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of objective, quantifiable fact, they’re instead misrepresentatively averaged numbers, based on massaged data, outdated assumptions, and fundamental misunderstandings of how your body actually processes food, and creates and uses energy.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.