Is Soy Good for Plant-Based Athletes? The Truth About Protein, Hormones & Muscle Building

Should plant-based athletes eat soy? Does tofu lower testosterone? Is soy protein actually good for building muscle?

In this episode, sports dietitian Natalie Rizzo, MS, RD, breaks down the latest research on one of the most misunderstood foods in sports nutrition. You'll learn how soy compares to animal protein for muscle recovery, what the science really says about hormones, and how much soy athletes can safely eat.

You'll also get practical ways to add tofu, tempeh, edamame, and soy milk to your daily routine, without relying on protein powders.

In this episode you'll learn:

•  Is soy a "complete" protein? 
•  Does soy help build muscle? 
•  Does soy lower testosterone or increase estrogen? 
•  How much soy should athletes eat? 
•  The best soy foods for recovery and performance 
•  Easy ways to eat more soy every day

Apply to work with Natalie for 1-on-1 nutrition coaching! 

Have questions or want to request a show topic? DM us @greenletes

Check out Natalie's book 📕: Planted Performance

Transcript

SPEAKER_00 0:05

Is soy really the best plant-based protein for athletes? Or have you been avoiding tofu because someone told you it lowers your testosterone or messes with your hormones? As a plant-based sports dietician, this is the one of the most common questions I get from athletes I work with. Should I be eating soy? And how much? And honestly, it's a little surprising because soy is one of the most researched foods around, and the most of the research, 99% of it, is positive. So why is there all this confusion about soy? We have decades of studies looking at everything from muscle growth to hormone health to athletic performance. So today I'm gonna break down what the latest science actually says. We'll cover whether soy is actually good for muscle building, whether or not it affects your hormones, how much soy athletes should eat, and why soy might actually deserve a regular spot in your training diet. If you're new here, I'm Natalie Rizzo, I'm a sports dietitian, I'm plant-based, I run marathons, and I am here to break down all the science-based information for people who want to incorporate more plants into their training diet, or as I always like to say, fuel their fitness with plants. Let's start with why soy gets so much attention. There's a lot of plant-based proteins out there: beans, lentils, nuts, seeds, but soy is unique. You probably hear a lot about high-quality proteins, things like chicken and fish. And what we mean by high quality is that they have a lot of the essential amino acids that your body needs that they cannot make. But believe it or not, soy is one of those high-quality proteins. You may have heard that soy is a complete protein, and what that means is that it has high amounts of the nine essential amino acids that your body needs, but it can't make. Other plant-based proteins also have these essential amino acids, but they have them in lower qualities. So when we look at the research on soy comparing it to animal foods, we see that it's about 75% bioavailable as compared to things like chicken and fish. So what does that mean? It means that your body's absorbing about 75% of the protein from soy and about 90% from animal foods. So it's pretty close in comparison. And because it has those essential amino acids in there, you're getting things that you need to help with muscle building, like those branch chain amino acids. So it's not incomplete, if you've ever heard that term. It's a complete protein and it's really bioavailable by the body. But here's the thing: protein isn't just about building bigger muscles. As an athlete, every time you work out, your muscles get these tiny little tears in them that need to be repaired in order for you to recover and feel good after a workout. Recovery is what allows you to come back stronger. And especially if you're training for something intense, like a marathon, a hyrox, an Iron Man, you need to do intense workouts over and over again. Recovery is incredibly important there. So that's a huge part of why protein is important. It's not just for making your muscles grow bigger and stronger, it's for helping them stay strong as you recover. It supports adaptations from training, it helps maintain lean body mass. And if you're an endurance athlete like a runner or a cyclist, getting enough protein also helps support immune function and reduce the risks of overtraining. Because as you're putting a lot of stress on your body, you can get sick a lot because your immune system starts to break down. Protein's important to that. And soy makes that easier because it's incredibly versatile and it's inexpensive. So three to four ounces of tofu will get you eight to nine grams of protein. A cup of soy milk has eight grams of protein. A cup of animame has almost 17 grams of protein. Three to four ounces of tempeh can get you 12 grams of protein. It's not the same as eating animal foods that may have 20 grams of protein in three to four ounces, but it's similar and these foods tend to have less calories overall, so you tend to eat a higher volume of food and end up with the same amount of protein per day. Here's something a lot of people don't realize: muscle growth doesn't depend on whether your protein comes from plants or animals. It depends on whether you're eating enough high-quality protein throughout the day to support your training. Research comparing soy protein with other high-quality protein consistently shows that soy supports muscle protein synthesis and training adaptations when athletes meet their overall protein needs. Meaning, if you get enough protein for your busy lifestyle and your training, if you eat soy protein or animal-based proteins, it doesn't matter. You can still recover it well. If you're eating enough protein and training consistently, soy absolutely supports muscle strength and building. It's not a second best protein. It works just as well as animal foods. Now let's talk about the elephant in the room. Hormones. Soy contains something called isoflavones. Those are often called phytoestrogens because they have a chemical structure that is somewhat similar to the estrogen that is in a female body. That's where the confusion starts for a lot of people. Estrogen is a hormone that fluctuates through a woman's life based on their menstrual cycle, based on menopause. Because of that, people hear the word estrogen and immediately think that soy must increase their estrogen or lower their testosterone. But that's not how biology works. Isoflavones are much weaker than human estrogen. Women have estrogen receptors in the body, but phytoestrogens interact with those receptors differently than regular estrogen. And researchers have spent decades studying whether eating soy actually changes hormone levels. And the answer is it does not. Large reviews of clinical trials have found that soy does not lower testosterone in men. So any man who thinks they are gonna get, quote, man boobs from eating soy, it's just not true. It has not been proven by science. It doesn't raise estrogen, it doesn't negatively affect fertility, and it doesn't disrupt hormone balance in healthy adults. So if you've been skipping tofu because you are worried about it making your hormones out of whack, you can clear that worry from your mind no matter what you see on social media or somewhere else. And here's the other part that gets overlooked. Soy isn't just a protein, it's an entire package of nutrients that athletes need. And a lot of times it's a package that costs less than $3. It provides healthy unsaturated fats, which help keep you full. It has fiber, which aids in digestion and heart health. It has iron, good for healthy blood flow. Potassium, important for blood pressure control, magnesium, also beneficial for your heart. And then soy, milks, and other soy products are fortified with calcium and vitamin D, which are beneficial for bone health. So there's a lot of nutrients in a small package. And then the isoflavones, a lot of researchers are looking into the antioxidant and anti-inflammatory properties of these compounds. Exercise naturally creates inflammation, and your body needs to get past that inflammation to recover. These isoflavones in soy can help the body reduce that inflammation and recover faster. So athletes need these antioxidants from whole foods to support their recovery. Hopefully, I've convinced you to add more soy to your diet, but you may be thinking how much soy is appropriate. The research doesn't show a certain amount that athletes need. There's no certain number. For most people, eating one to three servings of whole soy foods every single day fits comfortably into their healthy eating pattern. So for instance, a serving may be a cup of soy milk, three to four ounces of tofu or tempeh, half a cup of edamame. Personally, I have multiple servings of soy every single day. If you're training for something like a marathon or a Hyrux or an Iron Man where you are logging a lot of miles, you may need more calories and more protein than someone else who's not. So it totally depends on you and your training and your lifestyle and how frequently you can get these foods into your diet. The research doesn't show that eating soy several times a day is harmful. The bigger goal though is variety. You don't want to just rely on one sort of plant-based protein. Whether it is soy, beans, lentils, nuts, seeds, having a variety of foods in there gets you all the vitamins and minerals that you need, as well as fiber, different types of proteins, different types of amino acids to make sure that you are staying healthy during your training. If you're looking for some practical ways to eat soy, here are some of my favorites. Every morning after my run, long run, short one, whatever it is, or straight training, I tend to have a smoothie where I throw soy milk in there. And sometimes you can even put a soft tofu, like silken tofu, in there to add more protein. You can bake tofu in the oven, in the air fryer. You can also scramble it and have it in a breakfast bowl with some potatoes and stir-fried peppers. We have a recipe on greenleats.com that has a great scrambled tofu breakfast bowl. You can snack on steamed edamame after a workout. Edamame is one of the easiest things to keep in your freezer and it's delicious. There's simple ways to increase your protein intake without relying on protein powders, and soy is one of the best ways. So what's the bottom line? What do I want you to remember? I want you to remember that soy isn't just safe for plant-based athletes, it's one of the best foods you can eat. It has all the essential amino acids in a large variety, it supports muscle recovery, it helps you meet your daily protein goals, and it also provides important vitamins and minerals that athletes need. And decades of research show that it doesn't lower your testosterone or disrupt your hormones. If you've been avoiding soy because of something you heard online, hopefully you stop now and start incorporating more of it into your diet. If you found this episode helpful, I'd love to hear from you. What's your favorite way to eat soy? Are you a tofu person? Have you tried tempeh? Do you love edamame? Leave a comment below and make sure you rate and subscribe so we can get this info out to more people.