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.