Dear Mark: Exercising (and Eating) in the Heat; Post-Antibiotics Gut Health Support

For today’s edition of Dear Mark, I’m answering a pair of questions from you folks. First up, what with the crazy heat wave sweeping much of the planet, a reader writes in asking about the best way to eat and move in the heat. Should you cease all activity? Should you modify your normal movement patterns and eat and drink differently to keep the heat at bay? Read on to find out. Next, how should a person deal with and support the post-antibiotics gut biome? What can we do to mitigate the negative effects broad-spectrum antibiotics have on our gut bacteria?

Let’s go:

Hi Mark,

Thanks again for all the great work you’re doing here. Having been introduced to Primal living – my latest discovery has been kettlebells, and I’m so happy about it – has been one of the best things in my life.

Now, I have a question that might interest quite some people for this summer: do you have specific thoughts and recommendations about how to manage life during a heat wave? Drinking more of course; but should it impact our diet and exercise too?

Thank you so much!


Thanks for the kind words. Much appreciated.

When it’s hot out, you have to be really careful with your activity levels. Extreme heat stress reduces the blood flow to the brain by about 30%. By “extreme” I mean extreme — body temperatures approaching 40° C — but any degree of heat stress will reduce blood flow to the brain and increase it to the lower body, albeit not to the same degree. Throwing exercise into the mix, however, will raise your body temperature further, and throwing dehydration into the mix compounds the inhibition of cerebral blood flow.

You know how I rail against the potential dangers of dropping dead from heart attacks during extreme endurance events? Heat stroke is a larger danger. One study in a cohort of endurance athletes found that for every serious cardiac event occurring during a race, ten life-threatening instances of heat stroke occurred. Some researchers even think elevated core body temperature is the proximate cause of most endurance-related events.

A few things to keep in mind regarding heat stress and activity:

Larger people actually have a higher heat tolerance during exercise; whether it’s hot-humid or hot-dry, a larger body mass improves heat loss.

Stay hydrated. I’m not one to freak out about drinking eight glasses of water a day — that’s pretty silly and unnecessary in most cases — but things change when you’re dripping sweat. Exercise in said heat makes it even more crucial. I like a pinch of sea salt and a splash of trace minerals in my water, for the electrolytes and to replace any sodium lost. Coconut water is also helpful, especially if you’re working hard and burning through both glycogen and fluids. But the absolute greatest supplement for hard exercise in hot weather is a glass of fresh squeezed orange juice with a large pinch of sea salt mixed in. I know, I know. Juice is bad, right? No, it’s a tool to be used in special circumstances. And this is a special circumstance. It ends up tasting like really good Sunny-D (not purple stuff). I can remember a backpacking trip where a liter of salted orange juice cobbled together from materials purchased from a small stand along the trail saved me. When I say a pinch, I mean a pinch. Don’t put too much salt in, as its effects on blood flow during heat-stressed exercise are unclear.

Don’t do anything you don’t have to (like marathons). Keep exercising, but do the intense stuff inside, and if you do it outside do it under a tree or an umbrella. I’m totally on board with walking in intense heat. Just let your body be the guide when you do it. If your hands are swelling up, if you feel woozy or dizzy, if you’re not urinating no matter how much you drink, cool it. Take a break. Go inside, read a good book or watch a movie. Find a shady spot under a big green leafy thing growing out of the ground. Take a nap, even.

As for diet, I always heed the traditions of the people who live and cook and eat in warm climates for their entire lives: spicy food. Whether it’s Thailand, Mexico, Ethiopia, Nigeria, India, or Jamaica, the hotter the climate, the spicier the cuisine. Sweating is your body’s way of cooling off, and sweating because you just took a face full of chili paste works, too. I’m not sure if adding chilis to everything when it’s deep summer improves your physiological resistance to the damaging effects of heat stress (though it can reduce microbial growth, another problem common in the tropics), but it sure makes me feel cooler.


I’ve been paleo since I experienced the benefits of going gluten-free 30 months ago and dug deeper into the contrarian diet (counter to “accepted” diets) it espouses.  The benefits have been life changing (I promise I’ll send in a success story soon), but include blood pressure normalization (no more pills), “cured” my osteoarthritis, lowered my uric acid (no more episodes of gout), and I don’t get colds regularly.

Unfortunately, I’ve reinjured myself and see an arthroscopic surgery on the horizon, and I know this will include antibiotics. Is there anything I can do to protect my precious microbiome, hasten its recovery, and reintroduce what might have been compromised, outside of more fermented foods (dairy and vegetable), raw foods (including oysters and other bacteria-rich yummies)?


Great question. Prophylactic antibiotics given during surgery are broad-spectrum and thus target just about everything without discrimination. Good for infectious microbes, bad for commensals. There isn’t a ton of research into countering these negative effects. What the literature tends to focus on is preventing or treating antibiotic-related diarrhea. But, since antibiotic-related diarrhea is generally caused by the trashing of the gut biome, any treatment that works against it will probably also benefit your post-antibiotics gut in a more general sense.

Fermented dairy should absolutely appear in your post-surgery (and, heck, pre-surgery) diet. It just works. Yogurt is a good option to try, if a bit inconsistent. Kefir is probably better; it’s been shown to improve patients’ tolerance to triple antibiotic therapy during treatment for H. pylori infection.

Fermented vegetables like sauerkraut are also must-eats. The fermented cabbage contains ample amounts of L. plantarum, a bacteria strain that’s been shown to prevent antibiotic-related diarrhea in piglets. Good options exist in stores, and even more are available in farmer’s markets, but the best way to get the most bacteria-rich vegetable ferments is to make your own. While you’re at it, consider making your own yogurt and kefir as well.

Supplemental probiotics are fantastic here, too: large doses of the desired microorganisms delivered directly to your gut. Some of the strains used in Primal Flora, like B. clausii and S. boulardii, have been shown to be effective against antibiotic-related diarrhea, so that could be a good choice.

Don’t forget the food for your gut bugs: prebiotics. You need to eat fermentable fibers and other prebiotics like resistant starch. Consider eating cooked and cooled potatoes, unheated potato starch, leeks, garlic, onions, green bananas, apples, pears, berries, and pretty much any fruit or vegetable you can get your hands on. Here’s where a wide variety of plant foods becomes even more important to consume because it introduces a wide variety of fermentable substrates for your gut bugs. Oh, and dark chocolate is a great source of fiber and polyphenols which have prebiotic effects in the gut.

Get dirty, too, to introduce potentially helpful bacteria. Go out and garden. Go barefoot at the park and practice tumbling, or roughhouse with your kids (or friends). Don’t immediately rush to wash your hands before eating (unless you’ve been handling raw meat and/or dog poop).

Whatever you do, don’t stress too much about the antibiotics you have to take. Stress is awful for gut health and the antibiotics are actually helpful here. Overall, it sounds like you’re on the right track. Good luck!

That’s it for today, everyone. Thanks for reading and be sure to help out with your own answers down below.

Take care!

TAGS:  dear mark

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 three decades researching and 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 Primal/paleo, keto, and Whole30-friendly kitchen staples.

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