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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