Dear Mark: Exercise and Gut Diversity; Pregnancy and Calories

Rugby PlayerFor today’s edition of Dear Mark, we’ve got a two-parter. First up is a discussion of the latest (and only) exercise and gut flora study. It’s been making the rounds and even I gave it a brief mention in yesterday’s Link Love, but today I dig a little deeper. Find out what we can take away from it and apply to our own lives. Then, I cover a series of several questions regarding pregnancy, calories, weight gain, and exercise from a newly pregnant mother-to-be. Even though I’m a man, I offer my perspective and insight.

Let’s go:

Hey Mark,

What do you think about that new gut flora/exercise study that suggests that gut bacteria diversity improves with exercise?


It’s a great study, and the first of its kind in humans. A study in rats had previously shown that exercise has beneficial effects on gut health, with rats who engage in voluntary running on a rat wheel producing more colonic butyrate (the beneficial short chain fatty acid produced by prebiotic fermentation) than sedentary rats. For this one, the authors examined a group of professional rugby players from Ireland engaged in preseason training, along with two control groups – a normal weight (BMI <25.5) group and an overweight (BMI >28) group – to see how exercise (or a lack thereof) would affect gut microbial diversity. Gut microbial diversity is generally seen as a Good Thing. Higher diversity is associated with less obesity and overweight, better inflammatory profiles, improved immune responses, and a lower incidence of several diseases, including autism, inflammatory disorders, and gastrointestinal disease.

By now, chronic stress is widely accepted as having a negative impact on gut health. Exercise is a stressor, with more intense exercise being more stressful, so you’d think a group of professional athletes in a rigorous training camp might have suboptimal microbial diversity in their guts. But the opposite was true. Despite their huge exercise load, rugby athletes had far more microbial diversity in their guts than either of the control groups. They also had less inflammation and better metabolic markers.

What gives? How were they able to withstand all the exercise?

This study examined professional rugby players during preseason training, and I think that’s a salient variable. Preseason is less grueling than the season. There’s more strength training, more regimented rest periods, and more overall control of the schedule. They’re going hard, no doubt, and the actual training that occurs in the preseason is more intense than the regular season, but it’s more measured and predictable since they’re not juggling training with playing matches. They’re not just killing themselves on a daily basis with neither rhyme nor reason. Pro rugby players are treated very well. Coaches and trainers tailor individual training and recovery programs to their strengths and weaknesses. Dietitians build them diet plans. These aren’t amateurs holding down day jobs and destroying their bodies with excessive high intensity exercise in their free time. These aren’t Ironman hopefuls training four hours a day. They’re big, solid dudes walking around with low body fat, high lean mass (BMIs in the realm of 29 with 16% body fat, according to the study), eating good food, recovering well, getting massages, doing yoga, taking days off when they need them, and monitoring their progress/adherence with regular checkups. Their lives are dedicated to training and recovery. That makes a huge difference.

There were other differences that probably impacted microbial diversity:

Athletes ate more fruits and vegetables, a good source of fermentable fibers that promote microbial diversity. If you starve your gut bugs, they won’t diversify.

They ate way more protein than the other groups, and dietary protein was strongly associated with increased microbial diversity. Remember the study from a few months back showing that a zero-carb, zero-plant, all-meat and cheese diet resulted in “unhealthy” gut flora? Remember how people took that to mean this Primal stuff was a recipe for microbial disaster? I guess if you eat plenty of meat with plenty of fruits and vegetables – you know, like a normal person following a standard Primal way of eating – your gut bugs do okay. Better than okay, even.

They also snacked less than controls. Snack food is typically awful stuff, processed and full of industrial, gut-starving ingredients. I suspect that grazing all day doesn’t help, either. Eat solid meals, let your gut bugs go a little hungry in between feedings. They need to be fed, but they probably don’t need a steady, endless trickle of food. Some gut bugs often eat the byproducts created by other gut bugs upon fermentation of fiber.

To determine whether this study means anything for you, ask yourself a few questions. Is your training improving your life and health? Are you recovering from your workouts? Are you progressing? How’s your gut function? Are digestion and elimination going well, as far as you can tell? By and large, most people engaged in regular exercise experience improved gut function. If you can answer those questions in the affirmative, it’s a safe bet that your training is having a positive effect on your gut.

If you can’t dedicate your life to training and recovery, your gut probably can’t get away with a pro-rugby player’s level of activity. That said, your gut definitely can’t get away with being sedentary.

Hi Mark…..

I just found out that I’m pregnant and I really want to stick to primal eating during the whole 9 months. So far so good, but I would like to know what would be an ideal weight gain. Should I consider an extra intake of calories or should I eat intuitively?

Is it safe to hit the sweet spot?, fast 12-14hours in this stage? What about sprinting?

Sorry for so many questions but I really want to manage weight….I’ve seen too many friends gaining so much weight that after delivery over a year they are totally out of shape……

Thanks for sharing your knowledge,


Congratulations, first of all. Let’s address your questions one by one.

Calorie intake/weight: Eat when hungry. Sometimes your appetite will be non-existent. Other times you’ll be insatiable. Just go with the flow, and if that means you eat an “extra” 1000 calories one day and half that the next, it’s going to be okay. There’s a difference between nausea and lack of hunger, though. If you find yourself not eating much for a stretch of days because nothing sounds appetizing – but you’re still hungry – then you should make a point to eat something.

Try not to stress about the weight. I know, I know, it’s easier said than done. But “managing” your weight during pregnancy just turns the whole endeavor into a big chore. And from what I gather, pregnancy in and of itself isn’t the easiest thing in the world. Don’t add the stress of counting the calories you’re eating, fretting over the entire pineapple you just devoured, beating yourself up because you’re just too tired to exercise tonight, or worrying how stubbornly the weight will stick around after you’ve given birth.

Fasting: Eh. It might be safe, but I really wouldn’t risk it. You probably shouldn’t be trying to lose weight or diet down when pregnant. Your body is all about nurturing that baby growing inside you, despite what you might prefer. These days, we emphasize the primacy of the mother’s (and father’s to a lesser degree) desires. She’s the boss, she makes the decisions. If she wants to lose weight, she can do it because it’s her choice. But your choices may very well butt heads with your physiological directive. I’m not saying you can’t try to lose weight. You can. Go ahead. Just don’t be surprised when your body resists the weight loss and you start feeling “off.” We are baby-making machines (both men and women) as far as our base physiology is concerned. It’s a tough situation.

Sprinting: Sure, as long as it’s comfortable. First trimester should be fine. Don’t sprint if you feel the baby bouncing around in there. Make your sprints shorter than you normally would, like 5-10 seconds each with plenty of rest. Try not to turn it into a sprawled-out-on-the-floor-gasping-for-breath situation, because that indicates you’re really depleting your precious energy substrates. Keep ’em short and hard with lots of rest and you won’t be stressing your body out too much. Cycle (bump permitting) or swimming sprints are a good option if running doesn’t work.

Just keep moving as much as possible. When sprinting and heavy lifting and the more intense stuff get unfeasible, don’t stop walking every day. Do what you can, and make the daily activity a habit. And continue the habit after the kid arrives. I think a lot of women (and men) get overwhelmed with the immensity of parenthood and fall out of their activity habits – if they even had them to begin with. When you’re stumbling through the dreamy haze of the first few weeks, sitting on a comfy couch in front of the TV is far more attractive to your sleep-deprived brain than going for an afternoon walk. You shouldn’t expect to keep up at CrossFit as brand new parents tag-teaming a newborn, but you can still go for the walk. Do so. It will make a difference.

While we can learn a lot from the anthropological records, pregnancy and birth have never been minor, everyday affairs. The stories of pregnant women walking off into the bush to squat for a few minutes and come back with a healthy, happy, crying baby are nice and all, but I don’t know if they represent the universal birth. There is no universal birth experience. There are many experiences and many stories, and they differ greatly. Realize that women and babies alike died during childbirth. The variable we invoke to counter the common anti-Primal/paleo “cavemen only lived to 30!” talking point cannot be ignored here: infant mortality was high, and it’s possible that fetal malnutrition was a common predisposing factor. The 20-25 pounds modern OBs typically recommend women gain during a pregnancy might be on the higher side of historically/anthropologically normal, but it’s a good, safe buffer for many women on less nutrient-dense diets than Primal. It’s better to overshoot than undershoot when it comes to growing and sustaining a human.

I’ll see if Carrie has any other advice for next week or the following one. After all, she’s been there.

Thanks for reading, everyone. Let me know what you think in the comments!

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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