fasting. Oh yeah. Intermittent fasting. Yep. We need to distinguish between the two, of course. Perhaps the most common question I get as it relates to males versus females is, is intermittent fasting or time-restricted feeding as it's sometimes called, an eight-hour feeding window, a six-hour feeding window, a 10-hour feeding window.

Is that something that perhaps differs in terms of its impact and how well it works for men versus women? Yeah. That's a short answer. Great. Yeah, yeah. So I'll put some parameters around it, right? So if we talk about intermittent fasting, that's where you have like the 20-hour non-feeding window or you're holding a fast until noon or after.

And then we have time-restricted eating and that's the fancy way of saying normal eating where you're having breakfast and then you stop eating after or you don't have anything after dinner, right? So you're eating with your circadian rhythm during the day. If we look at intermittent fasting where you're holding the fast up till noon or you're having days of really low calorie restriction, we see in active women it's very detrimental unless you have PCOS or you have some other subclinical issue.

And the reason for that is we as women have more oxidative fibers. So we hear about all the things about fasting to improve our metabolic flexibility, to improve telomere length, to improve parasympathetic activation. But by the nature of women having more oxidative fibers, we are already metabolically more flexible than men.

Interesting. I didn't know that. Could you elaborate on more oxidative fibers, what that is and how it relates to metabolic flexibility? Sure, sure. So oxidative fibers are muscle fibers that are more aerobic capacity. So those are the ones that you can go long and slow for a very long period of time because it uses a lot of free fatty acids.

You need a little bit of glucose in order to activate those free fatty acids. So when we look, when a woman starts to exercise, she goes through blood glucose first and then gets into free fatty acid use, she doesn't tap so much into liver muscle glycogen, which is, I think, another misconception that happens.

So when we're talking about fasting or fasted workouts, trying to improve that metabolic flexibility, it increases stress on the woman. And so when we're talking about overall stress, we're talking about cortisol increase and they can't hit intensities high enough with no fuel to be able to invoke the post-exercise responses of growth hormone and testosterone, which then drop cortisol.

So from an overall stress perspective, that fasted workout and holding that fast for a long period of time increases cortisol. But then when we look from like a hypothalamic point of view and we're looking at how the brain reads it, we know that there's one area of kispeptin neurons in the brain for men, but there are two for women.

So the two areas are distinct where one controls appetite and luteinizing hormone and the other one is looking at estrogen and thyroid. So if you start having an exercise stress or a daily stress of getting up and going on with your day without fuel, you perturb those kispeptin neurons and downregulate them.

And so when you start downregulating them, we see that after four days, you have a dysregulation of thyroid. We have a change in our luteinizing hormone pulse, which is really important to maintain endocrine function. And we'll hear this, oh, I've been fasting for so many years and it does great for me.

But the other side of the question is, well, how much better would you be if you were to actually pay attention to your circadian rhythm and fuel according to the stress at hand and knowing that you're going to garner less stress that way? And if we're really tying in nutrition according to that profile, instead of following a fast, we see better brain improvements as well.

We see more cognitive function. We see less thyroid dysfunction. And overall, a woman does much better when we're not in that fasted state. Then when you look at population research that's coming out now, they're showing in both men and women who hold their fast till noon and then have an eating window from noon to maybe 6pm have more obesogenic outcomes than people who break their fast at 8 and finished their eating window by 4 or 5pm.

So it's coming back to the chronobiology of we need to eat when our body is under stress and needs it. Unless we have a specific issue like obesity, inactivity, PCOS, or other metabolic conditions, then we can look at using fasting as a strategic intervention to help with those modalities.

Super interesting. Two questions. Is there a protective effect of starting the eating window, and here I'm asking for both men and women, starting the eating window at say 11am or noon and ending it a little bit later? So not a six hour eating window or seven hour eating window, but extending that to 8 or 9pm.

Under those conditions, do you still see the obesogenic effect? Yes, because we're looking at the way cortisol responds. We know cortisol has lots of fluctuations throughout the day, and it peaks about half an hour after you wake up, right? So if you're having that cortisol peak half an hour after you wake up, but you're not eating, then that is that higher baseline sympathetic drive for women.

For men, it's not the same. So when we're looking at that obesogenic outcome, the actual timing hasn't been tested yet to see how can we expand or contract that eating window for men. But for women, because of that cortisol peak, that right after waking up, women tend to be already sympathetically driven.

So then they walk around more tired, but wired, and have a really, really difficult time accessing any kind of parasympathetic responses down the way. Where if you have something really small, where you're bringing blood sugar up, then it's signaling to the hypothalamus, hey, yeah, there's some nutrition on board, then we can start our day.

So again, it has to look at that circadian rhythm and those hormone fluxes, which people don't really either understand or talk about, because all of our hormones flux through the day. And so you have to look at where's the peak of cortisol, how does estrogen flux, how does luteinizing hormone flux, progesterone, all of these things that have this tight interplay.

And the more we're doing the hormone research, and the more we're understanding these perturbations, and how important it is to fuel for it to stay out of any kind of low energy availability stance. It sounds like intermittent fasting or time-restricted feeding, unless it's very well aligned to the circadian rhythm, is not going to be advantageous for women.

That's what I'm hearing. I'm also hearing that if a woman trains while fasted, so in the non-feeding window, so wakes up, maybe has some hydration and trains, that's going to further exacerbate the stress response in a way that's not going to be good. Exactly. And I have to imagine that if she also is drinking caffeine in order to do that training, because caffeine is a stimulant of the sympathetic arm of the autonomic nervous system, that it will further exacerbate all these issues.

So this is an eye-opener for me, because I've had female training partners for years. I don't eat until 11 a.m. I like to hydrate and caffeinate before I train in the morning, and then I like to eat starting around noon. Several of them have hopped on that schedule with me.

Some of them eat breakfast first, some of them don't. They do as they choose, of course. But now I'm thinking that's probably the worst way to go. And it gets worse as you get older, because if we're seeing as women are getting into perimenopause, which is in their 40s, and we have more fluctuation of those hormones and an increase in baseline cortisol anyway, then when you look at fasted training, it increases that cortisol drive and that sympathetic drive.

And because it's a point where you really need to polarize your training to get any kind of body composition change, not having any fuel before high intensity workout puts them in moderate intensity. They just can't hit the intensities they need to. Same with resistance training. Like you go in and a lot of women are now working on sessional RPE or rating perceived exertion, where you go in and say, okay, we need you to hit an eight on the squat.

So you have two reps in reserve and a sessional RPE of an eight. Well, if they're not fueled, then we're seeing trends that they're missing around two to 5% of that top load. So they're not really lifting in that zone that they need to be in. And they're not really lifting in that zone that they need to be in.

So they're not really lifting in it. And they're not really lifting in that zone. So they're gonna be lifting in that zone. So we're going to be lifting in that zone. So we're going to be lifting in that zone. So we're going to be lifting in that zone. So we're going to be lifting in that zone.

So we're going to be lifting in that zone. So we're