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

Episode Date: July 11, 2025

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 and use code Jen for 20% off Therasage: Head over to therasage.com and use code Be Bold for 15% off  TruNiagen: Head over to truniagen.com and use code HUSTLE20 to get $20 off any purchase over $100. Magic Mind: Head over to www.magicmind.com/jen and use code Jen at checkout. Bio.me: Link to daily prebiotic fiber here, code Jennifer20 for 20% off.  David: Buy 4, get the 5th free at 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

Hi guys, it's Tony Robbins. You're listening to Habits and Hustle. Crush it! 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. One of them? Quality. And when it comes to supplements designed for high performers, nobody does it better than Momentus. Momentus goes all in on NSF certification, which means every single batch is tested for heavy metals, harmful additives, and label accuracy. And that's why they're trusted by all 32 NFL teams and top collegiate sports dieticians across the country. Here's the thing, they don't sell every supplement under the sun because they believe

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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. How many times a week would you recommend someone doing this type of workout? 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 in dust dead. 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. Why is jump training so popular? Not popular, why is jump training so important? When we look at how bones respond to stress, we need multi-directional 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 uniplanar 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. What would be considered jump training,

like plyo jumps, like on a box? 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 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. So don't laugh, but how about just jumping on a trampoline?

Because you're still going up and down vertically. 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 the skeletal system. How about your nutrition scientists as well? So what is your take on women and fasting in perimetapause and metapause?

If I were to use 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 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, 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. 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. 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 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, autophagy 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. 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. Absolutely. And from a physiological perspective, women have two areas in the hypothalamus that is very sensitive to nutrient density.

The two areas are the arc areas and we have what we call chispeptin neurons that get expressed. When we don't have enough food coming in, we don't have all those chispeptin 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 as sensitive as it is for women. And I'd like to scope it down to calories per kilogram fat-free mass. When we look, women need a bare minimum of 35 calories per kilogram of fat-free mass to be able to maintain some endocrine health. Ideally, you want to see people up to 40 for minutes 15. When you start to drop below that 35 for women, we start to see a lot of

subclinical disturbance in endocrine and sleep and body comp. For men, when it's 15 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. But from a biological standpoint, women are more sensitive to no 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. And so you would recommend maybe a 12 hour window at best. And I have a 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'rebased? 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 40 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 50-55. 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 0.8 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. 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.

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and support your digestion and gut health every single day. And it fits effortlessly into your routine. Just mix it in the morning smoothie or tea or coffee or afternoon snack, and you're good to go. I love that it's so easy to prepare and you can incorporate with literally every busy lifestyle. So if you want to make getting fiber easy, visit BioMe. That's B-I-O-M-E dot com and enter code Jennifer20 for 20% off your first order of daily prebiotic fiber. That's biome.com code Jennifer20 for 20% off. Grab it today. 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, for me, is much more satiating, and plant protein, I found it harder to get enough of. Are you saying it's just equally as okay in terms of building the muscle mass? Were you an animal protein person and you switched or what was? When I was 15, we took a field trip to a pig slaughterhouse down the five.

That will do it. Yeah. So I'm well beyond 15 now. And that was the first 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 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 with whey. But when we're talking about meal and protein in a meal, if you're taking adenomy, 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. 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…

Absolutely. Right? And so, but you said that, what's the best sources of protein that you find for people who are not animal protein eaters? 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. 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. At the pig slaughterhouse. Yeah. Yeah.

It's crazy. I know it happened to me too. What about supplements? Right? Like, gross. What would you say are supplements that are fundamental for women's health? Or are you somebody who don't believe in supplements? Because supplements people think is food. It's not.

It's a supplement to what you're actually eating. To the things that you're eating, right. 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. 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-3 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 D3, because we live in a global community of 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 winter time. 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 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 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 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, where did she do it? 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 it 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. 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.