Dear Mark: 21-Day Challenge Rapid Fire Edition, Part 2

Questions finalLast week, I answered a bunch of the questions readers asked at the start of the 21-Day Challenge. You guys came up with so many great questions that I couldn’t get through all, or even most of them. So today, I’m back with another round of rapid fire answers to many of your questions. I talk about the utility (or lack thereof) of window-filtered sunlight in the winter, the need (or not) for 8 glasses of water a day, how fasting can impact fitness, why someone’s sleep might be suffering, which source of glycine is best, whether chlorine in a pool is bad for you, and many, many more.

Let’s go:

Living in Canada wondering what the effects are of sun if behind glass?
Sitting at home in the sun or in the car, does a longer exposure behind glass get me anywhere close to the 15 mins of sun?

Window-filtered sunlight can certainly feel good. There’s nothing like curling up by the window with a great book on a cold, but sunny winter day. Just ask cats. They know all about it.

Unfortunately, windows do not allow UVB through, only UVA. And as UVA penetrates deep into the epidermis, which causes aging, while UVB interacts mostly with the top layers of skin and is the wavelength response for vitamin D synthesis, this is a problem. UVB counteracts the UVA damage; UVA keeps the vitamin D synthesis from getting out of hand. You need both. If we upset the balance and get too much UVA without enough UVB, as you would through a window, problems arise.

Now, hanging out by the window on a sunny day won’t give you skin cancer. I’m not suggesting that. However, you won’t be getting all the benefits you normally associate with sun exposure, and you may do damage if you rely on window-filtered sun as a vitamin D source on a regular basis.

Do you subscribe to the “8 glasses of water” or “drink half your body weight in ounces” or do you think its better to focus on high-water-content foods and nutritious drinks like bone broth and kombucha?

No, I’ve long rejected that advice, which has no actual scientific foundation.

High water content foods are almost every food you can imagine.

Well, not every food. The average Westerner eating lots of baked goods, processed junk, fried foods, and other non-Primal, low-water fare may need extra water, but you? Nearly everything on the list of Primal Blueprint-approved foods—meats, seafood, fruit, vegetables, roots, tubers—contain more water than you’d probably think. After all, every organism on earth depends on and is comprised of lots and lots of water. So long as you consume those organisms without removing too much of the water, you’ll be getting plenty of water.

And yeah, liquids like bone broth work great. If you’re training, a pinch of sea salt in water helps.

Water needs do go up with hot weather and exercise. Even then, though, you don’t need to do any calculations to determine your needs. Thirst is the guide. Thirst is an accurate predictor of water needs, even in athletes engaged in physical activity. Thirst literally triggers the part of your brain that determines “physiological need states” and motivates seeking behavior. Thirst is the best barometer we have.

How does intermittent fasting play into Primal fitness, if at all? What are your thoughts on this practice?

Fasting is primarily a healthy aging buffer and occasional tool for fat loss. It’s not really meant to improve fitness.

However, just like “train low(carb), race high(carb)” can help endurance athletes retain glycogen for the final push in races, fasted training can improve glycogen retention during physical activity by training you to burn more fat and fewer carbs. If you’re in a fasted state, you’re burning fat.

One study exemplifies this phenomenon, pitting a group of untrained, carb-fed cyclists against a group of untrained, overnight-fasted cyclists and comparing both groups’ muscle glycogen content and V02 max. The fasted group improved their V02 max by nearly 10% and their glycogen content by over 54%, while the fed group improved V02 max by just 2.5% and glycogen by a paltry 2.9%.

Anecdotally, many people report feeling energized during fasted training. This is something to play with, and if you feel good training in a fasted state, it’s probably doing you good.

Mark, I basically eat primal and get seven to eight hours of sleep a night, along with exercising 5 – 6 days a week. I still have no energy. What can I do to increase my energy levels and feel rested in the morning?

Don’t exercise 5-6 days a week! That’s too much for most people. Lack of energy is a huge tell.

I’ve been trying to make sure I get a little bit of gelatin, collagen, tendon, or bone end caps with every meal. Is one form of glycine any better than the others? Putting some dissolved collagen into my tea is the easiest, but I’ve been mixing it up just in case.

Glycine is glycine is glycine.

What changes is the stuff that comes along with the glycine. So straight-up collagen hydrolysate is just that, while bone end caps and tendons have that plus other bone-related nutrients. You can go even further and get your glycine—and tons of muscle protein, B vitamins, and other nutrients—through gelatinous meats like oxtail or beef shank. They all work, although straight glycine infusions in the form of the collagen in your tea are the most “reliable” route if all you care about is the glycine.

Hi Mark,
I received a Himalayan salt block as a gift recently and I was thinking about grilling up some primal fajitas on it for the Primal Celebration Dinner that is to be planned for Saturday, December 6th. Any thoughts of this style of cooking? Is it Primal?


Never done it myself, but why not? This seems to be a great guide to using salt blocks.

Hi Mark,

Thanks for this easy opportunity to ask questions. The one that’s been floating around in my head is, “For those of us who haven’t done cardio work in many many moons, how best to reintroduce it in our exercise routines?” More specifically, I have small children, a very part-time non-desk job, and spend a good deal of time cooking. This means I move around slowly and fairly consistently all day most days, and don’t do much activity that gets my heart rate up. Happily, we’ve recently been able to join a gym, so now I have access to Zumba and a versaclimber and AMT machine. Should I do higher heart rate activity for a short duration until I get my respiratory system better conditioned, or is it fine to be red-faced and pushing my limit for a whole hour in Zumba twice a week? Thanks!

Start with high intensity intervals or sprints on the bike. For my money, they provide best ROI of any “cardio” exercise with the least amount of risk. Nearly everyone, from guys with meniscus surgery in their past to oldsters, can safely use a stationary bike, even at high speeds. You’re not gonna fall off or get hit by a car. You’re not pounding your joints. There’s a technique to it, but not so complicated as the technique required for proper track sprints. And they’re effective. They will increase your cardiovascular fitness quickly and reliably so that if you want to sweat it out in Zumba, you can.

The Versaclimber’s also a nice option. I love-hate that machine.

I have a desk job and drink a LOT of black, strong coffee. I start first thing, and drink 1/2 to 2/3 of a 12 cup pot. A good cup of hot coffee at my desk helps keep me focused doing work that I’d rather not be doing. In the afternoon or after dinner, I find that a cup of coffee helps me kill a craving for something sweet.

I know there’s a cortisol thing going on. When I don’t have coffee for a few days I don’t get any headaches, but I’m usually on vacation and am very busy doing non-work stuff. I don’t have trouble sleeping that I’m aware of, but at the moment I’m really focusing on eating primally and maintaining my physical activity. Right now coffee is a facilitator for that.

Should I focus on trying to reduce my coffee consumption now? or can I get to that later and still see success?

I’m not convinced you need to eliminate coffee at all. The evidence is pretty clear: coffee is consistently associated with health benefits.

And for you, it’s not just an association. You can reliably state that coffee improves your ability to work, focus, perform, and avoid eating junk food. It’s not affecting your sleep, as far as you can tell. The cortisol thing may be an issue, but you’ll usually feel that. A chronic cortisol issue from chronic coffee consumption usually manifests as stubborn belly fat, a “tired but wired” feeling, an inability to function without coffee, and an inability to get good sleep. If you’re feeling rested in the mornings, if you’re able reduce your coffee while on vacation and not get crippling headaches, you’re probably fine.

So yeah, focus on the stuff you’re not doing that you know you should, like eating vegetables and sprinting. These will provide measurable benefits.

After you’ve got everything else under control, you can always try cutting back. I sort of suspect 9 cups of coffee might be too much for most people, so it’s worth an experiment.

How do you know you are exercising too much?

Check out this post.

Mark, I have a few questions about carb sources and amounts as it relates to weight training. Most of the time I keep within the 100-150g a day for carbs. I lift 3 days a week, usually about an hour at a time. How many more carbs should I eat on those days? and is it a huge issue to use something like rice? I love sweet potatoes but at 25-30g per potato I’m afraid I’ll be full before topping off glycogen.

1. An hour of lifting is a good amount of exercise. Unless you’re lifting really intensively/heavily (CrossFit/powerlifting), the 150 grams should be plenty. Feel free to try another 50 grams or so just to see if it improves recovery and performance. But yeah, 100-150 grams should be enough for basic strength training.

2. Rice isn’t a big issue. Don’t consume bowls and bowls of it, and try to increase the nutrient density by cooking in bone broth instead of water.

I guess the real question is does the play requirement have to be physical in nature, or is it more for relaxation purposes? Would getting together for board games with friends be considered primal play time?

Absolutely. Play doesn’t have to be overtly physical to count. It’s whatever makes you happy and gets some part of you (brain, cardiovascular system, muscles) active. In this case, you’re using your mind.

Doing it with friends is a huge bonus.

Passive entertainment doesn’t qualify in my book, not that there’s anything wrong with watching a good movie from time to time.

I am on a swim team and practice everyday. I was wondering if the chlorine in the water causes health issues.

Maybe. Chlorine is a disinfectant. When chlorine comes into contact with foreign substances it’s designed to disinfect, disinfection byproducts (DBPs) are produced. Some of these DBPs appear to be toxic. For example, chlorinated pool water and urine or sweat beget nitrosamine carcinogens (the same type of compound that forms when we overcook bacon).

One study found over 100 chemical byproducts in swimming pools, many of them toxic. Before and after 40 minutes of swimming laps in such a pool, healthy subjects’ biomarkers were tracked and recorded. One marker that increased after swimming in the pool suggested increased lung permeability and inflammation, while another increased marker indicated DNA damage.

That said, a more recent study had swimmers swim 40 minutes in three different indoor pools, each with varying levels of DBPs. Swimming in pools with higher DBP content had no greater effect on oxidative stress indices. The one weakness is a lack of a true control group—swimmers in a saltwater pool.

Here’s what I’d suggest:

Rinse off before swimming. Most swim teams already do rinse off prior to practice, from what I gather, so this should mitigate some of the problem.

Find a salt water pool. A recent study showed that saltwater pools contain far fewer and safer (less genotoxic) DBPs than freshwater pools.

Find an outdoor pool. Indoor pools seem more problematic, since they concentrate chlorine fumes and introduce chlorine inhalation.

Don’t worry too much. The physical and mental benefits of training and competing on a swim team likely outweigh any potential problems from the chlorine.

Hi Mark! What is a reasonable tolerance level for the vegetable oils in condiments?

For example, at a work outing today I chose cucumber slices over free cookies ( YESSS!!!) but they came with a side of conventional Ranch dressing. Or my favorite garlic mustard dip from Trader Joes has soybean oil in it…I am not using it daily and do rely on organic butter and coconut oil and other primal fats most of the time.



If you’re truly eating them just occasionally—once, maybe twice a week at the most—and cooking with and eating good Primal fat the rest of the time, don’t freak out. Sure, you could do “better” but you’re way ahead of the game compared to everyone else playing.

If you find yourself starting to rely on these convenient foods more and more often, it may be time to optimize your choices. That’s actually why I created the Primal Kitchen line of condiments. I didn’t want to have to make mayo every time I wanted some tuna salad, or whip up a fresh batch of dressing every time I wanted a big bowl of greens. Convenient foods are, well, convenient and helpful. The reality of the world in which we live is that convenience matters. If you’re going to rely on that for a significant number of your meals, you gotta use the best.

I do have a question related to exercise. I have bad knees and have had bad knees my whole life. One has been replaced and the other needs to be replaced when I am ready. Sometimes I wonder how I will ever get in shape when I have these limits that do not allow for very much lower body exercise. What do you suggest, in the form of exercise, for people who might only be able to do upper body workouts?



Don’t give up. Maybe you’ll be stuck doing only upper body, but I doubt it. Stationary bikes, elliptical machines (for all the mocking they endure, they’re not the worst piece of low-impact exercise equipment), and the mighty Versaclimber can all give your lower body a workout that minimizes impact. Check with your doctor and physical therapist.

That’s it for this week, folks. Thanks for all the questions and be sure to leave your input down below!

How’s the Challenge going?

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending more than three decades educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates flavorful and delicious kitchen staples crafted with premium ingredients like avocado oil. With over 70 condiments, sauces, oils, and dressings in their lineup, Primal Kitchen makes it easy to prep mouthwatering meals that fit into your lifestyle.

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