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The Dose Transcript for September 12, 2024

Host: Dr. Brian Goldman

BRIAN GOLDMAN: Hi, I'm Dr. Brian Goldman. Welcome to The Dose. As our planet gets hotter year by year, we're getting a glimpse [at] just how hot temperatures can affect our health. Last week on our sister show 'White Coat, Black Art,' we spoke to an ironworker apprentice named Britnee Miazek, whose health was affected by extreme heat.

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BRITNEE MIAZEK: I was working there for three months up on top of the coke ovens, and my body stopped producing my monthly menstruation cycle.

BRIAN GOLDMAN: That show was part of a CBC special project called 'Overheated: Stories on Extreme Heat and How It's Affecting Us.' So this week we're asking what do we know about how women's bodies deal with heat? Hi, Amita. Welcome to The Dose.

DR. AMITA MAHAJAN: Hi, Brian. Thanks for having me.

BRIAN GOLDMAN: You know, as an emergency physician, I know a lot about the immediate effects of extreme hot weather on our bodies. You know, heat exhaustion, heat stroke. I know almost nothing on the health risk of chronic exposure to heat. Does that surprise you?

DR. AMITA MAHAJAN: I don't think it surprises me. I feel like you should know mainly about the immediate effects compared to some of the long-term effects. And I don't think we have actually spent much time as a medical community researching this, especially when it comes to women's health and kind of the impact heat has on female reproductive hormones.

BRIAN GOLDMAN: And that is why we want you on The Dose this week. But before we begin, can you give us a hi, my name is. First and last name, tell us what you do and where you do it. Just ad-lib.

DR. AMITA MAHAJAN: Hi, my name is Dr. Amita Mahajan. I am an endocrinologist here in Calgary. I am a clinical assistant professor with the University of Calgary. And within endocrinology, I have a special area of interest in women's health and the endocrinopathies of pregnancy and reproductive disorders.

BRIAN GOLDMAN: And it turns out when it comes to women's reproductive health, core body temperature plays an important role, which we are going to get to. But I want to begin with the basics. How does the human body deal with heat?

DR. AMITA MAHAJAN: Kind of starting at the basics, the most important gland in our body that is involved in body temperature regulation is a little gland in our brain called the hypothalamus. And the hypothalamus functions as, you know, somewhat of a control centre in our body, and is responsible for not only regulating core body temperature, but also really important functions like blood pressure, heart rate, hunger, sleep-wake cycles and reproductive function. And the hypothalamus works really hard to maintain our core body temperature and in a really narrow range, what we would call the thermoneutral zone. So it works hard to keep our body in this thermoneutral zone so that we can keep our body temperature constant over a wide range of activities and environments that we may encounter on a day-to-day basis. And it works through a number of mechanisms, but primarily through having our blood vessels dilate or get bigger, or constrict or narrow to help get rid of or retain heat. So when we're at the top of our thermoneutral zone and too hot, the hypothalamus will tell the blood vessels you need to start dilating and increasing blood flow, bringing it closer to the skin so that it can evaporate. And if that's not enough, it'll induce sweating. And then on the flip side, at the lower part of the zone, when you were too cold, it'll start constricting our vessels, making them more narrow, reducing blood flow, getting deep conservation. And if that's not enough, it'll induce shivering so that we kind of increase energy production and hopefully increase our core body temperature.

BRIAN GOLDMAN: So that's a great beginning. And up until now, you've been talking about human beings in general, men or women. How do women differ from men vis a vis the hypothalamus?

DR. AMITA MAHAJAN: There's actually not a ton of data on this. And the data that-- looking at the difference between men and women is primarily looking at how men and women respond differently during exercise. So how the hypothalamus differs is centred around, again, if and when it tells the body to change the blood vessel size and when it should start sweating. So women have a higher threshold for when the hypothalamus says, ooh, I'm too hot, you need to start sweating. Relative to men that start sweating quicker. So when we look at, you know, lab-created conditions of high heat environments or high intensity exercise, women deal better in kind of hot, humid environments because their body doesn't tell them to sweat as quickly as male bodies would. And conversely, men deal with hot, dry environments better because they can sweat quicker and lower that core body temperature a bit more efficiently than women can in those specific environments. But we have to kind of take that even with a grain of salt, because the data that I was looking at, looking at this specific question, even the authors of many of these studies say in your day-to-day conditions, like what we encounter as individuals, there's no major difference between how men and women deal with heat when it comes to low intensity exercise. Kind of moderate intensity exercise are our day-to-day activities, and there's really no evidence that women are at higher risk for heat-related illnesses or need to do different things to mitigate their strategies. It's just in these lab-created environments these differences do exist.

BRIAN GOLDMAN: What role do the hormones progesterone and estrogen play in all of this?

DR. AMITA MAHAJAN: Estrogen and progesterone play an extremely important role in regulating core body temperature. And we have, as a medical community and kind of in the general public, there's been so much more information and gaining popularity with temperature tracking. Especially for things like fertility, ovulation tracking and seeing how we essentially deal with heat stress. But how estrogen and progesterone affect heat is best illustrated if I start off by kind of going through what happens in the menstrual cycle, if you're okay with that.

BRIAN GOLDMAN: Sure. That's great.

DR. AMITA MAHAJAN: So, the menstrual cycle. Anywhere from 21 to 35 days. An average of about 28 days. A bit shorter in our 40s, because everything gets worse in our 40s. But what happens in the first half of the cycle is your body picks a follicle, tries to grow an egg, and that first half of the cycle is dominated by estrogen. In the middle of the cycle, our body releases that egg to go off and look for a sperm, and that is called ovulation. And now you have transitioned to the second half of the cycle, which is called the luteal phase, and is by definition, high levels of progesterone and relatively low levels of estrogen. So in the first half of the cycle, when the estrogen levels are highest, your core body temperature is actually on the lower end, with the lowest core body temperature documented immediately before ovulation, corresponding to when the estrogen levels are highest. And then in the second half of the cycle, assuming you've ovulated, progesterone levels are highest and your core body temperature is about .4 to .8°C higher in that second half of the cycle. The way estrogen and progesterone impact core body temperature, again, all centred around their influence on the hypothalamus. So estrogen makes the hypothalamus and the body very efficient at getting rid of heat, meaning you'll have a lower core body temperature at which it'll say, hey, I'm too hot and I need to do something about my blood vessels and I need to start sweating. Which will result in a lower core body temperature as you'd see in the first half of the cycle. And then in the second half of the cycle, progesterone is really good at conserving heat and increasing that core body temperature, because it acts to the level of the hypothalamus, increases the threshold to when our body says, ooh, I'm a little too hot, I need to start sweating. And then there's also some evidence that progesterone influences the mitochondria of each of the cells -- which are kind of the powerhouse of the cell -- by increasing their energy production, thereby increasing our basal metabolic rate and kind of increasing our core body temperature that way.

BRIAN GOLDMAN: And this is not senseless. This is all to a purpose to try to optimize ovulation and fertilization and ultimately implantation and creation of an embryo.

DR. AMITA MAHAJAN: Exactly right. I don't think it's a coincidence that the hypothalamus controls both core body temperature and reproductive function, you know. Those are closely linked because we need to have enough energy to really go through the processes you've just identified. You know, getting pregnant, staying pregnant and then lactating requires a huge investment of energy. And if the hypothalamus is, you know, I'm not thinking that we have that energy levels, it has the ability to shut down the reproductive axis. And having a high temperature increases energy demands, especially over time. And it can say, hey, I don't have the energy to expend on processes that are not necessary to life.

BRIAN GOLDMAN: In that second phase, that luteal phase when progesterone is being secreted, how do women feel during that time in the cycle vis a vis heat? Do they feel hot?

DR. AMITA MAHAJAN: Like I mentioned, your core body temperature can be anywhere from .4 to .8°C higher. Generally speaking, doing our day-to-day activities, women may feel subjectively a bit warmer. But truthfully, in the studies, they don't really show that this is a clinically significant finding, and really, the effect is more pronounced during exercise. And so during that luteal phase or that progesterone dominant phase, women may notice that they're more tired during that phase or that their exercise tolerance is reduced. Or that their perceived exertional effort is higher. And those are really important, and that's really seen in, you know, your recreationally trained female athlete. You know, somebody that's just trying to keep fit and be active. For higher intensity or elite female athletes, that effect is probably less pronounced, because our body gets more efficient with training when it comes to heat loss and hydration status and things. So day-to-day, women shouldn't feel significantly different in the luteal phase, but during exercise or times of stress, they may notice that their ability to cope with that stress is slightly reduced.

BRIAN GOLDMAN: What can women do during that time in their cycle to stay cool?

DR. AMITA MAHAJAN: I want to be a bit careful not to overstep on limitations, I would say, or restrictions we should have in our luteal phase, because the most important thing is how you feel. I can't give you a hard and fast rule to say, in your luteal phase you should not spend this many minutes outside. It's really important to listen to your body and understand that you may not be feeling the same way you do in your second half of your cycle as you do in the first, and to take precautions that you see necessary. So, really simple things that we should deploy, you know, any time we're experiencing heat stress, like staying hydrated. Really important both in the luteal phase but also in pregnancy and for all vulnerable groups. You know, loose-fitting clothing, and good sun protection. But also avoiding prolonged exposure to high heat and really humid environments, really would be the most important behavioural change I say we could make. Or I would suggest.

BRIAN GOLDMAN: Okay, now I want to move on to the whole subject of being overheated. And we heard a clip from Britnee Miazek, who said on our sister podcast 'White Coat, Black Art,' that she thinks her menstrual cycles were affected by the heat she experiences when she's welding on top of the coke oven. And it's hot. And certainly if you look at whatever information is out there, as meager as it is -- for instance, the Centers for Disease Control in the United States says that chronic exposure to extreme heat can potentially affect reproductive cycles. But what do we actually know about how high temperatures affect menstrual cycles, for instance?

DR. AMITA MAHAJAN: Yeah, that's a really good question. And I also tried to look, kind of do a deep dive on this, and I wasn't able to find really specific information. So a lot of the data that we know or, or talk about is based on what we know about physiology -- just how the body works -- and using kind of physiology-based study and not randomized clinical evidence-based controls [indistinct]... That data isn't really available for this. But without knowing specific details of the case you're describing or blood work or history or anything like that, my take on the case as an endocrinologist would be exposure to high heat over time -- or equatable to hypothermia -- increases the body's basal metabolic rate, which increases the energy demands on the body. And unless we meet those increased energy demands with actually increasing our energy intake, there's going to be an energy mismatch. And the hypothalamus, our control centre, is going to sense that. And because what we've alluded to about how much energy we need to go through the reproductive process... Because whether or not women want to, every month the body is trying to get pregnant when it comes to the menstrual cycle. So, the body recognizes that there's an energy mismatch and the hypothalamus will focus its energy on the vital functions that we need to survive. You know, heart rate, breathing, those kind of things. And it will, it will shunt energy away from reproduction or fat storage. And one of the consequences can be that women lose their periods.

BRIAN GOLDMAN: Amita, I want to make sure that people listening to this understand what you mean by energy. You mean calories, don't you?

DR. AMITA MAHAJAN: I do. Yeah.

BRIAN GOLDMAN: The energy mismatch means that women exposed to high heat on a chronic basis, by having a higher metabolic rate, they're expending more calories. And if they're not putting calories back in, the first thing that might get sacrificed hypothetically is the menstrual cycles.

DR. AMITA MAHAJAN: That's exactly it. And we see that in different circumstances of, you know, sometimes really high elite athletes. If they're not able to keep up with the expenditure from how much they're exercising, with calories, that mismatch exists and the body will choose what it needs to do to survive.

BRIAN GOLDMAN: I want to switch up topics a little bit. I want to ask you about women who are taking birth control pill or birth control IUDs. How might these affect the body's heat regulation?

DR. AMITA MAHAJAN: It is hard, truthfully, from a purely academic perspective, to interpret the studies looking at birth control pills and core body temperature. Because not all of the studies report which type of birth control these patients are using. So it may be a combined estrogen/progesterone pill. It could be an estrogen/progesterone pill with progesterone doses escalating each week, or it could be a progesterone-only pill. Keeping that in mind, the studies that looked at women... the same individual woman that was either on the pill some weeks and off the pill some weeks, they noted a core body temperature increase by about .3 to .5°C. And then women taking progestin-only contraceptive pills have not only noted a body temperature increase, but also authors have noted a basal metabolic rate increase. Perhaps due to the effect that progesterone has at the level of the mitochondria, by increasing energy production. So, those are objective findings seen in studies. But truthfully, Brian, in 2024 and the climate of information and misinformation around contraceptives, I'm hesitant to contribute to that because it isn't... it's hard to tell what is just objective findings and what's clinically significant to somebody's day-to-day life.

BRIAN GOLDMAN: Let's turn to women who are pregnant. I think we might know more about that. How does exposure to extreme heat or how might exposure to extreme heat affect pregnant women?

DR. AMITA MAHAJAN: Pregnant women, like other vulnerable groups, are more susceptible to intense changes in heat. And physiologically, women that are pregnant may notice a higher core body temperature in the first trimester, but in the second and third trimesters, their core body temperature actually can be lower. And this follows kind of temporally the same concentrations that progesterone and estrogen have in pregnancy. So in first trimester, when progesterone is highest, your core body temperature will be higher. And then in the second and third trimester, when estrogen levels are relatively higher, the core body temperature will be lower and your body will be more efficient at getting rid of heat, probably to protect the fetus from, you know, maternal high temperatures or hyperthermia. Pregnant women are more susceptible to changes in heat because, you know, they have a higher body surface area and body mass compared to when they were pre-pregnant; a significant increase in their blood flow; and then an increased basal metabolic rate. So, they do need to be cognizant that heat can affect them quicker and worse than it would have pre-pregnancy. To be safe, the advice isn't significantly different. And I want to preface it all with: listen to your body. There is no hard and fast rules. But remaining hydrated in pregnancy is of the utmost importance because it is extremely easy to get dehydrated because you are literally growing another human. And especially in hotter climates, it's very important to aim for that. About two and a half, three litres of water every day at minimum. And then, you know, the same kind of idea: looser clothing and really avoiding the prolonged exposures to high temperature or really humid environments. Important for pregnant women and really all vulnerable groups. And that's not really just the responsibility of pregnant women. I think as our planet gets warmer, we all need to think about this a bit more.

BRIAN GOLDMAN: Now how about women who are going through perimenopause?

DR. AMITA MAHAJAN: This is an interesting question. So, we've known temperature changes that occur across a woman's life cycle and especially during perimenopause and menopause. So, I think what you're referring to is really the presence of hot flashes and, and the bothersome nature of them and how they increase in severity as you get closer to menopause. The changes that occur around perimenopause and menopause are, again, strongly influenced by estrogen, and it's being in a state of estrogen withdrawal. So like we talked about before, when the hypothalamus senses a woman is too hot, it will start the mechanisms to get that woman to cool down, eventually ending in sweating. And the goal is to keep the woman's core body temperature in that thermoneutral zone. And in menopause, that thermoneutral zone is actually narrowed pretty significantly. And what that means is that even really subtle changes in our core body temperature, will start to induce these really intense responses of dilating our vessels and flushing. So, for example, for a pre-menopausal woman, she would have to have an increase in her core body temperature by about .4 to .5°C before her body would say, I'm too hot, I need to start sweating. But in a perimenopausal or menopausal woman, that same response can occur with just a .1 or .2°C increase. So they're just more sensitive to really subtle changes because the hypothalamus is working really hard to keep their temperature within a narrower thermoneutral zone overall.

BRIAN GOLDMAN: So given all that we've talked about and given the limitations in research, what's your advice to women, as our outside temperatures increase and there are more opportunities, more days when women at all stages of their reproductive lives are more likely to be exposed to higher temperatures in the years to come, what's your advice to them?

DR. AMITA MAHAJAN: The most important thing I want people to be aware of is the knowledge that your temperature does change -- second half of the cycle, in and around menopause. And to be aware of these changes and this increased sensitivity we have to very subtle differences in temperature overall. That would be my number one is just to be body aware so that you can leave room to understand yourself and to understand your limitations and not push too hard. And then really conservative things. Like, we've talked about hydration, we've talked about avoiding prolonged heat exposure and really humid environments. And then in and around menopause, there's good evidence to avoid dietary things like spicy foods or caffeine and alcohol. They can all trigger increases in body temperature. If you're susceptible to those, definitely using some of those pieces of advice would be helpful as well. But it really comes down to very individual susceptibility to feelings and changes in temperature, and understanding that we need to take measures ourselves to avoid exposure to those environments.

BRIAN GOLDMAN: Well, Dr. Amita Mahajan, this has been a fascinating conversation, one that we have not had on The Dose before, and I'm glad we've had the opportunity to do it for our Overheated series. Thank you so much for speaking with me.

DR. AMITA MAHAJAN: Thank you so much for having me.

BRIAN GOLDMAN: Dr. Amita Mahajan is an endocrinologist and clinical assistant professor at the University of Calgary's Cumming School of Medicine. Here's your dose of smart advice: rising temperatures in Canada and around the world have measurable effects on health. A hotter planet may affect women more than men. Compared to men, women have a higher percentage of body fat and a lower percentage of water. Those factors make women more prone to dehydration at higher temperatures. The hypothalamus, a gland found inside the brain, regulates core body temperature to keep it at around 37°C. It also helps regulate menstrual cycles and reproductive health. Extreme heat can affect reproductive health by causing women to use more calories than they consume. That negative caloric balance sends a signal to the hypothalamus to stop the menstrual cycle. That may explain in part how heat exposure can lead to decreased fertility. Pregnancy stresses the ability of women to regulate body temperature when it's hot. Women's bodies work extra hard during pregnancy to keep mom and fetus cool, to the point at which regular cooling mechanisms, such as sweating, are unable to fully get rid of excess heat. That's why pregnant women are at greater risk of exhaustion, dehydration, fainting and heat stroke when it's hot. High heat may also increase the risk of preterm births, stillbirths and low birth weight. For those reasons, women should be extra mindful to avoid, if possible, conditions of extreme heat, especially if they're pregnant. For times when heat exposure is unavoidable, women should make extra efforts to stay hydrated. Women who get lightheaded during times of extreme heat should use fans and AC to keep their environment cool. During perimenopause and menopause, women have increased sensitivity to excess temperatures. As more and more women work at occupations that expose them to high heat, employers and labour unions should be mindful of the unique impact of climate change on women's reproductive health and pregnancy. If you have topics you'd like discussed or questions answered, our email address is thedose@cbc.ca. If you like this episode, please give us a rating and review wherever you listen. This edition of The Dose was produced by Sameer Chhabra and Stephanie Dubois. Our senior producer is Colleen Ross. The Dose wants you to be better informed about your health. If you're looking for medical advice, see your health-care provider. I'm Dr. Brian Goldman. Until your next dose.

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What do we know about how women's bodies deal with heat? - Transcript

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