One of Greg Glassman’s big innovations in creating CrossFit (whatever you think of its many pros and cons) was to set out a clear definition of what fitness actually is, as well as a set of clear proposals for how we might test it.

One of those proposals was the hopper test: in short, you write every exercise, every sport, every possible physical feat on pieces of paper, and drop them into a bingo hopper. Then you randomly draw out, say, ten of them, and make people perform those ten, randomly-selected physical tasks. By Glassman’s definition, the ‘fittest’ person would be the one who performed best, overall, on those randomly selected tasks.

But Glassman also observed that the hopper test has an interesting side-effect. Because we all know what we’re good at, and what we aren’t, if you were to actually participate in a hopper test yourself, you’d have a very clear idea of what you most wanted to see come out of that hopper, and what you’d most dread.

Perhaps you crush heavy weightlifting, but can’t run to save your life. Or perhaps you’re great at moving yourself around in space, but atrocious at anything involving flying objects and hand-eye coordination.

From that, Glassman posited what he though would be the theoretically best way to improve your athletic ability: imagine the five things you’d least want to see come out of the hopper. Then work on those, deliberately and intensely, until you mastered them to the degree that they became the things you actually most hoped to see selected. Then move to working on the next worst five.

In real life, that approach doesn’t work. Glassman mentioned he’d tried it briefly with his early personal training clients. And, in short, it’s so demoralizing to suck badly at everything you do, takes such an emotional toll, that his clients would simply drop out rather than repeatedly face those most-feared tasks.

So, instead, CrossFit was built on the idea of broad variation. With a wide array of stuff thrown at you, you’re forced to address the things you suck at, while also feeling buoyed up by getting to excel at the things you do well.

Still, that always reminded me of an observation from my trumpet teacher at Yale, a professor in the School of Music. He pointed out that if you walked up and down the practice room halls, you’d think you were listening to the New York Philharmonic warming up. Left to their own devices, students spent time practicing what they already did well, rather than take on the hard task of improving the areas where they fell short.

For me, running has always been my biggest athletic weakness. I dreaded the timed mile in gym class, and would demure when invited to join friends for a weekend jog. Sure, I pushed myself to do it when necessary, at one point even (unexpectedly) doing a half marathon. But I sure as hell wasn’t going running if I had the choice.

That’s why, this year, I resolved to stop sucking at running. It’s the reason I took on several months of SEALFIT, and the reason I’ve been following Power Speed Endurance programming ever since.

So far this year, I’ve almost certainly run more than I did in the decade prior. And though I’m still not a good runner, still won’t be lining up at a road race start anytime soon (even for a 5k, much less a marathon), I can definitively say it’s paid off. My times have improved, and my distances have increased. But, more importantly, it no longer seems like something I tell myself I “can’t” do. Today, I ran a mile as part of my workout warm-up, and another as part of the cool down. And though that isn’t much, for the first time, I found myself setting out on each run with no trepidation. I knew I’d be totally fine. And I felt ready to consider what might come next on the most-feared list of my personal hopper test.

If you’re an average, 180-pound person, all the capillaries in your body – the smallest blood vessels, where oxygen and other nutrients are exchanged with cells – can together hold about 3 gallons of blood.

But blood, like water, is heavy. So you evolved into an evolutionary compromise. Your body only contains about 1.5 gallons of blood at a time; much lighter to carry, but only half of what you need to provide for your whole body at once. Fortunately, your body also evolved a smart system of hemodynamics, a combination of forces that sends that blood to capillaries as it’s needed.

At the front end, your heart pushes oxygen- and nutrient-rich blood through your arteries.

Then the movement of your muscles pulls that blood from your arteries into your capillaries, to feed individual cells.

In other words, while your heart is circulating blood all the time, the oxygen and nutrients only make it to cells when the muscles around them are moving.

That’s one of the major problems with excessive sitting: without movement, your cells are starving.

But that’s just one problem. After 30 minutes of sitting, your metabolism slows down by 90%. A few hours in, you’ve got increased blood triglyceride and insulin levels, and reduced (good) HDL cholesterol and lipoprotein lipase (an enzyme that breaks down fat in your body).

So perhaps it shouldn’t be surprising that people who sit more are sicker and fatter than people who don’t.

What’s more, that’s independent of exercise. Even between people who work out for the same number of hours weekly, a greater number of hours spent sitting each day correlates with an increase in both body mass and all-causes mortality. Studies have tied sitting to huge increases in everything from type 2 diabetes to cardiovascular disease and cancer.

For example, excess daily sitting increases your risk of lung cancer even more than the second-hand-smoke effects of living with a smoker.

All of which is bad news, because we apparently really love to sit. The average desk worker spends 7-8 hours a day sitting at the office, then comes home to sit down for another 5 hours of daily TV.

Fortunately, the solution is simple: get up frequently and move around.

Research has shown that even short breaks (a couple of minutes) at low intensity (walking to the bathroom, or simply standing up) make a huge difference. One study showed that, the greater the number of breaks taken, the lower the waist circumference and BMI, and the better the blood lipids and glucose tolerance.

Of course, once you get into the flow of work, it’s easy to forget just how much you’re sitting. That’s why you need a gentle nudge.

Breaktime for Mac or Rest for Windows will take over your screen at whatever interval you select, reminding you to stand up, shake it out, go the bathroom, grab a water or coffee, or similarly get that mini-dose of movement it takes to get your body back on track.

Getting up and moving every 30 minutes is a pretty small habit. But it pays big dividends in your short- and long-term health.

As I wrote in Part I of this series, fitness, nutrition, and medical authorities often reduce weight management to “calories in – calories out”. That sounds scientific enough, and it’s true at a very simplistic level. But it also glosses over a huge amount of real-world detail, hidden in the definitions of “calories,” “in,” and “out.”

So in Part II, I took a deeper look at what calories really are. Though they have the reassuring appearance of objective, quantifiable fact, they’re instead misrepresentatively averaged numbers, based on massaged data, outdated assumptions, and fundamental misunderstandings of how your body actually processes food, and creates and uses energy.

In that post, I eventually concluded that we can’t even really answer a basic question, like “how many nutritional calories are in this cup of strawberries?”

We’re not stopping there, though, because things keep getting worse. While we have real trouble determining the macronutrient content of that cup of strawberries (hint: it’s probably not the “24 calories – 0.2g fat, 6 g carbohydrates, 0.5g protein” asserted by the USDA), things go further downhill once we put those strawberries into our mouths.

That leads us to today’s topic, the first half of what ‘in’ means: digestion.

Before we even start to chew things over, though, are you cooking the strawberries, or eating them raw?

Cooking is a chemical process, which changes the molecular makeup of food. Consider a potato. When it’s raw, a large portion of the carbohydrates it contains is in a form our body doesn’t well digest. As we cook the potato, however, the starch gelatinizes, converting into a form that we can now digest more easily, allowing us to absorb more nutritional energy – more calories – from the same food. But let’s say you then put the potato into the refrigerator, to eat later. As it cools, a percentage of the carbohydrates converts back into ‘resistant starch,’ which digest differently than either of that carbohydrate’s prior states. Thus, a hot boiled potato (at 180ºF) has a glycemic index (a rating of your body’s insulin response to that food) about 20% greater than the same amount of white bread; whereas that potato cooled to 80ºF triggers about a 25% smaller insulin response than white bread. In other words, if we cook food, how we cook it, and what we do to the food after we cook it, all have huge impact on how our body absorbs the calories it contains.

Then, of course, you put the food into your mouth.

And you chew it. But how much do you chew it? In one study, people fed two ounces of almonds chewed each bite 10, 25, or 40 times. And, in short, those who chewed the almonds more times absorbed significantly higher amounts of healthy fat, and had longer hunger suppression and lower insulin response, then those who chewed the same amount of almonds less extensively.

And that doesn’t take into account how wet or dry your mouth is. Because your saliva also contains a variety of enzymes that actively digest food while you’re chewing. You can test this yourself, with a saltine cracker: simply put a whole saltine in you mouth, and wait. Your saliva contains the enzyme amylase, which catalyses the hydrolysis of starch into sugars. After a few minutes, the saltine will begin to taste sweet, because you’ve literally turned your low-sugar cracker into a high-sugar cookie through the power of drool.

Then, you swallow the food. In your stomach, digestion continues. But here, too, a huge number of factors impact how much digestion, and of what kind, takes place. For example, is your stomach empty or full? Did you eat those previously discussed strawberries alone, or with something else? Both of those impact digestion. So does stress. Your body’s fight-or-flight response prioritizes short-term survival over longer-term concerns like digesting food, so if your stress level is high, and you’re chronically stuck in a fight-or-flight state, the transit time through and acid level in your stomach changes. Also, do you have regular indigestion, GERD, or a history of ulcers? All of those imply too much or too little stomach acid (sometimes caused by the bacteria H. pylori), which further radically alters the degree to which you digest food in your stomach.

So, thus far, we have an unknown number of calories in our food, that have been changed in unknowable ways by cooking, chewing, salivating and stomach digesting. Let’s keep this party going!

Next up, we’re on to your intestines. This is where we start absorbing nutrients, as broken-down food particles pass through the gut barrier. How healthy are your intestines? A slew of factors affect GI health, which in turn determines how efficiently nutrients can pass through them into your blood stream. And again, how stressed are you? As with the stomach, stress changes the time it takes food to pass through your intestines, similarly affecting absorption. Finally, how long are your intestines? It turns out that varies substantially from one person to the next, and the amount of nutrients you can absorb through your intestines is to a large degree determined by their length. (That’s perhaps why, though intestinal length doesn’t correlate with height, it does correlate closely with weight.)

As a last stop, whatever’s left of the food enters your colon. Here, it’s a team effort. Your colon is home to literally ten pounds of bacteria, which help you break down nutrients (like “indigestible” fiber) that you couldn’t on your own. For example, if you have the right bacteria, and they’re healthy and active, they can convert certain kinds of unusable vegetable fiber into the short-chain fatty acid butyrate, a very usable (and neuro-protective) fuel for your brain. As we’re just beginning to learn, we have a huge number of different strains of gut bacteria, their relative percentage varying starkly from one person to the next. Depending on the number of each bacteria and their overall health, and the amount of mucin (the natural protective layer) coating the inside of your colon, the kinds of nutrients that get processed, how much of each does, and how well each passes through the gut barrier, all vary hugely as well.

At that point, you poop out the leftovers. (Squatty potty, anyone?) As discussed, an array of nutrients from the food have now passed into your body along the way. But due to all the aforementioned factors, we have basically no idea what percentage of the ingested nutrients that represents (and of food where we similarly already have no idea how many calories, let alone how much of specific macronutrients, it contains).

Or course, digestion is just the first half of what ‘in’ actually means. Once those nutrients pass into your body, you have to do something with them. So tune in shortly for Part IV, when we look at how your body puts incoming nutrients to use, and (perhaps not surprisingly) the already convoluted plot just continues to thicken.

A couple of years ago, I was talking to a friend who had just built an IKEA bookshelf.

“It looks great,” she told me. “And I barely had any extra parts!”

These days, wise in my old age, when I put together furniture, I tend to read the instructions through once or twice first, then follow them to the letter. And, invariably, all of the included parts end up in the finished product.

Simply put, manufacturers are controlling costs to the penny, and they don’t just throw in a few extra bolts for good measure.

Evolution works the same way. Developing biological structures in a growing body is extremely physically costly. Nature is parsimonious. So it would be surprising to discover that we have major physiological structures ‘by mistake’.

Indeed, over the last few decades, we’ve increasingly discovered that organs we once believed were vestigial – evolutionary ‘leftovers’ – actually serve important functions that we simply hadn’t yet discovered.

Consider the appendix, which paleontologist Alfred Romer once joked served primarily “to support the surgical profession.” In the last fifteen years, we’ve discovered that it’s crucial in early childhood, aiding in the maturation of B lymphocytes and the production of antibodies. And a growing group of scientists has suggested it also serves as backup reservoir of good gut microbes, so that we can healthily ‘reboot’ after illness. Individuals without an appendix, for example, are four times more likely to suffer from C. diff. colitis, a bowel irritation caused by the overgrowth of bad bacteria.

As a result, I’m particularly dismayed by the number of surgical interventions that ‘fix’ problems by simply disposing of structures that are the current source of problems.

Consider gastric bypass, the current state of the art in bariatric medicine. To help people lose weight, a bypass reduces the size of the stomach, to decrease appetite, and then routes the vast majority of the food around the large intestine entirely, to keep people from absorbing the majority of the food they eat. Problematically, different parts of your intestines absorb different kinds of food, so that ‘reduced absorption’ caused by arbitrarily skipping parts is actually a short path to unbalanced malnutrition. And the skipped gut serves all kinds of immune and neurological functions beyond simple digestion, which we lose when we indiscriminately cut it out of the picture. Further, dumping food into later parts of the intestine in a less-digested state causes additional problems, because those later parts weren’t designed to function properly with that kind of undigested input.

You can argue here as to whether the doubtless deleterious effects of wrecking your digestive system is outweighed by the more pressing disaster of morbid obesity. But you can also lose weight by cutting off your legs, yet fewer medical centers have popped up around that idea, mainly because the missing legs are a more immediate problem than those caused over time by a bypass.

Similarly, I’ve talked to a number of orthopedists who proudly ‘treat’ plantar fasciitis surgically by cutting the plantar fascia. Your plantar fascia is tight and painful? We’ll just snip that thing, and everything will be great! Except, obviously, it won’t. Your plantar fascia plays an important role in the structure of your foot, and in the way you stand, walk and run. With yours missing, you’ve created a slew of new problems. Whatever underlying disfunction caused the plantar fasciitis hasn’t disappeared; it will just slowly start wrecking a different portion of your body, further up the biomechanical chain.

You can see this clearly in surgeries like total knee replacement. Your knee is meant to bend in a straight line, but if you spend enough years putting weird torque into the system due to wonky movement patterns, you’ll eventually wear through the cartilage that’s meant to serve as side-rails, rather than as a primary support surface. After that, you’ll start to grind away the bone, which really hurts. So an orthopedist can solve the problem by removing your knee completely, and replacing it with a metal version that’s simply too strong for you to torque it in a damaging way. Problem solved! Except, of course, it hasn’t been solved here, too. Your wonky movement patterns persist, but because you can no longer get play in the system at the knee, your body start compensating at the hip or low-back instead. Indeed, after total knee replacement, the odds of needing surgical intervention at the hip or low-back skyrockets. But don’t worry, we can also fix those. We can replace your hip with a metal version, too. Or, if the bendy parts of your spine aren’t moving the way they should, we can just fuse them together so they can’t move any more. We’re great at this!

All of which is to say: your body is a hugely complicated system, which operates holistically in ways that we’re just beginning to understand. Giving real thought and research to how different inputs change the output of that system is probably a smart route to finding solutions that are helpful in the long-term. But removing ‘extra parts’, and then not expecting that to cause all kinds of new and unexpected problems seems a rather myopic way to go.

Yesterday, I wrote that the basic ‘thermodynamic’ equation of weight loss (calories in – calories out = net calories) glosses over a lot of important information, mainly by obscuring the definitions of ‘calories’, ‘in’, and ‘out’.

So let’s clear things up a bit, starting today with calories themselves.

When we discuss “calories” in food, we actually mean “kilocalories”, as nutritional calories are based on 1000 thermodynamic calories. Outside of the nutritional world, a kilocalorie is a well-defined measure of energy: the amount of heat required to increase the temperature of one kilogram of water by one degree celsius at a pressure of one atmosphere (i.e., at ground level).

We test calories in food using what’s called a ‘bomb calorimeter’. Basically, it’s a device for blowing up food, and then seeing how hot the explosion makes surrounding water. To operate a bomb calorimeter, you take a small amount of food, and put it into a metal canister filled with pure oxygen, with a fuse that extends out to an electrical ignition. Then you submerge the metal canister, floating it in a kilogram of water. You carefully check the temperature of the water at the start. Then you hit the ignition, and the food explodes. As it does, the heat from the burning food begins to raise the water temperature. By tracking how much the temperature increases at its peak, how many degrees celsius the water temperature rises from the starting point, you’ve got the number of kilocalories in the food you just blew up.

Odds are, you aren’t a biologist. But you’re likely still aware that this isn’t really what happens inside of your body. You don’t walk around with a series of explosions detonating in your stomach all day long. (However, insert fart joke here.)

Tomorrow, when we look at ‘calories in’, we’ll try to get a better sense of what your body actually does to extract the energy from food. But before then, there are a few even more fundamental problems.

Towards the end of the 19th century, Wilbur Olin Atwater, a scientist at Wesleyan University, set out to understand the connection between heat calories (the kind you measure when blowing up food) and nutritional calories (the metabolisable energy your body derives from that food), through a series of experiments.

Atwater tested the heat calories in a wide variety of foods. And then he tested the heat calories in the feces of people who had eaten those same foods, to determine ‘apparent digestibility’, the percentage of the calories absorbed by the body rather than excreted.

The results of those experiments are the “Atwater factors”, the well-known idea that a gram of fat contains 9 calories, while a gram of protein or a gram of carbohydrate each contain 4 calories.

Most people don’t realize, however, that those numbers are averages. Not all fats, proteins and carbohydrates have the same amount of heat calories per gram. Similarly, not all fats, proteins and carbohydrates have the same apparent digestibility. So Atwater devised weighted values for the gross heat of each macronutrient, based on what he thought to be their prevalence in the standard diet at the time.

Similarly, Atwater first tested the apparent digestibility of individual foods. Then he started combining foods together, the way we do when we actually eat things. At that point, however, the numbers started to muddy up. Unfazed, Atwater simply came up with ‘adjustments’ that he hoped might account for the discrepancies in the mixed diet numbers.

When Atwater was doing his experiments, the best-selling Fannie Farmer cookbook was just introducing newfangled kitchen technology like the measuring cup and spoon. The cookbook featured perennial favorites of the time, like turnip croquettes, tomato cream toast, and Washington-style terrapin (turtle served in white sauce with its liver, small intestine, and “any turtle eggs”).

In other words, the ‘standard diet’ on which Atwater based his average computations is more than a bit different from what most of us eat today.

Similarly, Atwater’s ‘adjustments’ to account for mixing foods are difficult to justify in retrospect as anything other than liberal massages of the raw data to better align with his intended conclusions.

So even before we begin to look at how our bodies digest and assimilate foods, it’s clear that perhaps calorie math – which has the reassuring appearance of incontrovertible science – isn’t quite as objective and accurate as we might hope.

Indeed, calorie math doesn’t really take into account the difference between, say, a gram of carbohydrates from a strawberry and a gram of carbohydrates from a pear, or what happens when we eat them together, along with some sugar and cream. (Which, as an aside, sounds delicious.)

Further, it doesn’t even take into account the substantial differences from one strawberry to the next. A huge number of factors in the life of each strawberry can affect its nutritional content. At what time of the year was the strawberry planted and picked? How often was it watered, and how much? Was it fertilized? How much direct sunlight did it get, and how close was its nearest strawberry-plant neighbor? As a result of these and countless other questions, any given strawberry might have different amounts of fructose, glucose, sucrose, soluble and insoluble fiber, and micronutrients than the next.

The result of all of this is that we can’t even answer a very basic question, like “how many nutritional calories are in a cup of strawberries?” We have pretty much no idea.

So, calories. Not really what they’re cracked up to be.

But wait, it gets worse! Tune in for Part III, to learn why, even if we knew how many nutritional calories were actually in that specific cup of strawberries you’re holding, it still has only a small bearing on what happens once you actually put them into your mouth.

A lot of nutritionists, diet book authors, and health experts love to point out that losing (or gaining) weight is a simple matter of thermodynamics: calories in – calories out = net calories. If net calories are positive, you gain weight; if net calories are negative, you lose it.

And, at a very high level, that math is correct. But, like with most real-world processes distilled to simple models, it’s also missing a whole lot of important detail.

In short, there are three main problems with the calories in / calories out formula: we don’t accurately define ‘in’, we don’t accurately define ’out’, and we don’t accurately define ‘calories’. Other than that, it works great.

Over the next few days, I’ll be unpacking each of those three definitions in separate posts, to see if we can better understand how people lose and gain weight. As you might expect, the reality is a bit more complicated than a six word equation. And the devil – as well as our salvation – is all in the details.

A month back, I mentioned that I’d been following SEALFIT, an over-the-top approach to CrossFit-esque workout programming targeted at the military special operations crowd, for the past four or five months.

In a lot of ways, it was a great experience. First and foremost, I learned that I could push myself through vastly harder and longer workouts than I would have expected. Several times a week, halfway into a workout, I’d realize there was just no way I could make it to the end. But then I’d put one foot in front of the other, and inch my way through. And, somehow, I managed to finish every single one.

At the same time, the volume was insane, sometimes involving hours a day of workouts that beat me into the ground, one after another. Paired with the stresses and time commitments of real life, it proved to be too much. I got sick, and then got sick again. My resting pulse climbed and my heart rate variability plummeted. I had clearly veered deep into overtraining territory.

So, a few weeks ago, I moved laterally to try out the programming from Power Speed Endurance (née CrossFit Endurance). While it’s similarly biased towards running (and therefore in line with my 2016 goal to become a less terrible runner), and still includes two-a-day workouts twice a week, it also follows sensible progressions, includes a lot of pre-hab and mobility work, and generally seems like something you could do for an extended period of time without a death-wish.

In parallel, I’ve been writing a bunch of programming through Composite, and at some point it seems inevitable that I’ll circle around to eat my dog food. But, for the moment, I’m enjoying testing out another fitness guru’s best thinking. If you’re looking for some smart, challenging workouts (and especially if you’re a runner, cyclist, or triathlete), it’s worth checking out.