Dear Mark: Leg Fatigue, Too Much Muscle Meat, Starch Persorption, and Hadza Study Author Responds

Road CyclingFor today’s edition of Dear Mark, we’ve got four questions. First is a question from a reader who exemplifies the “between a rock and a hard place” situation inherent to chronic cardio. Second, I address a reader who worries that I’m not worried enough about exclusive muscle meat consumption. Third, I give my thoughts on whether starch persorption into the blood stream is a real problem for most people and a black mark against resistant starch. And finally, one of the lead authors of the fascinating Hadza gut biome study mentioned in a recent Weekend Link Love clarifies the precision of their sample preservation methods.

Let’s go:


After years of aggressive road cycling, the lone aspect I am struggling with since starting the Primal Blueprint last year is leg fatigue. Granted I am 55, but a very healthy, lean and active 55. I find through repeatable experimentation that heading out on the bike on the normal daily Primal menu results in leg fatigue, but if I carb up the night before and again 2-3 hours before the event, I am just fine, back to my old self. This is how I did it before Primal. Am I missing something? By the way, though, when I do carb up, the muscle inflammation spikes up too, dang it!

Didn’t you mention that you were writing a book on Primal for endurance athletics?



You’ve just stumbled onto the big problem with chronic cardio. In order to stay competitive, maintain the training, and perform like usual during your rides, you need to carb up – usually with the junkiest stuff you can get your paws on. Short term, it feels good. You kill the ride, you improve your time, all is right in the world.

But if you carb up enough to support your massive training load, you may end up suffering from systemic inflammation from all the training and all the sugar. Long term, your body suffers and starts to fall apart at the seams. You’re noticing that yourself.

If you ask me, this is simply more evidence that “aggressive road cycling” isn’t something we should be doing on a regular basis; at least not if your goal is optimal health. To do so requires a suboptimal, inflammatory diet that’s almost as difficult to maintain as the training.

Also, when you go low-carb and start fat and keto-burning, you lose your top end for a while. There may be a lengthy adaptation that’s even lengthier if you’re not strict about it. Carbing up, particularly with non-Primal sources, may extend the adaptation period. If you’re not already, definitely start lifting heavy once a week in the gym, focusing primarily on the lower body. Squats, deadlifts, leg presses, lunges – that sort of thing. Doing heavy maximum output leg training in the gym once a week may help with adaptation and can actually increase the resistance of your muscles to fatigue during training.

There are ways to “carb” up the night before planned hard workouts (and you shouldn’t make every workout hard – maybe just twice a week) that minimize inflammation and maximize usable glycogen. Note the term “usable” that means that you might not need more than 150 grams total to top off muscle glycogen stores, especially since you’re only going to be drawing on (and depleting) leg glycogen on the bike. Fruits are a good way to do that, as are tubers.

Ideally, you just limit this kind of training. At any age but especially yours (and mine), that aggressive riding is hard to maintain and still thrive. Hard long rides for fun? Sure, I might even go for those once a year or so. If you insist (and I know you will because I know the feeling!), there is a better way to do this kind of training, which I’ll be exploring in my upcoming book, Primal Endurance. It’ll be right up your alley.

Dear Mark,

I know you’ve talked a great deal about what we lose out on when we shy away from the fattier cuts. However, I’ve never heard you say anything about the possible hazards of eating too much muscle meat. I think people read about how hunter-gatherers ate so much animal and think “Oh, well I can eat 2 pounds of steak every night” but hunter gatherers would never have eaten that much muscle meat at a time. Is it possible that a lot of the red flags about meat consumption are true and just misunderstood? Is it unhealthy to eat so much meat and so little of the fatty organs?


I completely agree. The only point I contest: I have addressed this in previous posts.

I’ve come up with methods to trick oneself into eating organ meat because I recognize the nutritional importance of eating them.

I even took readers on a visual tour across the edible landscape of an entire animal carcass.

When I analyzed the latest “meat will kill you” study, I wondered if some of the (tenuous and often insubstantial when you correct for unhealthy lifestyles, cooking temperature/method, processed meat intake, etc.) connections between meat intake and early mortality can be explained by our widespread tendency to only consume muscle meat and eschew organs, bones, skin, and gelatinous cuts.

I don’t know that it’s “unhealthy,” but I do think that if you intend to base your diet around animal products, you can’t expect to get the best – or even good – results avoiding 75% of the animal. From Paleolithic hunter-gatherers eating the still pulsating colon packed with fermenting vegetation to grandmothers who’d visit their favorite butcher every Tuesday for bones, oxtails, liver, and shanks, our ancestors both distant and near ate everything. That’s been the normal way to “eat meat” up until a generation or two ago, and I suspect there’s something to it.

Hi Mark,

I’ve been following your posts about resistant starch and my husband and I have both started taking potato starch in the last couple of days.

Then I read this today from a podcast by Ray Peat:

‘Adding butter or cream slows the digestion so it isn’t such a powerful insulin stimulant, but it also reduces the chance of what’s called persorption of starch granules. […] A potato starch granule happens to be very big. Other starches are more the size of a red blood cell, but a potato starch granule is several times fatter than that. But even these huge granules bigger than cells can get squeezed right through the wall of the intestine, enter the lymphatics and the blood system, so within 30 minutes after you eat starch without fat, you see the starch grains circulating through your blood, and if they’re big they’ll plug up your arterioles. Studies in mice showed that a high raw starch diet accelerated their aging. You can demonstrate areas of every organ that were being killed by plugging up the arteries.’

It freaked me out a bit and I’m wondering if this is true. I’ve always found your posts to be well researched and I trust your opinion. Is this something we should be concerned about?



I like reading Ray Peat’s stuff, you know, I really do, but I think he fixates on the minutiae way too much, often using obscure animal research and extreme conditions to support his positions.

Persorption of starch occurs, but it’s totally normal. It happens with any starch, not just raw starch, and humans clear any persorbed starch from the blood within a couple hours. In infants, it’s probably a feature rather than a flaw: human milk oligosaccharides, normally valued only for their prebiotic qualities, get into the bloodstream to mop up and clear pathogens while the immune system is still in its, ahem, infancy.

Persorption may be increased in cases of leaky gut, but what’s ironic (if you take the view that persorption is evil) is that the butyrate produced as a result of consuming resistant starch and other prebiotics lowers intestinal permeability. So if anything, eating more raw potato starch will reduce problematic amounts of persorption.

This is where taking a step back from the minutiae to look at the big, evolutionary picture can help. We can argue about how much starch humans have historically eaten, but everyone agrees that we have eaten starch for tens of thousands of years. Some cultures have eaten more than others. Many seemingly healthy cultures eat a fair amount of starch. There are problems with starch (over)consumption, especially in the modern sedentary inflamed population, but I don’t think persorption of starch granules into the bloodstream is one of the major ones.

In the end, you have to weigh your options. Do you continue taking potato starch for the direct, observed, experienced, empirical benefits to digestion, sleep, gut health, and metabolism? Or do you hang your hat on a theoretical problem, on the off-chance that persorption of starch into the bloodstream is slowly, inexorably, silently killing you? I know what I’d choose.

Dear Mark,

First, I’ve been a long time reader of your site, have met you in person, and greatly admire the effort you put in to educating the community about how to achieve personal health and wellness. In the last couple of years, I’ve only managed time to read the link love days, which are very useful for me to see what kind of news aggregates on your radar. This week I am honored that one of those news pieces you linked is to the study that I and my colleagues recently published on the Hadza gut microbiome. Thank you for promoting further interest in this field of research and in our work.

I wanted to say, though, that while I support healthy discourse in science and think it is fair to offer the conclusions with the criticisms, in our case I am concerned about the emphasis on the criticism by Rob Knight that our results may be not valid due to the ethanol/silica storage procedure, and that the soon to be media zeitgeist on our work may be ultimately dismissive of our contribution. And this because of a comment wrought out of conflicting interests (Rob Knight works with Jeff Leach on their own Hadza microbiome study). Instead, I would like to encourage anyone with criticisms to read our methods in which we of course took measures to ensure this protocol would not invalidate our results. We first validated the method by sequencing and amplifying test samples both frozen and dried (ethanol) and obtained comparable results.

So, I suppose what I am saying is that while criticisms are welcome and a natural process of science, it is not helpful and instead harmful to give platform to unfounded criticisms that have already been thoroughly addressed in our original methodology and reporting. The paper is completely open access and I welcome any and all people to read it for themselves!

Gut microbiome of the Hadza hunter-gatherers

All the best,


I appreciate the email. From what I can tell from reading your methods and your explanation of the issue, you took all necessary precautions to ensure the validity of your results was not compromised. It seems that the Hadza do indeed throw into disarray our previously conceived notions of what constitutes a universally healthy gut biome.

Great work, Stephanie, and thanks for bringing that to our attention. Please do it again whenever something seems amiss!

Thanks for reading, everyone. Be sure to keep the questions coming and I’ll do my best to answer them.

Primal Kitchen Ketchup

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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