Dear Mark: Brain Caloric Expenditure, Muscle Preservation, Potassium, and Iron Supplements

ThinkingFor today’s edition of Dear Mark, I’m covering four of your questions. First up is a question from Rachel, an English major who finds herself growing ravenous once exam time rolls around. I discuss whether or not her increased cognitive activity is increasing the amount of fuel her brain is burning through, and whether this affects her hunger. Next I provide a few tips to a young athlete who’s injured and has to rest, but doesn’t want to lose any muscle mass or curtail his fitness pursuits in the process. Luckily, there are a few dietary modifications he can make to preserve that lean mass. Third, I discuss the importance of potassium. It plays a few vital roles in the maintenance of our health, and if you want enough, you’ll have to consume some plants. Finally, I field a question from a woman who’s worried about iron supplementation. She’s not doing it, thinks she might need it, and wants my take on the subject. I explain why it’s probably better to get your iron from food, rather than supplements – which may do more harm than good.

Let’s go.

I LOVE your site!!! I’ve been reading it for a while, but just made the change from a CW “healthy” diet to a primal one this past October and I’m NEVER looking back!

My question is this: I’m a college student, and I always get SUPER HUNGRY during exams. I’m an English major, so exam time actually usually means LESS stress and MORE sleep for me since papers and readings are finished. Does studying intensely actually increase your fuel demand?


Our brains are incredible energy hogs, making up only 2% of our body mass while using up about 20% of the energy we take in. So during a period of heavy thinking, say, finals week, you’d expect the energy demands to increase and our overall caloric intake to follow. It seems like that should be the case, but the evidence is mixed:

In one study, subjects performing a verbal fluency test increased the cerebral glucose metabolic rate by 23% (for the duration of the test) compared to the resting control group. Subjects who did better on the test burned less glucose, probably because their brains were more efficient at performing the task. Oddly enough, this other study found that high aptitude subjects (people for whom the tasks were easier) used more glucose than lower aptitude subjects (people for whom the tasks were more difficult). Clearly, it’s not quite as simple as “hard task = more fuel required.”

Another study found that a period of “intense cognitive processing” led to measurably lower levels of blood glucose than a period of resting, indicating that actively using your brain burns more glucose than not using it. In one of my favorite books, Why Zebras Don’t Get UlcersRobert Sapolsky notes that top chess players engaged in intense competition often have metabolic requirements similar to top athletes during training.

Walking, which uses fuel, has been shown to improve cognition. And I don’t mean walking regularly makes you smarter through a training effect (although that’s also true!); I mean that walking improves your brain function immediately. If improving brain function were as simple as increasing glucose availability, this shouldn’t happen.

I’m not totally convinced that mental tasks have no effect on fuel consumption. I think part of it is that recreating the frantic mental atmosphere of finals week can’t really be replicated in a lab setting. One to two hour studies where subjects are being paid to memorize numbers or colors or perform “arduous” mental tasks just don’t measure up to the reality of studying as if your future depends on it (it probably doesn’t, but it sure does feel like it!) for a week straight. The Sapolsky mention of chess players’ metabolic demands lends credence to this.

There’s also the stress component. You say you’re not very stressed out because you’re getting plenty of sleep and don’t have to worry about papers, but studying for an exam – or multiple exams – is generally not a pleasant, enjoyable experience. Watching a gripping documentary or reading a great novel? It’s stimulating and requires brain activity, but it’s ultimately experienced as a pleasurable encounter, and you don’t leave the theater exhausted or the library starving. Scary movies, on the other hand, do increase caloric output, probably because they are slightly stressful. By definition, studying for the big exam is placing stress on your system, and the body responds by increasing cortisol, adrenaline, and turning on a bunch of physiological processes which burn through fuel reserves.

The point is that if studying is making you hungrier than normal, studying is increasing your fuel demand, whether it’s because of increased stress, increased metabolic demand by the brain, or both. Maybe you forgot to eat. Whatever the cause, you should probably eat.

Hi Mark,

I am an 18 year old male from Australia who regularly competes in middle-distance athletics (800-3000m) events, but recently rendered sedentary with a painful varicocele. In order to best maintain muscle mass and CV fitness should I up my fats, limit any carbohydrates, and up protein? What do you think are the most essential foods/drinks for fitness maintenance whilst completely inactive?

Thanks a lot,


Sorry to hear it – that’s tough. I know how hard it is to sit out, and I have a few bits of advice:

When it comes to retaining lean body mass while on bed rest, protein is king. Try to get a gram of protein per pound of lean body mass.

You might also look into branched chain amino acids (BCAAs), which have been shown to attenuate the loss of lean body mass and protein wastage during bed rest.

Try not to restrict calories too much. Since you’ll be inactive, your needs will probably be lower, but dropping them even further will only speed up the catabolic process and cause you to lose even more lean mass.

Other than that, just keep eating nutrient-dense food. You probably won’t be as hungry as usual, so make sure you’re not “wasting” your calories on junk.

Unfortunately, there’s not much you can do to maintain your cardiovascular fitness while doing absolutely nothing. The good news, however, is twofold: keeping as much lean mass as possible and being a young guy will help you bounce back faster. You won’t lose much.

Dear Mark,

I’ve have begun to use FitDay to ensure I’m getting all my essential vitamins and minerals. It seems I’m consistently low in potassium (around 80% RDA). My diet consists entirely of meats, seafood, veggies (mostly raw), nuts, and berries (every other day). I’ve been cramming veggies to make up the imbalance. Is this what I should be doing? Perhaps the RDA is higher than I need on this diet? Or should I consider a supplement?


Mark in DC

That sounds like a solid, healthy diet, for the most part. However, if you’re coming up short on potassium, you probably want to take care of that. Potassium is a very important mineral, performing a range of functions in the body:

It regulates blood pressure, with some studies even showing that it’s more important in the fight against hypertension than sodium.

Potassium regulates the excretion of calcium in the urine, which is probably why its consumption is associated with high bone mineral density. More potassium, less bone in your pee. You don’t want to urinate bone, do you?

Humans also aren’t very good at preserving potassium in a state of deficiency, which indicates to me that we likely evolved in a dietary environment where potassium-rich plants were readily available. It also indicates that potassium is a crucial micronutrient.

Considerable evidence exists that it’s not the absolute amounts of potassium or sodium that are so important, but rather the potassium:sodium ratio – particularly in men. Track your sodium intake for a week. Shoot for around three to four times as much potassium as sodium, but preferably more than that. Bonus of eating more potassium? You can eat more salt without negatively affecting your blood pressure.

Besides the usual potassium-rich suspects (spinach, sweet potatoes/potatoes, avocados, bananas, most plant foods), dairy products like yogurt are surprisingly high in potassium. If you do dairy, consider adding a cup or two of dairy a day. Fresh meat also has potassium, but we end up cooking most of it out. Consume your meat on the rarer side, or toss it into stews and soups, and you’ll consume more potassium that way.

Should women eating a Primal Blueprint diet and taking your damage control supplements take additional iron supplementation?

Thank you for all you do!


Probably not. Iron supplements have a generally poor track record, which is partially why I kept them out of my supplements. They have a history of exacerbating gastrointestinal distress, and, if you don’t actually have a need for supplementary iron, iron overload. Having too much iron is associated with atherosclerosis, cartilage degeneration, fatty liver, type 2 diabetes and a host of other maladies. Among anemic women, iron supplementation increases oxidative stress even as it corrects the anemia, making “blind supplementation” all the more dangerous.

Besides, there are so many easy ways to obtain iron from your diet, particularly one of the Primal variety. Eating meat, even one that isn’t particularly famous for its iron content like pork, is as good as or better than iron supplements at repleting iron stores with fewer side effects. Something like beef, lamb, or organ meats will be even better sources of iron (in addition to tons of other important micronutrients like zinc and vitamin A).

Before you worry about iron, figure out if you even have an iron issue. Get a full iron panel done, and be sure to test for ferritin (the storage form of iron). Only then should you explore actively increasing your iron intake, whether by food or supplementation.

Thanks for reading, guys. Take care and Grok on!

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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42 thoughts on “Dear Mark: Brain Caloric Expenditure, Muscle Preservation, Potassium, and Iron Supplements”

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  1. #3 and #4 = supporting evidence for my rare steak habit.

    (Cooked rare, not rarely eaten, of course).

  2. I’d like less folic acid in the Damage Control Master formula, too. I understand that it is not indicated for men or women past childbearing. It may even contribute to osteoporosis or fractures, right? A Primal eaters should be getting some from green veggies. Supplementing 800mcg a day is too much.

    1. From what I’ve read (e.g. from Chris Kresser), folate seems to be the better form to take. There is some indication that folic acid increases the risk of cancer.

    2. It can’t be all that bad. After all, it’s presence is only indicated as B-nign(9) on the label.

      1. Haha this is great. If only I loved scary movies… 😛 Maybe I’ll burn more calories though since my stress levels would be through the roof.

  3. I second the sweet potatoes and full fat yogurt! I think everyone should be eating sweet potatoes…even eating one a day it’s hard to approach 150 g carbs, much less enough for a refeed day…

    1. On the subject of sweet potatoes, I’m struggling to find Mark’s post regarding the amount of carbs he recommends adding to your daily intake per hour of cardiovascular exercise. Anyone have a link/know off the top of their head?

      Thanks and apologies for the somewhat off-topic post.

      1. Not sure of the specific post, but there is the PB carbohydrate curve. Trying to lose: <100 g, hold steady: ~150 g, etc…I do a lot of strength training and like to ramp up the carbs post workout on those days—I actually treat any cardio I do (all of which is a means to another end, ie mountain biking)as a rest day and scale the carbs back and increase the fat. Anyway, I also think Paul Jaminet's 1 pound of safe starches a day works very well if you're an active person. Experimentation is really the only way to find out.

    2. Any difference in starch availability between raw and cooked sweet potatoes?

  4. It’s always best to get your vitamins and minerals from food, but that can be expensive! Wish I could eat all the meat, seafood and veggies that I wanted, but it’s just not in the cards right now. I would never take an iron supplement though. That’s an easy one to obtain from food.

  5. I definetly burned through some fuel reserves in highschool. Was always hungry and tired after the rare class where I actually applied myself (lol).

    And still to this day in college it happens, but luckily my knowledge of nutrition has grown in direct proportion to my appetite, phew!

  6. Just a quick note about Potassium: the food with the highest amount is beet greens (about half the RDA, in a one cup). Most people suggest bananas, but I think that’s just because people love to eat sugary things. In reality, bananas don’t come close to beet greens for potassium.

    1. Also coconut water is really high up there on the potassium chart. It tastes great after some sprinting or a bodyweight workout! The colder the better.

  7. Folic acid is dangerous for people with MTHFR and since almost 1/2 of the population has MTHFR and does not know it-all should stay away (or get tested). Folic acid is synthetic and inactive and people with MTHFR store it in their bodies to the detriment of their health. Folate (as metafolin) is active and a far superior form and can be utilized by all.
    I am convinced that the rise in cancer in the last decade is due to the fortification of folic acid in bread, rice and cereal that began in 1998. Luckily,being Paleo we avoid most fortified folic acid.

  8. “Walking, which uses fuel, has been shown to improve cognition” So true. Anytime, I’m stuck at a problem, the solution comes much faster if I’m walking. Even if its around the house.

    1. I always walk in the morning and at the end of the day. My best thoughts and ideas occur when I’m walking.

      I never thought it was improved cognition. I thought it was just me. 🙂

  9. As a psychophysiology grad student, who is more interested in the body as opposed to the brain. Our nervous system is about 80-90% afferent (from body to brain) and only 10-20% efferent (from brain to body). It’s being found more and more that interoception (perception of bodily signals) can impact very complex emotions and cognitive facilities. So working your brain is pretty much the same as working your body. In order to access thoughts and memories, your body relives them at a low level in order to reactivate the afferent parts of the brain that were responsive when they were stored. Granted the brain does a lot of work, but that isn’t close to the whole story. Not that we have much more than a clue about the whole story.

    1. Could you say that again in a different way? Like say for an accountant reading.

    2. Hi Sean

      I think this is really interesting and it makes sense too. I’d like to read more. Could you please provide some links or references?


  10. You’re making me think about the iron issue! I was severely anemic before being diagnosed with Celiac, & still feel iron anxiety. But now I eat more meat & zero gluten, so I may be overdoing it.

    1. If you’re celiac you should look at GAPS by Dr. Natasha Campbell-McBride.

  11. In his book The Future Eaters, Tim Flannery suggests that evolution has caused koalas to shrink their brain-size in relation to their cranium, ie there is a gap between the brain and the bone. His explanation is that the couple of eucalypt tree species that the koalas prefer to eat, don’t ‘want’ to be eaten because of the naturally low nutrient availability of most Australian soils. Therefore, they have chemicals in their leaves that are poisonous. The koalas’ evolution had to weigh up between eating enough to survive: without poisoning themselves and basically falling out of the tree to become dingo food. This also explains the koalas’ habit of sleeping for a distinctly large proportion of their lives. The brain size reduction was to save energy.
    Flannery: “Marsupials are notorious for having tiny brains, but the koala has really gone out on a limb because it’s the only mammal on earth whose brain doesn’t fit its skull, 40% of the space inside there is just fluid, and that’s because the brain is a real extravagance. It takes more energy to run than any other organ in the body. And the koala has traded brain power for survival”.

  12. Ever since I went paleo my iron increased quickly! I went from a ferratin level of 33 to 44 over the transition…awesome! I added in grains again a few months ago (when I went to a dietician for my eating disorder) and my blood results came back with a ferratin level of 25…Not sure if it’s just the increase in mileage or grains or what, but I’m pretty sure it was from including gluten in the diet again. Ever since I’ve moved back to potatoes for my carbs (loving it so much more) and we’ll see what my ferratin levels are soon after my next blood test!
    Thanks again for all your posts, these have great info!

    1. Rachel, I looked over a few of your past posts on your blog. Do you have a Friday success story?

  13. What happened to Unignoranted comments? I was interested in what he had to say about starches etc and ALL of his comments have been removed?

  14. I have some issues just like Rachel the first to be addressed by Mark. I also notice that every time I think a lot, or need to study for exams, I end up getting hungry more often than usual. Thanks Mark for addressing this concern.

  15. Very nice posts mark, keep up the good work!
    I also have noticed that when studying, i get more hungry than usual and sometimes it tires me out more than physical work.

  16. Those of us eating a Paleo diet usually do not need to worry about iron, because we eat enough meat and we do not eat grains or legumes. I wholeheartedly agree with Mark that iron is best obtained from food.

    However, if you find your ferritin levels are low despite eating a Paleo diet, you may want to consider removing nuts and seeds, which are very similar to grains and legumes in their composition, and therefore contain the same anti-nutrients, including phytic acid, which interferes with iron absorption. Also take a look at your oxalate consumption–oxalates are found in veggies like spinach and rhubarb and can also interfere with iron absorption.

    If you choose to take a supplement, traditional iron supplements are very irritating to the gastrointestinal tract and are poorly absorbed, so they don’t work very well. “Heme” iron is the form found in animal foods and is available in supplement form on the internet. Heme iron is gentle on the digestive tract and easy to absorb.

  17. Timely advice on nutrition while recovering! I’ve been laid up for a week and lost a pound, and was suspicious that it was muscle weight. Now I know why I was craving beef and fish. Thanks!! It’s hard being hurt and laid up, and now I won’t worry so much about the effect of this inactivity.

  18. I read somwhere that cooking in a cast iron pan is also an easy way to add iron. Anyone know if this is true? Curios as unless baked all of my meat and eggs are cooked in cast iron.

  19. I have been taking iron supplements for a year now but after reading your article about its negative effects, I got a change of mind. I will go with what you said. Maybe I’ll start obtaining iron from the foods that you have suggested.

  20. They say energy can’t be destroyed, just transformed. So if heavy thinking can convert glucose into thoughts, where does it go? Does a thought have a physical form that could be converted back into another form of energy? Or does all the used glucose get turned into heat, or light or sound in the brain and the thought is a by-product?

  21. I’m currently reading the excellent book called “Willpower” by Roy Baumeister and John Tierney.

    Willpower is something we all struggle with.

    The studies in the book found that a lack of glucose in the body has a huge impact on your ability to focus and concentrate. Basically your brain needs a good dose of glucose to stay focused and get the job done.

  22. Women need to take iron supplements since they are at risk for blood loss due to their menstrual cycle every month. Taking iron supplement will prevent diseases such as anemia, also, you can just eat green leafy vegetables, organ meat especially liver. These food are high in Iron which would help you to become stronger.

    Potassium on the other hand is not just associated with the Sodium in our body, it is actually related to the contraction of the heart, which means, if you have a low level of potassium, you are at risk for experiencing cardiac arrhythmias. So, for you to avoid this scenario, eat fruits such as orange, Apple, Banana, etc.

  23. The ratio of potassium to sodium in fresh meat is 5:1. In most plants the ratio is more like 80:1.

    I also noticed that when I ate a very low sodium diet with lots of vegetables, that I would get muscle cramps after eating a rare bag of pretzels. Like Mark said, human evolved in a potassium-rich environment (meats and vegetables) but had to hunt for salt. As a result, the body’s ability to rapidly sense and respond to changes in potassium intake never developed to the extent that it did for sodium, where a deficiency could be dangerous. As such, the body developed an exquisite balancing mechanism for sodium and could rapidly increase or decrease sodium excretion. I believe (but have no proof) that my muscle cramps were caused by sodium-induced potassium excretion since it attempts to maintain constant optimality. When I eventually increased my sodium intake to more normal levels, the problem disappeared.

  24. I was on the chess team in elementary school and played in tournaments. I think the games were 20 or 25 minutes. It was a draining endeavour.
    The one day I could feel the bags under my eyes becoming more prominent from solid concentration, verified by my friends and a mirror. Cortisol levels must have been high.
    I played a lot games that day, got frustrated by someone who cheerily and perhaps intentionally distracted me by saying it smelled like rain.and so on. (the gym doors were open). Lost that game.
    There was a local chess master from one of the schools, not competing in the tournament, 12 years old or so, who had four boards set up and he was beating people on a whim moving from board to board.

  25. just wanted to say what a wonderful site, just downloaded all the free books

    when I’ve digested all the info and know what I am going to do I will start ordering – amazing resource

    I had gotten into the ‘We Want To Live’ lifestyle but let it slide as it was impractical

    thanks for the inspiration

  26. In his book, The Brain that Changes Itself , Norman Doidge, the author suggests or quotes that walking improves brain function in part because the brain anticipates the challenge of mapping the walk (hunt or gather), finding way home as well as memorising landscape, fauna & flora.