comfortable that soy will not interfere with your hormones. Instead, you can reap the many other benefits soy and soy isoflavones have on your overall health, while still improving your strength and growing your muscles.