Episode 466: Dr. Stacy Sims: Why Women Can't Fast Like Men + The Training Intensity You Should Avoid

Why do women struggle more with intermittent fasting than men? And what's the real secret to building bone density that most women are missing? In this Fitness Friday episode, exercise physiologist Dr. Stacy Sims and I break down the biological differences that make popular health trends work differently for women. We explore the concept of "polarized training"—why women need to go super hard or super easy, avoiding that moderate-intensity middle zone that can actually increase cortisol and visceral fat. We also discuss the fascinating brain differences between men and women that make fasting potentially harmful for women's hormones, sleep, and body composition. Dr. Stacy Sims is an international exercise physiologist and nutrition scientist who specializes in sex differences in training, nutrition, and environmental conditions. She's the author of "Roar" and "Next Level" and has spent decades researching how women's unique physiology requires different approaches to health and fitness. What we discuss: The polarized training approach: why moderate intensity is the enemy Minimum effective dose for women's training (hint: it's less than you think) Why jump training beats running for bone density The specific type of jumping that builds bones (it's not what you expect) Why trampolines don't count for bone building The brain differences that make fasting risky for women Why women become "anabolically resistant" at 40 and need more protein The top 3 supplements every woman needs Thank you to our sponsor: Momentous: Shop this link https://www.livemomentous.com/?srsltid=AfmBOorDUIZz3psr1Qsq_n1yE8_6JTq_QUFabOO9lH65HDEXCIoJOPLj and use code Jen for 20% off Therasage: Head over to therasage.com http://therasage.com/ and use code Be Bold for 15% off TruNiagen: Head over to truniagen.com http://truniagen.com/ and use code HUSTLE20 to get $20 off any purchase over $100. Magic Mind: Head over to www http://www/.magicmind.com http://magicmind.com//jen and use code Jen at checkout. Bio.me: Link to daily prebiotic fiber here https://bio.me/?utm_source=Jennifer, code Jennifer20 for 20% off. David: Buy 4, get the 5th free at davidprotein.com/habitsandhustle http://davidprotein.com/habitsandhustle. Find more from Dr. Stacy Sims: Website: https://www.drstacysims.com/ Instagram: https://www.instagram.com/drstacysims Find more from Jen: Website: https://www.jennifercohen.com/ Instagram: @therealjencohen Books: https://www.jennifercohen.com/books Speaking: https://www.jennifercohen.com/speaking-engagements

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Speaker 00

  Hi guys, this is Tony Robbins. You're listening to Habits and Hustle. Crush it.

• 0:06Jen Cohen?

  Hey friends, you're listening to Fitness Friday on the Habits and Hustle Podcast, where myself and my friends share quick and very actionable advice for you becoming your healthiest self. So stay tuned and let me know how you leveled up.

• 0:28Jen Cohen?

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• 1:11Jen Cohen?

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• 1:53Speaker 02

  Women underestimate recovery all of the time. So polarized means that you're staying out of that middle zone. So you can go super hard when you need to and you recover super easy. So we look at the moderate intensity stuff as it's too hard to be easy and it's too easy to be hard to invoke change. Stay out of that. You want to be hard to invoke change and you want to go easy to recover so that you can go hard again.

• 2:16Jen Cohen?

  How many times a week would you recommend someone doing this type of workout?

• 2:21Speaker 02

  A bare minimum, we see two sprint interval sessions or one sprint and one high intensity session and three lifting sessions a week. But you can combine the sprint and the lifting for one day in the gym. So you might do a lower body posterior chain work or doing hip thrusts and deadlifts. And then we finish off with some sprints on the bike and then you're done and dusted. Or maybe you do box jumps instead of sprints on the bike as your high intensity work and then you're done and dusted. So like I was saying earlier, it's about the quality of the work that you're doing rather than the volume of the work that you're doing.

• 2:57Jen Cohen?

  Why is jump training so popular? Not popular, why is jump training so important?

• 3:02Speaker 02

  When we look at how bones respond to stress, we need multidirectional stress to invoke actual bone regeneration and increasing our bone density. Jumping does that because you're landing and it's complete stress in all the different planes that go up through the skeletal system, which then causes a cascade response of I need to be stronger through the entire bone. If we look at just running, it's very uniplainer and it doesn't cause that multidirectional stress. We look at walking, it doesn't either. Strength training does, but not to the extent of jump training. So if people can't jump, strength training is going to help improve bone density, especially the heavier work that you should be doing. But just plain running doesn't do it.

• 3:47Jen Cohen?

  What's what would be considered jump training, like plyo jumps, like on a box?

• 3:52Speaker 02

  You you can do that. When we're looking specifically at building bone, it's a landing, not how we've been taught with soft knees, but absorbing the impact through our bones. We're not jumping really high. We might be on a low box and jumping off as a depth jump and landing kind of flat footed hard or doing pogo, pogo jumping where you're flat footed and absorbing the impact through your skeletal system. And it only takes 10 minutes, three times a week at the most to invoke change.

• 4:23Jen Cohen?

  So this, don't laugh, but how about just jumping on a trampoline? Because you're still going that you're still going up and down vertically.

• 4:31Speaker 02

  But you're not getting the impact from the ground. Because our body moves when it hits the ground, the ground doesn't move. Whereas the trampoline, it moves. So you're not getting the same kind of reactive force through this skeletal system.

• 4:48Jen Cohen?

  How about you're a nutrition scientist as well. So what is your take on women and fasting in perimenopause and menopause?

• 4:57Speaker 02

  If I were to use the bud words or the buzzwords of fasting, I would say you do your 12 hour overnight fast. That's what you do for fasting. But when we look at it from a hormonal response, reducing stress, improving body composition, brain health, all the things that people want with fasting. For women, we need to eat within a half an hour of waking up because we have a cortisol peak and we need to drop that peak. We also see from circadian research that fueling throughout the day improves sleep, but it also improves the feedback for increasing lean mass development and dropping body fat. So when we have a big hole of no food and what happens for the most part is women

• 5:41Speaker 02

  will start a fast and they'll try to hold their fast till noon and then they end up working out fasted. And the brain, especially the hypothalamus is like, hmm, what's happening here? There's no fuel for this exercise. I'm going to start breaking down lean mass because I need some amino acids for some fuel and I can't support really metabolically active tissue when there's no fuel coming in. So when we start looking at what's the best way to counter the body comp changes that are happening in perimenopause? Train smart, eat. Eat during the day, stop eating after dinner, so you don't have nighttime snacks, and making sure that two to three hours before you go to bed was your last meal, so that you can get into a deep reparative sleep.

• 6:26Speaker 02

  And I know sleep is fleeting for lots of people in perimenopause. So we need to work on the sleep hygiene and maybe it's adding supplements like apigenin and L-theanine, maybe it's cycling progesterone to help with sleep, so that you do get into that deep parasympathetic activation, so that your body knows that it can change body comp. Because you cannot create change without enough calories and without good sleep.

• 6:54Jen Cohen?

  Yeah, but again, another huge trend, as I'm sure you know, is this whole idea of fasting, fasting, fasting. And I, you know, I don't understand how with hormonal issues and/or just as someone who is someone who is active, how do you not eat and then also be active? If you're someone who doesn't move all day, right? Okay. And I know that, I think she was on my podcast, I think you did her podcast, and it all, you know, she goes on about like, etophagia and how it's actually really important for women to be fasting in their 40s. And this whole idea, like this is, it's actually much healthier to do it.

• 7:35Jen Cohen?

  And we went back and forth because, you know, I see, I can see how it is for men. I see how men respond to the fasting differently than how I've seen women respond.

• 7:47Speaker 02

  Absolutely. And from a physiological perspective, women have two areas in the hypothalamus that is very sensitive to nutrient density. It's, the two areas are the arc areas, and we have what we call kisspeptin neurons that get expressed. When we don't have enough food coming in, we don't have all those kisspeptin neurons being expressed, so we have a hit on our entire endocrine system. So that's not just estrogen and progesterone, it's also things like thyroid and our appetite hormones. Men have one area, so their sensitivity to nutrition density is not nearly.

• 8:23Speaker 02

  As sensitive as it is for women. And I like to scope it down to calories per kilogram fat free mass. We look, women need a bare minimum of thirty five calories per kilogram of fat free mass to be able to maintain some endocrine health. Ideally, you want to see people up to forty. For men, it's fifteen. When you start to drop below that thirty five for women, we start to see a lot of just subclinical disturbance in endocrine and sleep and body comp. For men, when it's fifteen and below, we start to see that disturbance. So there's a massive threshold difference. So when we start talking about fasting, yes, men are going to respond because their hypothalamus is not as sensitive to low calorie.

• 9:04Speaker 02

  But from a biological standpoint, women are more sensitive to low calories because we're the ones that are or were responsible for reproduction, for carrying a baby, having a proper menstrual cycle, being able to support the ongoing aspect of survival of the species. So from a biological standpoint, there are specific sex differences in the brain that people don't acknowledge when we talk about fasting and fasting protocols.

• 9:29Jen Cohen?

  And so you would recommend maybe a twelve hour window at best. And how about protein? I mean, what's your idea? Because I know, I think I also see that you're not someone who eats animal protein, right? You're, you're, you eat plant, you eat, you're not a...

• 9:44Speaker 02

  Yep, I'm primarily plant based. When I travel, because I travel so much, I'll use organic Greek yogurt and/or whey protein because it's readily available. So that would be the only kind of animal product I put in. For protein, we see that there is an age and sex difference in the way your body responds to exercise and protein. We see that when women start to hit forty onwards, we are more what's called anabolically resistant to exercise and protein. So that means that we need more protein and we need a stronger dose of resistance training to get our bodies to build and maintain lean mass. For men, that starts about fifty, fifty five.

• 10:27Speaker 02

  So when we talk about protein and protein intake, women really need to dial it up because that recommendation that is based on the bare minimum to prevent malnutrition is still circulating as the needs for people. If you're a sedentary person who's in bed all day, every day, then yeah, the recommended of point eight grams per pound. That might work, but for women and men who are active and trying to rebuild and promote that body comp, we're looking at that one to 1.1 grams per pound as a bare minimum. And that is to stay healthy, maintain our endocrine system, and keep building bone and mass.

• 11:13Jen Cohen?

  Let's quickly talk about a health issue that affects almost all of us: fiber deficiency. Did you know that 95% of people don't get enough fiber in their daily diet? I was shocked when I learned this because fiber is truly the foundation of overall wellness. It's not just about keeping our digestion smooth and regular, although of course, that's super important. But fiber also nourishes the good bacteria in our gut. It supports a balanced microbiome, helps us feel fuller for longer, which makes managing our weight even easier. And it even improves our energy by optimizing nutrient absorption and stabilizing our blood sugar.

• 11:54Jen Cohen?

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• 12:32Jen Cohen?

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• 13:00Jen Cohen?

  So I'm so surprised to hear that you are not an animal protein person because A, you're so fit. I mean, it's insanely, you're insanely fit. But I mean, just in terms of the satiation piece of it, right? Like, animal protein is, for me, is much more satiating. And plant protein, I found it harder to get enough of. Are you saying it's just equally is okay in terms of building the muscle mass? Have you, were you an animal protein person? And then you'd switched? Or what was?

• 13:32Speaker 02

  When I was 15, we took a field trip to a pig slaughterhouse down the five.

• 13:40Jen Cohen?

  That will do it.

• 13:43Speaker 02

  Yeah. So I'm well beyond 15 now. And that was the first, like, four way into it. I had issues back in the day because there was no such thing as plant-based. And so I've kind of fought my way through. But I've been plant-based for a very, very long time. And it's, you go through the whole, you have to have complete proteins at every meal, you have to have X this, X that. But it's not about that. It's about the total amount of protein you have through the day and making sure that you have all of your essential amino acids. The important part, yes, is leucine content post-exercise. And if we look at pea protein isolate, it's just on the cusp of having enough leucine. So you have a little bit of a bigger dose of the pea protein than you would of the whey. But when we're talking

• 14:23Speaker 02

  about meal and protein in a meal, if you're taking edamame, green peas, nuts, seeds, other beans, maybe some tempeh, then you're going to get your 40 or 50 grams in one meal. And it's going to be a mix of all your essential amino acids and you're golden. It's just really understanding nutrition. And I think that's one of the lacking points is the education around it.

• 14:46Jen Cohen?

  That's right. Well, because even when you said that, I'm like, well, aren't you also getting a lot more carbohydrates, a lot more fat when you're saying you're eating edamame and all these other things? It's easier to eat a piece of chicken, let's say, than to...

• 14:59Speaker 02

  Absolutely.

• 15:00Jen Cohen?

  Right? And so, and so, but you said that. What's the, what's the best sources of protein that you find for people who are not animal protein eaters?

• 15:10Speaker 02

  The big ones that I try to get people to put in are tempeh spirulina. Pea Protein Isolate, yeah, spirulina is really good in iron and protein. And so for the supplement is pea protein isolate. We look at some of the fortified almond or coconut yogurts. They can be highly fortified in protein as well. So there's lots of different options, but when we're looking at carbohydrate and fat, women are afraid to eat carbohydrate and for the most part, they don't eat enough. And if we're looking at the plant-based proteins, we're also getting a lot of fiber, which is really super important for our gut microbiome. So when we're looking at all the animal sources, yeah, they're high, high in protein, which is a great hit.

• 15:52Speaker 02

  But we also have to look at how are we keeping that gut diversity and also getting enough carbohydrate? So it's not one or the other. Ideally, it would be a mix, but for me, I've been plant-based so long. And through the years, I've tried to put in egg or fish and I just can't do it. It's just brings me right back to my time when I was 15.

• 16:11Jen Cohen?

  A pig slaughterhouse. Yeah. It's crazy. That would happen to me too. What about supplements? Right? Like gross. Well, what would, what would you say are supplements that are fundamental for women's health? Or just, are you somebody who don't believe in supplements? Cause supplements, people think is food. It's not. It's a supplement to what you're actually eating.

• 16:36Speaker 02

  Things that you're eating. Right.

• 16:37Jen Cohen?

  Yeah.

• 16:38Speaker 02

  So there, I would say the big three would be creatine monohydrate for sure, because you can't eat 22 chicken breasts in a day to get enough creatine to support brain and gut and heart health.

• 16:50Jen Cohen?

  Yeah.

• 16:51Speaker 02

  There's so much evidence about creatine being so beneficial for men and especially for women, even in pregnancy. So that is probably my number one. Omega three fatty acids, really, really important, especially for perimenopausal women who are active to help with the antioxidant capacity as well as the actual cell membrane and cellular capacity. And vitamin D three, because we live in a global community of, you know, sunscreen, hats, clothing, avoiding the sun. And we live in the, you know, I live in the very, very. Southern part of the world and we don't get a lot of sun in the wintertime.

• 17:30Speaker 02

  And vitamin D is so important for every system of the body, including things like iron and iron absorption. So if we look at vitamin D, that's, that's the third one. So those would be the top three. And then of course you can add things like your adaptogens if you want, your protein powders are good. We talk about the extremes of performance enhancement type supplements. There's no real evidence for women. Things like beet juice, where you know, beet juice became a thing a few years ago. For postmenopausal women, sweet, it works well. It helps with vasodilation, it helps improve VO2 max. But for premenopausal women, including perimenopause, it has a backwards effect. Because we have estrogen that's tightly tied to our vessels and that's

• 18:15Speaker 02

  part of the nitric oxide cycle that causes vasodilation and constriction. So if you're introducing nitrates, it interrupts that system and you end up with a disconnect in what we call orthostatic hypotension or poor blood pressure control. Kate Wickham out of, um, where did she do, she's in Copenhagen now. She did research on this, looking at the differences between premenopausal and postmenopausal women in nitrates. And saw that yes, it's beneficial for post, but not for pre. And then things like beta alanine, it may or may not have an effect for women. So it's kind of in the, there's not enough to elucidate the evidence for, to be pro women. So that's why I'm always like, okay, let's stick with the big three. And then we can do an individual basis.

• 19:00Speaker 02

  Did you test low for magnesium? Maybe you need magnesium. Are you on a big training block? And we need to look at how we're going to adapt to the heat or how we're going to adapt to altitude. There's some things that we can do in there from a supplement standpoint. But for the most part, it's those big three and then some protein. And then we can kind of pepper other things in on an individual basis.