What Does it Mean to Be Fit?

(This is the first part of a four part series on fitness. Part 2: Could You Save Your Own Life?, Part 3: Modern Fitness Standards)




  1. The capacity of an organism to survive and transmit its genotype to reproductive offspring as compared to competing organisms
  2. The ability to conduct oneself in physically demanding situations; to function effectively in emergencies; to display superior body composition and aptitude in matters of strength, cardiovascular capacity, power expression, reaction time, speed, agility, flexibility; to evince generally superior health and resistance to injury and disease

In Grok’s time, both definitions of fitness were inextricably linked. In fact, I’d argue a Paleolithic hominid organism’s reproductive fitness almost completely relied upon his physical fitness level, whereas today’s humans wield various currencies, both immaterial and tangible, that predict their reproductive fitness irrespective of their physical strength, stamina, or endurance. A person’s bank account, education, or employment status are all considered to be better predictors of reproductive fitness. The ability to whip out a debit card and pay for a cart full of groceries matters more than the ability to kill, butcher, and carry a deer. We pay monthly rent to a landlord rather than having to build a domicile out of heavy stones with our own two hands. With regard to reproductive fitness understand that evolution doesn’t so much care how strong you are or how fast you can run when all of your needs are met. Another way to illustrate this is to look at early examples of agrarian societies (e.g. Egyptians). You find that as soon as Homo sapiens had abundant sources of calories that were easy to cultivate and store (read grains) they became shorter, and exhibited bad teeth, decreased bone density and diseases that weren’t seen prior. The irony is that the cheap sources of calories that enabled us to easily reach reproductive age and eventually populate the world with billions upon billions of people is the root cause of modern man’s ill health and poor fitness. I could go on with examples, of course, but my main point is this: physical fitness no longer determines reproductive fitness. It has changed from requirement to elective. Being big, strong, fast, and agile is certainly beneficial to us (and even attractive to the opposite sex), but it isn’t necessary – let alone expected.

What, then, happens to our definition of physical fitness? If physical fitness is no longer a vital aspect of our essential humanness, what does it mean, exactly, to be fit?

I often talk about “functional fitness,” or fitness that enhances one’s ability to effectively function in a given environment. But that functionality is malleable, and the form it assumes is totally dependent on the environmental pressures being exerted. In other words, the required “functions” are always changing based on circumstance, and the “fitness” that allows these functions to be performed must change along with them.

I’ll give an example to illustrate my point. The body compositions of Roman gladiators were actually a far cry from those of the sub-10% BF, muscle-bound model-actors depicting them in movies; helped along by a diet high in barley and other grains, real Roman gladiators were sheathed in a substantial protective layer of subcutaneous body fat. To us, they would have looked like your average CW-touting slob, but in reality, they possessed incredible functional fitness – it’s just that they existed in an extremely narrow environmental niche, wherein the right amount of adipose tissue protected against serious wounds without compromising one’s ability to swing a mace or thrust a trident. Thus, a functionally fit body composition, for the Roman gladiators, was pudgy, bulky, and dense. But were they fit, in the broader sense? They might survive the arena, but would they reach old age, or would the effects of a grain-based diet eventually catch up with them?

That opens up another can of worms: isn’t overall health an aspect of fitness? The gladiator example is an extreme one (heck, their entire fitness regimen was predicated upon the assumption that they would be bludgeoned, stabbed, and sliced on a regular basis; these guys weren’t exactly thinking about their long term health!), but the extreme endurance athlete might be more suitable and applicable. As you guys know, I used to be one of them, running a hundred miles a week and training for hours daily at my peak. And unlike the gladiators, I actually looked to be in great physical health. I had almost no body fat, an impeccable resting heart rate (38 bpm), a relatively high VO2 max and I qualified for the 1980 Olympic trials as a marathoner, but I wasn’t healthy. I was constantly sick, my joints ached, my feet hurt, and my body was inflamed from all the simple carbs I had to eat to support my training. For my environment niche (endurance training), yeah, I was fit, even functionally so. But in terms of that other, somewhat wider environmental niche, the one that encompasses health, happiness, longevity, physical strength, agility, power, and resistance to disease? I was a mess. I wasn’t fit at all. Grok would have kicked me out when he saw I couldn’t haul a hundred pounds of bison back to camp without wincing and complaining about my knees.

I think we have to include health in the definition of proper fitness, especially if we’re talking about Primal fitness. There’s no point to lifting twice your body weight, running a sub-6 minute mile, and doing twenty consecutive pull-ups if you aren’t going to live a long, full life. With that in mind, I think true physical fitness must be functional across a broad spectrum of environmental pressures – no highly specialized gladiators or marathoners allowed – while still promoting optimum health and longevity. And I’ll admit – I can’t think of a time period in which greater varieties of functional abilities were demanded than the thousands of years before agriculture. Grok and company were the ultimate practitioners of a type of functional fitness that encompassed most, if not all of the parameters laid out in the dictionary definition up above. Competent strength, power, speed, agility, balance, flexibility, and cardiovascular endurance were essential attributes for our hunter-gatherer ancestors as they hunted, stalked, foraged, lifted, hauled, threw, climbed, and jumped. They were essential attributes for reproductive fitness and survival of the species.

Okay, but we don’t live in the Paleolithic anymore and, as I said earlier, we don’t have those same environmental pressures demanding we be able to jump high, run fast, and lift heavy things. How, then, has fitness changed from the theoretical past to the present?

Isn’t fitness, in an objective sense, entirely dependent on environment? If finances matter more than might, and education predicts success more often than does foot speed, does that render the old ideals of fitness irrelevant?

Or has the objectively ideal physical fitness remained the same? Just as our bodies do better when we eat, sleep, and mitigate stress like our Primal ancestors, do they also improve when we achieve Grok’s physical fitness? Is a balanced, measured, all-around competency suited for a wide range of environments and experiences the best marker for general fitness, regardless of financial status?

Is physical fitness truly necessary, or is it just another form of tourism?

I think you probably know my answers to all these questions. I’m eager to hear your thoughts, so hit me up with a comment. Thanks, everyone!

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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25 thoughts on “What Does it Mean to Be Fit?”

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    1. I was thinking the same thing. This echoes the important third dimension they added which looks at how much work capacity you retain over your lifespan, not just at your peak or a specific point in time.

  1. If you are not fit you will have to become a slave when you age. Your definition of self will be to align yourself with all the tangential trappings of supposed happiness.

    “Ahhh the luxury the sweet life it is”

    But don’t expect to be one of those 70 year old men and women I used to see on Saturdays at Les Grands Montets heading out for those weekend excursions across the Alps.

  2. Being fit dramatically improves the quality of a life, plain and simple. This is particularly true as we get older.

  3. Keeping muscle and bones strong and healthy now (at 41, I am building bone and muscle while I can! after years of sugar-eating) is important. It has a big pay off when a chap can look forward to many years more of independence while grain/sugar/alcohol and sedentary living put other’s into an assisted living situation sooner.

    Glad to have gone Primal when I did, better than never!

  4. Mark,
    I’m glad that someone finally addressed these two concepts of “fitness.” Great job laying out the contrasting definitions and their connections.

  5. keeping up with Grok is easy until you are 30 or so.

    I think fitness after that is the simple question do we keep you around as a productive member of tribe — or do we put you out on the iceberg?

  6. Great read but…

    “Grok would have kicked me out when he saw I couldn’t haul a hundred pounds of bison back to camp without wincing and complaining about my knees.”

    Well maybe, but not necessarily!

    Remember, Grok’s mission was to survive. He may have seen and utilized your skills (as a better runner than himself) to be a game runner.

    It could have went down like this:

    1) You & Grok go hunting together.
    2) Game is seen.
    3) You run down and make the kill shot.
    4) Grok carried the bulk of the meat (his job).
    5) You carried what you can (obviously).
    6) The butcher/preserver is waiting back at camp.
    7) Everyone wins!

  7. As a measurement for fitness and health for fitness I would measure Cardiovascular Efficiency and for health a key measure would be your insulin level.

    Working towards improving those two numbers will help open the door to a longer and healthier life.

  8. Interesting post, Mark. Food for thought. If you don’t mind a few questions that I hope you address in the next part of this series:

    Even Phil Maffetone and Doug McGuff, two guys with some pretty wildly divergent views on the proper modality of exercise for promoting health, insist that it’s possible to be fit without being healthy. Maffetone even suggests (and McGuff seems to agree) that one can be healthy without being particularly fit. It seems like you’re on your way to rejecting the duality of fitness and health.

    [quote]I think we have to include health in the definition of proper fitness, especially if we’re talking about Primal fitness.[/quote]

    Are you suggesting that fitness and health are really just two different names for the same underlying thing or that it’s simply impossible to be fit without being in a state of health?

    Also, how are you defining “health”? Maffetone and McGuff both spend a good bit of time working up definitions of “fitness” and “health” and use those definitions as the cornerstones of their method. Not to be a pedant, but you admit that the definition of “fitness” you provide at the outset of this piece is now internally inconsistent:

    [quote]physical fitness no longer determines reproductive fitness[/quote]

    Your definition of “functional fitness”:
    [quote]Competent strength, power, speed, agility, balance, flexibility, and cardiovascular endurance were essential attributes for our hunter-gatherer ancestors as they hunted, stalked, foraged, lifted, hauled, threw, climbed, and jumped. [/quote] is better since it doesn’t conflict with itself in present day humans. Still, it’s not clear to me how all the aspects of functional fitness relate to health. Is it not possible to exhibit a “competent” level of those traits unless you’re healthy? Are you healthy if you simply possess those traits at a “competent” level? What’s “competent” to mean these days anyway?

    1. Geoff,

      Lots to discuss over the next few days. I don’t have a solid answer so much as an informed opinion as to what I consider fitness, but I want yours (all you readers) as well. It was in 1982 that I first wrote about the notion that fitness and health are incompatible at an elite athlete level (The Runners World Triathlon Training Book). I even had a graph I made up to illustrate the concept. Still, somewhere down the scale from “elite” there is a point at which the two are certainly compatible and reinforce each other.

  9. I believe Mark is asking HOW MUCH fitness is enough for the environment we live in and for functional retention as we age.

    Is squatting two-times bodyweight necessary for the above parameters, or even necessarily healthy long-term? How about a marathon?

    Consider the many cultures that have never seen the inside of a gym, or had a barbell on their backs, or run a marathon-distant in less than 3 hours — they are for the most part free of chronic diseases and live long lives.

  10. I don’t know about the gladiator analogy, I’ve seen 2nd century mosaics with men that looked like Steve Reeves.

  11. Increased work capacity over broad time, modal, and age demands.

    Doesn’t get much more simplified than that. Work capacity means nothing if it is not sustained over a lifetime. If you cannot incorporate rock-climbing, Judo, weightlifting, and running into the same day then there is a flaw in your fitness. Also, being able to sustain efforts of strength and endurance are imperative.

  12. Thanks for this another wonderful article….the principles I’ve learned from this blog has made my successful transformation into reality…you can see my pictures of transformation in my blog…


  13. Great post about fitness Mark! Mmm, food for thought.
    Just a quick personal note below:
    I went gluten free about a month ago because I have been looking for a solution to thyroid and infertility problems. I feel so much better already, but after two weeks of reading your blog, I am so inspired to go beyound looking for a solution for my problems, and now just want to improve my overall health. I’m ready to go grain-free, so I’ve just purchased your book on Amazon UK, and I’m so excited about getting started (and moving more to get fitter). Now I just need to persuade my husband to join me. Thanks for a fantastic and inspirational blog!

  14. You make a really good point, Mark. It’s something that I’ve started taking to heart within the past year. I’ve always been thin and active, but now that I have chronic knee problems, I’m realizing that being “healthy” doesn’t cut it. I need to work toward the type of functional fitness you talked about in this post. I only wish I realized this before I brutalized my knees!

  15. Almost everyone agrees that physical fitness enhances mental fitness. Most (admittedly not all) people need a healthy body to have a healthy mind.

  16. Nice Blog…….It has a big pay off when a chap can look forward to many years more of independence while grain/sugar/alcohol and sedentary living put other’s into an assisted living situation sooner.

  17. Great article! When I think of fitness, I think of a full expression of your genetic blueprint as it relates to your environment. I find it interesting that although our environment is drastically different than it was in Grok’s time, our genetic make-up remains largely the same. I think it is in our best interest in terms of wellness to respect the way our physiology has been shaped after hundreds of thousands of years, BUT does this mean that we are resisting an evolution to adapt to our current and ever-changing environment? In fact, Ive read a couple of books that suggest that most types of cancer are a form of natural selection that would elminate those who dont have a certain type of genetic “flexibility” that would allow them to adapt to our current environment. Dont know if I full subscribe to that theory, but its an interesting point.