Dear Mark: Growing Appetite, Boxing as Cardio, and Ammonia Sweat

BoxingFor today’s edition of Dear Mark, I’m covering three questions from readers. First, how does a Primal family handle the growing appetite of a growing prepubescent without resorting to cheap fillers? It may involve reassessing our definition of “filler,” for one. Second, does boxing – an intense, demanding sport by any measure – qualify as chronic cardio? It’s intense, to be sure, but what if you really, really enjoy and thrive doing it? And finally, should you worry if your sweat smells like ammonia? Some say it’s a sure sign of impending doom, others wave it off as totally benign. Find out what I think below.

Let’s go:

Dear Mark,

We have a growing 12 year old boy whose appetite has also been growing with him. His diet is 75% primal as he eats what we eat with a few exceptions (bun with burger, occasional pasta etc). We obviously want to stay away from the easy cheap fillers – white rice, potatoes, pasta so do you have any suggestions as to how we can feed his appetite without having him eat us out of Primal house and home? What foods/dishes would you recommend we incorporate into his diet that will satisfy his hunger while also continuing to live a healthy lifestyle?

Thank you,


Honestly, don’t underestimate the power of the filler. Growing kids sometimes just need sheer caloric density to maintain and support that growth. Yeah, micronutrients are extremely important, but you can’t satisfy all his caloric requirements with kale, chard, oysters, liver, and wild blueberries. Provide ample amounts of healthy fats, animal parts, and vegetables at every meal – which it sounds like you’re already doing – and round out the calories with some “filler” foods that contain starch to provide caloric bulk.

I can hear you balking. Adults for whom growth means bigger belts and added risk of chronic diseases? Filler foods should be limited, absolutely, in favor of the most nutrient-dense foods you can find. Kids are a different story. Growth is good, is physiological rather than pathological, and they need both. Some ideas:

White potatoes: Believe it or not, white potatoes are quite nutritious. And they make a good source of resistant starch when allowed to cool overnight in the fridge. They get a bad rap because of french fries fried in two day-old reheated oil, hash browns cooked in vegetable oil, potato chips, and other junk food. That’s how most people eat potatoes (and it’s why they’re the most widely consumed vegetable in the US).

Sweet potatoes: Primal darlings, the sweet potato family is both carb and nutrient-rich and perfect for a growing boy in need of energy. Check out an Asian grocer for the purple Okinawan sweet potatoes for a massive dose of anthocyanins (the purple pigments with antioxidant effects, also found in blueberries and other purple/blue foods).

White/wild rice: Yeah, rice. I said it. It’s easy to make, it’s relatively devoid of antinutrients, and it can be a great vehicle for nutritious ingredients. Best of all, rice absorbs anything you add to the cooking water. For example, instead of water, use homemade bone broth, a pat of grass-fed butter, and some sea salt to cook your rice.

Winter squash: Who doesn’t love a nice butternut squash? The downside to these is that quality really matters. With tubers, quality is fairly constant. It’s hard to find a really bad sweet potato, or a truly awful Russet. But a bad butternut squash is bad. It’s bland, watery, fibrous, and not even really worth eating. To avoid such tragedy, go for heavy squash, dense ones that just feel weighty in your hand. Hold two squash up of comparable size, one in each hand, and choose the heaviest.

Hope it helps!

Hi Mark,

Before embarking on my primal journey I was a keen boxer. Once I started to learn about the primal principles I gave up boxing in favour of weights and the occasional sprints, fearing that my boxing was too much like chronic cardio. Fast forward 3 years and a stall in my weight loss, I have recently taking up boxing again – and quickly remembered everything I loved about it – the sense of power, confidence, lightning quick reflexes and participating in a group. My weight is finally shifting again and after an intense boxing class I feel relaxed for days.

So I was wondering – is a 1 hour boxing class really like chronic cardio? Or is it more like ‘play’? I would love to continue to box guilt free.

Thanks so much


There was a huge physical component to my abandonment of endurance training – the arthritis, the tendinitis, the inhuman amounts of junk carbs it required me to eat and the subsequent metabolic fallout – but a big reason I stopped running marathons and triathlons was because I stopped loving and began hating it. Endurance grew into a chore that seeped into the rest of my life, intruding on my thoughts when I was trying to enjoy myself with my family and friends. You say boxing relaxes you for days after a session; all I could feel after a training session was the dread of having to do it all over again the next day. I couldn’t even read a book or enjoy a romantic dinner without worrying I was wasting my time, valuable time that I should be spending in preparation for some event off in the future. That psychological component in addition to the physical effects made it chronic cardio.

You should never feel guilty doing something you truly love, especially if the objective, empirical effects on your life are positive. You’re losing body fat – that’s an undeniably positive effect (as long as you’re not losing too much body fat). You’re more relaxed. You’re part of a community. When I read about the power and confidence and quick reflexes you feel when you box, it sounds like you’re hitting that “flow” state we’re all after where you merge with the activity itself and exist only in the moment. And maybe most importantly, you’re having fun.

Besides, the intensity of boxing isn’t like the intensity of training for high intensity endurance events. In one experiment, an hour of “boxing training” expended about as much energy as running 9 kilometers in 60 minutes (a bit slower than a 10 minute mile pace). That’s pretty easy and not too stressful just looking at total expenditure, but they don’t explain what the training consisted of. Was it lots of short bouts of sparring interspersed with short bouts of rest, as most sparring goes? That’s a far cry from ceaseless pounding of the pavement.

Definitely stick with it. Boxing can get incredibly intense, so depending on how you train, boxing may be able to replace or supplant the sprints. I would continue with weight training.

Just watch out for the head trauma.

Ammonia smelling sweat: Is it just normal (assuming your daily carb intake trails around 50g & approx. 1g of protein per kg body weight and a 40:60 protein to fat consumption ratio) or an alarm signal? Comments on forums (including yours) range from “Nah, take it easy, dude …” to “This is a serious warning signal and it should scare the sh*t out of you!”. Can you shed some light on this? Thank you!


Ammonia is a natural byproduct of protein metabolism. Whenever protein is metabolized for energy (rather than incorporated into muscle tissue), we produce ammonia. Normally, the liver converts it into urea, which is less toxic than ammonia and more easily excreted. But if urea is saturated or the liver is experiencing problems, ammonia can back up and require additional excretion avenues (like sweat).

Ammonia excretion in sweat during and following exercise is normal because our muscle protein is constantly being broken down. It’s also constantly being replenished, which is why lifting a barbell doesn’t result in a net loss of muscle, but rather a net gain over time. But the point is simple: exercise breaks down protein, and broken-down protein releases ammonia. And some of that ammonia shows up in our sweat. Cycling at just 40% of maximum heart rate – a relatively easy pace – produces ammonia-tinged sweat. It really doesn’t take much.

Low carb diets lead to increased ammonia production during exercise, and this is completely expected. Low-carbers often have reduced muscle glycogen, which leads to a greater reliance on protein during exercise. Not only are you creating ammonia via exercise-driven protein catabolism, you’re also creating it by converting protein into glucose via gluconeogenesis. All totally normal.

If this is worrying you, you can always eat a few more carbs on workout days. You can make sure those 50 grams of carbs you’re already eating come from starches and fruits, rather than green vegetables (which don’t really count toward your carb count). You can also get a liver function test to rule out hepatic insufficiency.

My bet? You’re turning to gluconeogenesis to fuel your intense training. You can get by and even thrive in endurance training on low-carb, provided you’re fat-adapted. But in the context of high intensity, glycogen-intensive training, your body’s going to get the glycogen however it can, either through gluconeogenesis from protein or by utilizing dietary glucose. You might as well just eat glucose, its direct precursor. It’s cheaper that way and minimizes the buildup of ammonia. Consider carb refeeds two or three times a week to saturate your glycogen stores; it doesn’t take much.

Another factor may be insufficient fat adaptation. I’ll be dealing with this very subject in my upcoming book, Primal Endurance, which is all about fueling and optimizing performance on a high-fat diet. The focus is endurance performance, but with a few tweaks you can get pretty close to optimal high-intensity, normally glycogen-dependent performance with full fat adaptation. Stay tuned for that one.

That it for today, everyone. Thanks for reading and keep the questions and comments coming! Be sure to chime in if you have any additional advice for today’s questioners.

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.

If you'd like to add an avatar to all of your comments click here!