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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