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Physiologically Speaking Q&A #1: CGMs, Time-restricted Eating, Endurance Training Structure, and More!

You asked, I answered.


Today I'm releasing the first installment of an audience question and answer!

This Q&A includes questions that I've received throughout the previous month related to nutrition, health, science, my routines, my training, etc.

These are questions I’ve received from my Substack subscribers, people on X who may send me direct messages, or question I’ve received about things I’ve posted on social media or my blog.

Physiologically Speaking is a reader-supported publication. To receive new posts and support my work, consider becoming a free or paid subscriber.

For this Q&A, I took a sampling of questions that I would be able to expand upon and provide some more context about, but also things that I thought you all might find interesting to know the answers to.

If you enjoy this Q&A, I would love to make this a monthly thing and at some point, I may start to submit requests for questions from my audience.

For now, the Q&As will just be a compilation of random questions that I have received.

This Q&A will be free to all subscribers so you can get a taste of what future Q&As might look like.

Watch the video above or, if you’re so inclined, I’ve provided a (rough) draft of the questions and my responses below! You can also download/listen to the podcast on Apple Podcasts.

Alright, let’s get to your questions.

Brady, how useful do you find CGMs? I have used them on and off for a while, and I have very mixed feelings. Sometimes I think they make eating more difficult. (Peter) Attia and others don't think that one should exceed a blood glucose of 140. I'm not sure that makes sense.

Tl;dr: Continuous glucose monitors can be extremely insightful to use for a month or two, but unless you have diabetes or are at a high risk of having diabetes, it’s probably not something you need to use all the time. I learned a lot about my body and responses to food and exercise by using a CGM For a month.

CGM is referring to continuous glucose monitoring. These are or once were only available to individuals with diabetes as a way to manage their glucose control. CGMs are a device placed on the back of your arm. A little needle is inserted and it goes into your interstitial space and it measures not blood glucose, but rather interstitial glucose.

Nonetheless, it can give a 24-hour reading of your blood glucose concentrations. CGMs are a lot better to use than just say a snapshot of your blood glucose after a meal. They allow you to see what your blood glucose levels are doing throughout the day.

So how useful do I find that they are?

I think that everyone should probably use a CGM at least once for maybe two weeks to a month or maybe up to two months. I think they can be very valuable, perhaps invaluable tools for learning about how your body responds to certain foods, learning about how your body responds to exercise or sauna or different types of stresses, and even what your blood glucose levels look like before you go to sleep at night or while you're sleeping or when you wake up in the morning.

I think that using a CGM for a month or two could give you an insight into here's what happens to my blood glucose when I eat some french fries or here's what happens when I'm fasted or you know in the middle of a high-intensity workout.

I used a CGM for about two weeks maybe even more like a month and I learned some interesting things. The most interesting thing was that not a lot of things food-wise, I guess, raised my blood glucose as much as I thought they might, maybe due to my exercise levels or the fact that, I mean, generally I tend to eat fairly healthy even though I do, you know, eat some carbohydrates and sugar from time to time.

But I found that it was very useful. I learned a lot, but I don't know if it's something that I would always use because in a way it did seem to get in the way of, or I guess make eating a little bit more difficult. I think that CGMs may not be good for people who are prone to having an eating disorder or who have tendencies to maybe avoid foods just because they're going to elevate your blood glucose a little bit.

An increase in blood glucose in response to a meal is a normal physiological response. And of course, if you measure your glucose two hours after a meal and it's over 140 or near 200, that could be a sign that you might have diabetes. But if your glucose levels elevate to 120 in the 30 minutes after a meal and then start to decline, that's a normal response. That's what's going to happen when you eat something that contains carbohydrates and it's probably nothing to be scared of. I don't think that transient rises in blood glucose are dangerous per se.

So again, I think that using it all the time, may, at least for me, it would probably be a nuisance and kind of a burden because I would always just be paying attention to my blood glucose.

And I just don't think it's probably healthy, at least for someone like me in that case.

If you're worried about diabetes or diabetes runs in your family, or you're worried about metabolic issues, I think CGMs can be super valuable.

Regarding the last point on Attia and others, not thinking somebody should exceed 140, you know, I won't necessarily comment on that. I'm not a clinician, I'm not a physician, but again,

I think that, you know, elevations in blood glucose in an otherwise healthy person are just a normal response and they're not necessarily indicative of disease or underlying pathophysiology.

So short answer to that question is I'd recommend, you know, CGMs are becoming available over the counter. I think the FDA just approved the first one of those, so we're all going to be able to have access to it probably very soon. I would recommend that most people, you know, try one out for a month or two.

Brady, do you have a sense of which benefits of intermittent fasting or time-restricted eating arise from going a certain amount of hours without eating and which arise from having a certain amount of acute energy deficit? This is a practical/lifestyle question for many endurance athletes because if most of the benefits come from an energy deficit, then I could just do a long run or workout. bank a 1,000 to 2,000 deficit plus another from my resting metabolic rate, maybe skip breakfast and have a 3,000 calorie deficit by lunchtime.

Tl;dr: For weight loss, it’s all about the energy deficit. However, time-restricted eating may have unique benefits for metabolic health independent of weight loss.

I think a shorter version of this question is: Are there unique benefits to intermittent fasting independent of calorie restriction? Or is it all just about calorie restriction? Are all of the benefits just due to calorie restriction?

I think it's a little bit of both, but I also think that it depends on what your goals are.

When it comes to just weight loss, I think that it's all about your energy deficit and it's all about calories. This is evidenced by the fact that when we look at randomized controlled trials or meta-analyses of randomized controlled trials, comparing time-restricted eating versus a calorie-restricted diet that doesn't have time-restricted eating, both tend to result in similar amounts of weight loss. Neither of them seems to be better for weight loss.

Time-restricted eating does appear to be an effective way of reducing calories because when you limit the window in which you're able to eat, say to four to six hours during the day, you're probably just going to eat less naturally versus if you allowed yourself 10 to 12 hours to eat during the day, of course.

So for some, I think time-restricted eating represents this way to artificially or not artificially, but naturally reduce how many calories they're eating. But if you're trying to lose weight, it's all about just energy deficit and however you achieve that, you know, it's kind of up to you.

Where I think time-restricted eating may have unique benefits though is in terms of fat-burning ability, increasing mitochondrial content, mitochondrial enzymes, and improving your ability to burn fat.

I think that going, say, 10 to 12 hours per day, maybe even more, maybe 12 to 16 hours per day, doing that sort of a 16-8 time-restricted eating type of thing can be beneficial in its own right, independent of calories for certain metabolic benefits there. I think time-restricted eating can be great for that.

But again, if your goal is just to lose weight, there doesn't seem to be anything special about time-restricted eating.

What are your thoughts on following training plans by distance versus time? For example, a 7-, 9-, or 12-mile run in a training plan is very different for someone who averages 7 minutes per mile versus someone who averages 12 minutes per mile. So should the 10 to 12-minute miler be running the same distance or the same time?

Tl;dr: If you’re training to race a particular distance, basing your training on distance makes the most sense.

This is a great question. I think that overall, time should probably be a consideration maybe for someone who is just starting out. Say, you know, I want to increase the time that I'm running.

We use running in this case since that was used in the example.

I want to increase the time that I'm running per week from, you know, one hour to three hours.

But I think that I always like to train by distance and this would kind of be my

opinion just based on how I like to train but also I think it makes logical sense that if you're training for a particular race. Take somebody who's trained for a marathon I guess well that marathon is 26 miles so you're going to want to train yourself to be able to run 26 miles. And so obviously you could just train by time and not think about miles.

But in my opinion, you want to probably base your training on mileage run for the week. So I want to run 30 miles this week or I want to run 40 miles this week or get a 20-mile long run in.

If you tell two runners training for a marathon to run 3 hours in a week, the person who's running the 7-minute miles is going to get a lot more miles covered in that time than the 12-minute miler. I think training by miles is probably better and makes more sense.

If you're training for a distance, I think prescribing your training based on distances kind of makes the most sense there.

Do you usually run early in the morning? And if so, do you eat a very carb-heavy dinner?

Tl;dr: I run at 5:30 in the morning every day and don’t eat beforehand. The dinner the night before is usually substantial, but not purposefully more “carb-heavy” than the other meals during the day.

I do run very early in the morning on the weekdays and typically on the weekends too, although I do sometimes sleep in for maybe one to two hours extra on the weekend because I have a little bit more time.

During the week, I typically wake up around 4:45. So my alarm is actually for 4:43. Don't ask me why. It's just random and I chose that time.

But I'm usually out the door for my run by about 5:30 on Monday through Friday it might not always be a run it might be a bike but I'm working out at 5:30 in the morning during the weekday almost every morning so I don't eat anything before those sessions. That's just due to mostly logistics not necessarily a reason that I'm like trying to do those workouts fasted. I don't have enough time to wake up and eat breakfast and let it digest before I go and run.

I'm typically not running or riding maybe for more than like two or two and a half hours. And if the workout is of a low enough intensity, I probably don't need to eat anything. So I'm usually just doing those workouts fasted. But I will have a pretty substantial dinner the night before.

We typically eat dinner around 6, and finish sometime around 7. 7:30 is kind of when I have my last bite of food during the day.

Is it carb-heavy?

It's not more carb-heavy than usual. Dinner usually is the meal of the day that has more carbs than the other two meals like breakfast or lunch, but I don't necessarily make it a point to stuff myself with carbs during that dinner. I just have a normal dinner and that tends to carry me through the workout the next morning. Sometimes I'll put honey in my coffee if it's like a workout in the morning and that tends to help as well with a little bit of the energy issues there.

How do you exactly record walking? Do you go out for a specific walking session or do you record every time you walk?

Tl;dr: If I go on a walk for the sole purpose of going on a walk, I’ll log it using my GPS watch just like I would a normal activity. I like to quantify all of my purposeful activity during the week and typically log about 4–5 hours of walking per week.

This I believe is about, typically I'll post like a screenshot of my weekly activity, including my running, my biking, sometimes strength training, and how much I'm walking during the week.

So I'd like to, it's something I've recently started doing.

I recently only logged my formal workouts. So if I go on a run or if I go on a bike ride, I'll log that on my GPS watch. Walking was kind of just something that I did and didn't log, but recently I've been going on dedicated walks.

So I'll take my son in the stroller and sometimes my dog too and go out on walks for 30, 45, or 60 minutes sometimes if it's nice out and I have some time to spare.

I have been logging those recently because I figure it's a kind of training and I would like to see how much walking, how much purposeful walking I'm doing during the week. So I log all of my walks.

If it's going to be like 15 minutes or more and I'm going on the walk for the sake of the walk, I will log it. If I'm just, you know, going out to take my dog around the block maybe for a few minutes I'm not necessarily going to log that so purposeful walks I log all of those and count that kind of in addition to my other training that I do. Again, this is something that I've recently just been doing to try to quantify all the stuff that's going on during the week and I found that to be pretty fun.

Have you ever had a full thyroid and hormonal panel done? For example, cortisol. If I start approaching more than 10 miles in a week, my sleep goes to sh*t. I typically lift three times a week too.

Tl;dr: The more I train, the better I tend to sleep unless I’m training too hard and risk overtraining. Working out at night can sometimes negatively impact my sleep unless I’m able to cool down my body beforehand. I try to avoid working out too late and doing high-intensity training too close to bed.

I sleep like a baby.

Recently I've been training a lot and we have an almost one-year-old son in the house so by the end of the day I'm typically just exhausted along with working during the day. 

I never have had sleep issues. I've always slept pretty good.

There are some certain circumstances though where I think sleep has maybe been affected by training. One of those instances would be, say, perhaps during the summer or if I'm increasing my mileage a lot and I may be overreaching a bit and maybe in a bit of sympathetic overdrive, perhaps combined with maybe not eating enough, that can typically lead to my sleep being less than ideal, I have found.

The second time when my sleep might suffer is if I time my exercise too late in the afternoon or make it too high intensity, and then it's too close to sleep, that can typically interfere with sleep as well. If I finish that workout before like 6:00, typically, and don't eat an insanely big dinner afterward, sleep is typically fine, as long as I can get my body temperature back down to normal.

If I do a high-intensity workout and it's a bit later in the evening than I would like, and you eat a big dinner and you're trying to go to bed, you're hot, you're kind of over-activated, your heart rate hasn't come down yet. That tends to make sleep bad as well.

But regarding just overall training, I mean, the more I train, the more I tend to want to sleep and to think the more sleep that I need. So no, I've never had sleep issues.

I never really had a thyroid and hormone panel done either. I mean, I've had some blood work done in the past, but I don't remember specific numbers or anything for that, but is probably something that I should do. Maybe a yearly test might be something that's probably valuable for me to do and something that I will look into but regarding strategies, maybe, you know, if you are training hard and are suffering or having a hard time sleeping.

I think the best strategy you could do is to try to work out in the morning if you can if you're not already doing that. So evening workouts could be affecting your sleep.

The best thing that I can do to improve my sleep after a workout is to get my body temperature as low or back as close to baseline as possible before I go to sleep.

So if I do an evening workout, if it's cool outside, if it's the spring, go on a little walk outside, maybe without a shirt or in minimal clothing, just to like cool down your body temperature because a high body temperature is going to be one of the main things that influences how well you sleep and how easily you're able to get to sleep, how easily you can stay asleep, and like the quality of that sleep.

So that's the biggest thing that I think will interfere with sleep, especially, you know, with the summer coming. Try to find some strategies to cool down, not just a physiological cooldown, but, you know, cool down your body after your workout. And I think that that will hopefully help with sleep.

Do you follow an 80-20 routine with a bike? I would like to add this to my running more. And do you find programming your week ahead of time works best for you?

Tl;dr: I bike about twice as much (time-wise) as I run during the week. Right now I bike for about 6–8 hours and run for 3–4 hours. The bike has helped me increase my exercise volume and fitness as a runner. Programming my week(s) ahead of time is something I’ve recently started being more diligent about and has been wonderful for training progression and avoiding injury.

So this is about how I split up my week, how I do a majority of my endurance training, splitting it between the bike, which is mostly on the indoor trainer, and running, which is outside.

I guess it tends to probably fall to around 80-20. I would say maybe it's more like a 60-40 kind of split, or like maybe a two-to-one ratio of biking to running.

So right now, I'm doing probably seven to eight hours of bike riding per week. And then running tends to be about four hours now, hoping to increase that a little bit in the future.

And that's intentional. I mean, I would like to run a little bit more, but struggling with injuries in the past few years has just made me change my tune a little bit on how I approach training and getting the volume per week that I want without, you know, increasing the impact. And that has led me to find the bike. I'm able to improve my fitness a lot by biking.

And so I would recommend that all runners, you know, you don't need to do a majority of your training on the bike. But if you're like me and maybe struggle with injuries at higher volumes and want to find a way to exercise a bit more without, you know, just running, I think the bike is great. I would recommend all runners bike.

I think that you know, currently I'm training for a marathon, and just running three days a week or four days a week, even for like 30, 40 miles, isn't going to cut it. Biking is allowing me to get that extra volume, the metabolic work that I need to kind of train for a marathon.

I'm curious to see how the race turns out when I'm able to, you know, with this training that I'm doing right now.

So yes, I do kind of follow an 80-20 routine, but again, that will just depend on what kind of volume of running that I'm doing. Typically, if run volume goes up a little bit, naturally bike volume is going to to fall a little bit.

In reference to programming my week ahead of time the answer to that would be definitely yes. It’s not something that I have done a lot of in the past.  I've been a runner like my entire life and so I tend to just fall back on kind of training um training tendencies and protocols that I have used in the past regarding like how the week is structured and what workouts you're going to use and how much I want to run during a particular week. It all has tended to be the same and I just have tended to do what works.

I've kind of just done my own thing and been my own coach, but I haven't done much coaching for myself rather than just, you know, kind of go with the flow in terms of training.

But now in the past month or this year, I guess, is when I started to formally sit down and plan out some more structured training for myself.

Sometimes that involves planning something 8 to 10 weeks out, but more often than not, it's like, let's just sit down and look at what the next month looks like. What does my fitness look like? Where am I in training? What kind of volume can I do? Am I training for a race?

And then just plan out the next month.

I think I find that valuable, not only because it kind of prevents me from doing maybe too much or not doing enough and like getting the right workouts in, but also because it just offers a lot of psychological flexibility and that you don't have to wake up every morning and figure out what to do or plan your workout for the day. It's already there.

And so this can be something, again, as simple as like Sunday night, typically I'll just sit down and write in my planner what I'm doing Monday through Sunday this next week. That just frees up a lot of psychological space as well and gives me a little bit of confidence going into the week that, I know, you know, these times are set aside for working out.

I know what workouts I have to do to get where I want to be at the end of this week. So yes, it's a great thing to do. Program your week ahead of time.

It doesn't have to be 8 to 10 weeks in advance, but I would recommend everybody on Sunday or Saturday, depending on when your week starts, sit down and do some sort of semi-structured plan for the week. It can be very helpful.

How do you titrate running volume coming off injuries?

Tl;dr: I typically start by running 3 days per week or running every other day for a few months. I’ll add 5–10 minutes to each run every 2 weeks, and eventually add another day of running. It’s always a combined progression of volume and intensity.

Progression of running coming off of an injury is, is always hard and everyone will have unique situations based on what your injury was and how much time you took off.

So most recently, and I think this question was about my most recent injury was a stress reaction in my femur that forced me to take, uh, about a month off of running.

So I took a month off. I was still able to cycle during that time and do some strength training.

So it wasn't, you know, a completely one month of inactivity.

I was able to, I think, maintain some strength and fitness. But when I came back from that, you know, what I typically do is just I'll run every other day. And I will kind of start at the minimum dose of running that's at least somewhat substantial.

So I did my first run back was 30 minutes. I did another 30-minute run on Wednesday. So I ran 30 minutes on Monday, and 30 on Wednesday. On Saturday, I did a 45-minute run, I think.

Again, that might be a lot for some, and even for me coming back from some injuries, but this one was different. I don't think I had lost a lot of fitness, so I was able to kind of start at a higher baseline than previously.

But run three days a week. So control running frequency. Don't go running every day during the week when you first come back.

And then obviously intensity. I don't do workouts for like a month for when I'm first coming back. I just do a lot of like easy running.

And then obviously volume.

So the way that I like to progress is to do two weeks of the same amount of volume and then kind of step up every two weeks. So that will pretty much include adding 5 to 10 minutes to every run. And that ends up just, you know, increasing your overall weekly volume by maybe 30 minutes to 45 minutes or something like that. And that tends to be like a safe progression for me.

I know that sometimes people will recommend that you don’t increase by more than 10% per week.

If you're running volume is very low, you know, I think you can probably go beyond that and maybe go up to even like 20% per week.

But again, your mileage, uh, your mileage may vary on that one, but progression is always the key and slow conservative progression, even though it's hard, you know, especially for me, especially I have a hard time progressing with the running. When I come back, you get excited. You want to run a lot, but, um, just being patient is important.

Do you ever do post-run glucose levels?

Tl;dr: I have a glucose monitor and if I’m curious, I’ll measure my glucose levels during the day or after a run, but not normally. It’s not something I’m too concerned about.

So do I measure my glucose levels after I run? This kind of is in relation to our first question on continuous glucose monitors.

I don't typically.

I do have a glucose monitor at home. It also doubles as a ketone monitor, which is kind of interesting. I'll measure it from time to time if I'm curious. If something feels off, I'll measure it.

If I'm just curious, like if I've maybe fasted for a long time and did a particularly long run, I will measure my glucose levels, but more often than not, I'm not typically worried about it.

That's an instance where I think a CGM would be fun and handy to have, you know, what are my glucose levels during a run? What do they look like after, you know, if they're too low, maybe you need to, you know, add more carbs to like your post-workout meal or something like that.

But, um, so that the answer to that would be, no, I don't, I don't find a need to, but I will do it in special cases if I'm curious or if something seems amiss and I think measuring glucose levels might provide some useful information or something like that.