The Truth About Fasting for Women | Dr. Stacy Sims & Dr. Andrew Huberman

00:00:00.000 | fasting. Oh yeah. Intermittent fasting. Yep. We need to distinguish between the two, of course.

00:00:08.280 | Perhaps the most common question I get as it relates to males versus females is,

00:00:15.280 | is intermittent fasting or time-restricted feeding as it's sometimes called,

00:00:20.480 | an eight-hour feeding window, a six-hour feeding window, a 10-hour feeding window.

00:00:24.220 | Is that something that perhaps differs in terms of its impact and how well it works for men versus

00:00:35.680 | women? Yeah. That's a short answer. Great. Yeah, yeah. So I'll put some parameters around it,

00:00:42.480 | right? So if we talk about intermittent fasting, that's where you have like the 20-hour non-feeding

00:00:47.860 | window or you're holding a fast until noon or after. And then we have time-restricted eating and

00:00:54.760 | that's the fancy way of saying normal eating where you're having breakfast and then you stop eating

00:00:59.200 | after or you don't have anything after dinner, right? So you're eating with your circadian rhythm during

00:01:03.940 | the day. If we look at intermittent fasting where you're holding the fast up till noon or you're

00:01:09.560 | having days of really low calorie restriction, we see in active women it's very detrimental

00:01:15.720 | unless you have PCOS or you have some other subclinical issue. And the reason for that is

00:01:24.360 | we as women have more oxidative fibers. So we hear about all the things about fasting to improve our

00:01:32.200 | metabolic flexibility, to improve telomere length, to improve parasympathetic activation. But by the

00:01:38.720 | nature of women having more oxidative fibers, we are already metabolically more flexible than men.

00:01:45.240 | Interesting. I didn't know that. Could you elaborate on more oxidative fibers, what that is and how it

00:01:52.040 | relates to metabolic flexibility?

00:01:53.300 | Sure, sure. So oxidative fibers are muscle fibers that are more aerobic capacity. So those are the

00:01:58.880 | ones that you can go long and slow for a very long period of time because it uses a lot of free fatty

00:02:04.640 | acids. You need a little bit of glucose in order to activate those free fatty acids. So when we look,

00:02:10.220 | when a woman starts to exercise, she goes through blood glucose first and then gets into free fatty

00:02:14.700 | acid use, she doesn't tap so much into liver muscle glycogen, which is, I think, another misconception

00:02:19.860 | that happens. So when we're talking about fasting or fasted workouts, trying to improve that metabolic

00:02:25.300 | flexibility, it increases stress on the woman. And so when we're talking about overall stress, we're talking

00:02:31.200 | about cortisol increase and they can't hit intensities high enough with no fuel to be able to invoke the post-exercise

00:02:40.120 | responses of growth hormone and testosterone, which then drop cortisol. So from an overall stress perspective, that

00:02:47.280 | fasted workout and holding that fast for a long period of time increases cortisol. But then when we look from like a

00:02:55.460 | hypothalamic point of view and we're looking at how the brain reads it, we know that there's one area of

00:03:00.300 | kispeptin neurons in the brain for men, but there are two for women. So the two areas are distinct where one controls

00:03:07.340 | appetite and luteinizing hormone and the other one is looking at estrogen and thyroid. So if you start having an exercise

00:03:15.480 | stress or a daily stress of getting up and going on with your day without fuel, you perturb those kispeptin neurons and

00:03:22.900 | downregulate them. And so when you start downregulating them, we see that after four days, you

00:03:27.780 | have a dysregulation of thyroid. We have a change in our luteinizing hormone pulse, which is really important

00:03:34.040 | to maintain endocrine function. And we'll hear this, oh, I've been fasting for so many years and it does

00:03:40.140 | great for me. But the other side of the question is, well, how much better would you be if you were to

00:03:46.160 | actually pay attention to your circadian rhythm and fuel according to the stress at hand

00:03:51.260 | and knowing that you're going to garner less stress that way? And if we're really tying in nutrition

00:03:57.720 | according to that profile, instead of following a fast, we see better brain improvements as well.

00:04:05.220 | We see more cognitive function. We see less thyroid dysfunction. And overall, a woman does much better

00:04:12.080 | when we're not in that fasted state. Then when you look at population research that's coming out now,

00:04:18.260 | they're showing in both men and women who hold their fast till noon and then have an eating window from

00:04:23.900 | noon to maybe 6pm have more obesogenic outcomes than people who break their fast at 8 and finished

00:04:31.080 | their eating window by 4 or 5pm. So it's coming back to the chronobiology of we need to eat when our body

00:04:37.660 | is under stress and needs it. Unless we have a specific issue like obesity, inactivity, PCOS,

00:04:46.820 | or other metabolic conditions, then we can look at using fasting as a strategic intervention to help

00:04:53.720 | with those modalities. Super interesting. Two questions. Is there a protective effect of starting the eating

00:05:02.040 | window, and here I'm asking for both men and women, starting the eating window at say 11am or noon and

00:05:08.980 | ending it a little bit later? So not a six hour eating window or seven hour eating window, but extending

00:05:14.260 | that to 8 or 9pm. Under those conditions, do you still see the obesogenic effect?

00:05:19.920 | Yes, because we're looking at the way cortisol responds. We know cortisol has lots of fluctuations

00:05:26.020 | throughout the day, and it peaks about half an hour after you wake up, right? So if you're having that

00:05:31.460 | cortisol peak half an hour after you wake up, but you're not eating, then that is that higher baseline

00:05:37.600 | sympathetic drive for women. For men, it's not the same. So when we're looking at that obesogenic outcome,

00:05:43.860 | the actual timing hasn't been tested yet to see how can we expand or contract that eating window for men.

00:05:52.200 | But for women, because of that cortisol peak, that right after waking up, women tend to be already

00:05:59.620 | sympathetically driven. So then they walk around more tired, but wired, and have a really, really

00:06:05.420 | difficult time accessing any kind of parasympathetic responses down the way. Where if you have something

00:06:11.320 | really small, where you're bringing blood sugar up, then it's signaling to the hypothalamus, hey, yeah,

00:06:15.540 | there's some nutrition on board, then we can start our day. So again, it has to look at that circadian

00:06:20.940 | rhythm and those hormone fluxes, which people don't really either understand or talk about,

00:06:25.900 | because all of our hormones flux through the day. And so you have to look at where's the peak of

00:06:29.680 | cortisol, how does estrogen flux, how does luteinizing hormone flux, progesterone, all of these things that

00:06:35.520 | have this tight interplay. And the more we're doing the hormone research, and the more we're

00:06:40.100 | understanding these perturbations, and how important it is to fuel for it to stay out of any kind of low

00:06:45.840 | energy availability stance. It sounds like intermittent fasting or time-restricted feeding,

00:06:52.360 | unless it's very well aligned to the circadian rhythm, is not going to be advantageous for women.

00:06:58.420 | That's what I'm hearing. I'm also hearing that if a woman trains while fasted, so in the non-feeding

00:07:04.660 | window, so wakes up, maybe has some hydration and trains, that's going to further exacerbate the

00:07:11.400 | stress response in a way that's not going to be good. Exactly.

00:07:14.600 | And I have to imagine that if she also is drinking caffeine in order to do that training, because

00:07:23.600 | caffeine is a stimulant of the sympathetic arm of the autonomic nervous system, that it will

00:07:28.240 | further exacerbate all these issues. So this is an eye-opener for me, because I've had female

00:07:36.340 | training partners for years. I don't eat until 11 a.m. I like to hydrate and caffeinate before I train

00:07:42.740 | in the morning, and then I like to eat starting around noon. Several of them have hopped on that

00:07:48.240 | schedule with me. Some of them eat breakfast first, some of them don't. They do as they choose,

00:07:52.680 | of course. But now I'm thinking that's probably the worst way to go.

00:07:59.100 | And it gets worse as you get older, because if we're seeing as women are getting into perimenopause,

00:08:04.220 | which is in their 40s, and we have more fluctuation of those hormones and an increase in baseline

00:08:10.640 | cortisol anyway, then when you look at fasted training, it increases that cortisol drive and

00:08:17.520 | that sympathetic drive. And because it's a point where you really need to polarize your training

00:08:23.740 | to get any kind of body composition change, not having any fuel before high intensity workout puts

00:08:30.140 | them in moderate intensity. They just can't hit the intensities they need to. Same with resistance

00:08:34.840 | training. Like you go in and a lot of women are now working on sessional RPE or rating perceived

00:08:40.240 | exertion, where you go in and say, okay, we need you to hit an eight on the squat. So you have two reps

00:08:46.140 | in reserve and a sessional RPE of an eight. Well, if they're not fueled, then we're seeing trends that

00:08:52.600 | they're missing around two to 5% of that top load. So they're not really lifting in that zone that they

00:08:58.800 | need to be in.

00:08:59.460 | And they're not really lifting in that zone that they need to be in. So they're not really lifting in it.

00:09:01.580 | And they're not really lifting in that zone. So they're gonna be lifting in that zone. So we're

00:09:03.520 | going to be lifting in that zone. So we're going to be lifting in that zone. So we're going to be lifting

00:09:05.840 | 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

The Truth About Fasting for Women | Dr. Stacy Sims & Dr. Andrew Huberman

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