Dear Mark: Strong but Stiff, Denatured Proteins, Weeklong Fasting, and Oxalates

Back PainToday’s edition of Dear Mark is a bit of a grab-bag. First, I give advice on how to loosen up after strength training. Being able to deadlift however many pounds is nice and all, but what if you’re too stiff on your days off to do anything with the strength? Next, I briefly discuss the effect of heat on protein powder and raw cocoa powder. Raw fans will try to scare you away from anything heated, but are they exaggerating (or outright getting it wrong)? Find out below. After that, I sort of try to convince a reader not to embark on his planned week-long fast. Or, at the very least, to reconsider the week duration and try something a little shorter first. And finally, I discuss whether or not dietary oxalates are a toxic substance that should be avoided at all costs.

Let’s go:

Dear Mark,

I go to the gym three times a week and lift weights. Once a week in addition I do some sprinting on the treadmill, and once a week I do a high-intensity bodyweight workout. After many of my workouts I feel very stiff the next day. What can I do about this? Is it worth it to build up my strength if I’m so stiff I can’t do anything with it?



No, I don’t think strength is worth very much if you’re hobbling around stiff-legged all day, too sore and functionally decrepit to translate that added “strength” into kinetic energy. Since you wrote in about this, you probably feel similarly. It’s a common intuition to have.

Luckily, there’s a simple fix that doesn’t require you stop exercising, or even cut back much: mobilize.

Start your day with a brief but effective movement session. I highly recommend drawing on this VitaMove routine by Angelo dela Cruz, PrimalCon presenter, bodyworker, and ninja warrior, for inspiration.

As Angelo says, rather than dynamic, be fluid and thorough. Don’t use momentum to throw your body parts around time and space. Seek full range of motion with every movement. Be deliberate, even slow. When you squat, for example, bend at the waist, grab your toes until your hamstrings stretch, and pull yourself down as deep as you can. Oscillate back and forth, shifting weight between your feet and really feeling the stretch in your thighs. Extend your arms overhead and stand up, squeezing your glutes and bringing your hips to full extension at the top as you stand.

My favorite hip opener when I’m short on time is the walking lunge stretch. Take a big forward step as you lunge, back knee hovering above the floor. Hold that position, extend your arms out in front, and place your palms against each other. Keeping your arms straight, move them as far to the left as you can, then as far to the right, then repeat it. Think big, sweeping, expansive movement; you should feel your fascia stretching. Lunge forward with the next foot and repeat the arm movements. Experiment with different arm movements, like chopping motions.

On your off days, go for a walk. Do about an hour, if you can. Keep those joints fluid and moving.

Also, consider dropping one of the weight lifting or bodyweight days. I think three days of strength training plus a day of sprints is plenty.

I’ve made homemade protein bars using both whey and egg white protein powders. Does baking these have any adverse effect on the nutrition or protein content? I also have a raw cocoa powder that I use in a few different things, but I also use when I make these baked bars – any issue with heating raw cocoa powder?


If heating protein resulted in an inability of the hominid digestive system to access and assimilate said protein, I don’t think we would have gotten very far as meat-eating, fire-starting, barbecuing apes. Raw foodists will talk about heating resulting in “denatured proteins” as if they’re a bad thing (and I gotta admit, they do sound kinda scary), and that’s all well and good, but denatured proteins are generally more digestible than undenatured proteins. We’re always denaturing the proteins we eat before we eat them. When we cook egg whites, the proteins become denatured and more digestible. When you stick seafood in a lime juice bath to make ceviche, you’re denaturing the proteins. That doesn’t “destroy” the proteins; it just rearranges them. They’re still broken down in the gut into amino acids.

The confusion may arise from the fact that denaturing proteins in living tissue (like, say, you) often causes cell death. Denaturation of living protein, bad. Denaturation of dead protein that you’re about to eat, good. If anything, baking your protein powder will make them more available in the body, rather than less. And besides: most whey proteins have already undergone a heating process. You’re in the clear.

As for the cocoa powder, I wouldn’t worry about heating it, either. For one, some of the earliest cocoa fans – the Mayans – consumed it roasted, not raw. And second, the roasting process actually increases the antioxidant activity of the cocoa bean. All those cocoa polyphenols we’re so interested in are actually boosted by heat. It seems the Mayans got that one right. Furthermore, many supposedly “raw” cocoa powders undergo plenty of heat stress, whether it’s during the crushing process or because there’s little consensus on what “raw” actually means.

I am considering attempting a week long fast. I have been fasting intermittently for some time now, and thought it would be a cool challenge to go an entire week. As a fat adapted “caveman” should I worry much about muscle loss, or any other potential problems for that matter?


Amino acid scavenging from existing stores will start to happen when your liver glycogen is depleted, which will occur at somewhere around the 24-30 hour mark (depending on a number of factors, including your activity level during the fast). Any dietary protein available will go toward gluconeogenesis and liver glycogen replenishment. After that’s all used up, you’ll draw on your own tissues – probably less so than the average person thrown into a week-long fast, given your ability to use fat and spare glucose, but muscle loss will still occur. A week is a long time.

Short term fasts with refeeds generally prevent any metabolic slowdowns, but a week-long fast will put your body on alert. Thyroid activity will downregulate and leptin will drop. By all accounts, you’ll officially be in “starvation mode.” This will be highly unpleasant.

In my fasting series, I wrote about how severely obese patients have had success with year-long fasts, but they were under medical supervision and had a lot of extraneous tissue to burn through. I take it that you do not, so I would advise against a weeklong fast. If you insist on pushing the limits (can’t blame you there), start smaller. Try a day-and-a-half long fast, first, and see how you respond. Go from there, and stay cognizant and realistic about how you’re feeling during the fast. Be willing to cut it short if you feel terrible.


I just wanted to ask you to add to your Primal Blueprint a warning of the dangers of oxalic acid. We hear we need to eat a lot of plant food, like dark, leafy greens. Oxalic acid in many fruits, vegetables, nuts and berries can cause kidney problems and even death. I would like you to share with the public that we should be aware of foods high in oxalic acid, not eat the same plant foods regularly, and not consume too much of it. It doesn’t take much oxalic acid and it doesn’t need to be over a long period of time, before you might get ill from oxalic acid! It’s highly toxic! There should be a warning to people about this, especially when they choose to eat Primal! (And this is why I think we were born with a sweet tooth! To avoid plant food that are high in oxalic acid!)

Best regards,


I don’t know that I’d characterize oxalic as “highly toxic,” at least not unequivocally for everyone.

In healthy guts, oxalates will generally pass through the GI tract into the stool without being absorbed and causing problems. Just between 2-15% of dietary oxalates ever get absorbed in healthy people, depending on the inherent solubility of the oxalates (almond oxalates, for example, are more absorbable than black bean oxalates). In compromised guts, oxalates will be absorbed at greater rates. People with celiac disease, which is usually characterized by a perpetually permeable gut lining if left untreated with a gluten-free diet, are at a higher risk of calcium oxalate kidney stones due to (in part) greater oxalate absorption. Certain species of gut flora also contribute to oxalate degradation (PDF), so individuals without the right species will experience impaired oxalate breakdown, leaving more oxalate available for absorption. Probiotic supplementation with the right species has been shown to reduce urinary excretion of oxalates and the formation of stones in patients with hyperoxaluria.

When oxalate is absorbed, however, the vast majority of it is excreted through the urine. Too high a concentration of urinary oxalate can lead to impaired dissolution of oxalate and the formation of kidney stones; less oxalate in the urine by volume means the oxalate is easier to dissolve.

Some people definitely have problems with oxalates, either because of intestinal permeability, hyperoxaluria (excessive urinary excretion of oxalate caused by impaired oxalate degradation enzymes and/or increased oxalate absorption), or dysfunctional gut flora. Those people will want to limit oxalates and cook their greens. Other strategies include taking a calcium citrate supplement with meals (calcium citrate binds to oxalate in the gut and reduces its absorption), taking a magnesium-potassium citrate supplement, and supplementing with probiotics.

The benefits of “high oxalate” foods like leafy greens, nuts, and other vegetables are such that I wouldn’t give up every oxalate-containing food. If you’re worried about oxalates, rotating the greens you eat (kale and collards are some of the lower-oxalate greens) and limiting the amount of raw vegetables you eat should reduce your oxalate absorption.

If you do have leaky gut, celiac, any intestinal disorder like Crohn’s or IBS, generally poor digestion, or have a family/personal history of kidney stones, oxalates are worth paying attention to.

That’s it for today, folks. Thanks for the questions and keep them coming!

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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