Your body is a phenomenally complex machine. Right now, as you sit here reading this, it’s busy digesting your lunch, making new cells, fighting infections, converting food to energy, regulating your temperature, filtering your blood and thinking about getting milk on your way home… not to mention keeping your heart beating, your lungs pumping, and translating the series of black marks in front of your eyes into words for you to read.
Sometimes it’s easy to forget that it does all of this with one set of inputs – the food that you eat.
Prior to embarking on my health journey, my rough understanding of how my digestive system worked went something like this: eat food; food gets mushed up into glucose; body uses glucose for energy.
I knew very vaguely that things like vitamins were important, but not really how or why. I also ‘knew’ that a calorie was a calorie, because all food gets mushed up into the same unit of energy – so all that really mattered was how many calories you ate, not where they came from.
It is true that all calories have the same amount of energy – a calorie of broccoli is the same as a calorie of french fries. But that’s because a calorie is a unit of measurement, not a nutrient. A calorie, to get technical, is the energy needed to raise the temperature of 1 gram of water by 1 °C.
But, in a plot twist that seemed to have escaped my younger self, I am not actually a machine that boils water.
If all I needed was the right input of energy, I could consume all my daily calories in table sugar and be perfectly healthy. I’d like to think even teenage me would have understood that that’s some very faulty logic – but it’s taken me a painfully long time to realise something that should have been obvious: everything your body needs to do, it resources from the nutrients in your diet.
Your body literally makes itself out of what you put into it.
We’re prone, I think, to thinking of our bodies as either far too complex to ever understand, or as so simple they’re basically machines. Sometimes we have both thoughts at the same time – like when we think our bodies ‘just’ use food to make glucose for energy, while also having absolutely no idea what our organs of digestion actually do.
So let’s digest my dinner together (ew) to figure it out. Tonight, because I’m the meanest mother in the world, we had fakeaway fish and chips – oven chips and pan-fried fish, wrapped in paper and served with coleslaw and tartare sauce.
Our bodies start digesting before we even take our first bite of food. Just thinking about the food, smelling the aromas of hot, salty chips… feel the saliva start to fill your mouth? That’s your brain getting your body ready to start digesting the food to come. And when the first bite hits the tongue, our taste buds start relaying all sorts of messages to our brains so they can coordinate our bodies to respond to what’s in our food.
Chewing breaks down the food into smaller pieces so that we can swallow it, but our saliva also contains enzymes that start to break down fats and simple carbohydrates as we chew, making it easier for our bodies to digest them fully later.
Once the chewed up fish and chips reaches the stomach, the real magic begins. Sensing the protein in the fish, our stomachs release a substance that produces more stomach acid to help break it down, as well as disinfecting the food to kill off any pathogens. Between the acid and the churning and grinding of our stomach walls, our food is mashed into an acidic pulp, which also releases vitamins and minerals so that we’re able to absorb them.
From the stomach, the puddle of mush formerly known as dinner enters our small intestines. The fat and protein in the meal signals our bodies to produce a hormone that tells the gallbladder to release bile, the pancreas to release enzymes, and also lets our brains know that we’re full.
As the mush works its way through around 7 metres of small intestine, our bodies absorb everything we need to use. Carbohydrates are broken down into simple sugars (ol’ mate glucose) and proteins are mostly broken down into amino acids. Both of these are then absorbed into the bloodstream (more on where they go next later). Fats are broken down by bile and then absorbed into either the bloodstream or the lymphatic system, depending on their type.
In the large intestine, we enlist the help of our microbiomes to wring any remaining goodness out of our meal. Our microbes consume the things we can’t use (like fibre, polyphenols and resistant starches) – and then make things from them (like vitamins and short-chain fatty acids) that we can. We absorb any excess water and electrolytes, and whatever’s left is what comes out as poop.
That’s the basic summary of how the body takes food and converts it into nutrients. Next, let’s cover what it does with those nutrients once it’s got them.
Glucose = energy?
We’ve broken down the carbs in our chips, fish batter and veg and released them into our bloodstream as glucose. That’s our energy sorted, right?
Turns out it’s a bit more complex.
Our central nervous systems, sensing the rise in blood glucose, tell the pancreas to make insulin. Think of insulin like a bouncer – the glucose can’t get out of the blood and into a cell without insulin escorting it. Because high blood glucose is dangerous, we can’t leave the glucose that we don’t currently need for energy in our bloodstream – we have to take it and put it somewhere.
Since we’ve taken a big hit of starchy, salty chips, we’ve probably got a bit more glucose in our blood than our cells currently need. So after our cells have had their fill, we store some of the rest away in our liver and our skeletal muscles. And once those stores are full, we convert whatever’s left into triglycerides to store as fat.
(Then, later in the evening, we’re probably going to start feeling peckish again. We ate a lot of energy, but we had to store it because we couldn’t use it right away, so our blood glucose will soon drop again and we’ll start craving a little something sweet to tide us over until bedtime. If we resist the chocolate calling to us from the pantry and our blood sugar dips too low, our pancreas will release a different hormone called glucagon, which will tell the liver, the muscle and then our fat cells to release their stores for us to use as energy. (Our adrenal glands also release hormones that kick this process into gear, both when we have low blood sugar and when we’re under stress.))
Fat = fat?
We’ve just covered what happens to the glucose, but what about the protein (now in the form of amino acids) and fat (now fatty acids) in our meal that also passed from our small intestines into our bloodstream?
We only use protein for energy if we don’t have enough carbohydrate or fat, so mostly we’ll use those amino acids to build things – like enzymes, hormones, neurotransmitters and immune system cells. But, like the glucose, our cells can use the circulating fatty acids for energy – and then store any extra for later in our fat cells.
Over the last few decades, one thing we could all agree on was that fat made you fat. If I’d been asked to think any deeper about it, I imagine I’d have pictured some kind of kids-book cog machine – fat goes in, cogs whirr, the fat gets deposited straight on my thighs. No one had ever indicated to me that fat had any deeper purpose, or did anything beneficial in my body.
But dietary fat is, in fact, really important for our health. It’s true that fat is a denser form of energy, with more calories than carbohydrates or protein – but that’s a good thing. Think of carbs like kindling, providing a quick burst of fuel that burns out quickly. Fats are the big logs on our energetic fire – they provide long-burning energy that keeps us going and keeps our blood sugar stable between meals and overnight. They also help us to feel full, and make food taste better.
Our bodies also use fats to build our cell membranes and make hormones – and we need fat to be able to absorb the fat-soluble vitamins (A, D, E and K).
What about the other stuff?
We’ve covered what happened to the fats and the carbohydrates in our dinner. What about the vitamins and minerals that we broke down and absorbed along with them. What are they for?
We’ll cover vitamins in detail another time, but minerals make up about 4% of our bodies, and we can’t make them – we have to get them from our food. They include things we need quite a bit of, like calcium and sodium, and things we only need a tiny amount of – but can’t function properly without – like iron, iodine and zinc.
Here’s a handful of ways minerals are essential to our health:
- We need zinc to make stomach acid
- We need iodine to produce thyroid hormones
- We need iron to make hemoglobin (red blood cells), which carry oxygen around our bodies
- We need calcium to contract our muscles and magnesium to relax them.
There’s one other big player in our meal that we haven’t discussed yet – water. We can survive about 8 weeks without food, but more than a few days without water will kill us. This is because water makes up 55% of our body mass (60% if you’re male) and contributes to just about everything your body needs to do.
Water enables all the digestive processes we’ve just talked about because we break our food down in water. It’s also the way we transport nutrients around our body – we move things through fluid. It lubricates our joints and absorbs shocks, regulates our body temperature through sweating, and removes waste and flushes toxins out of our bodies through our urine and faeces. Typically, you get about 20% of your daily water from food (that cabbage in your coleslaw was 92% water), and the rest you need to drink.
In conclusion: suck it, kids
My kids might complain that they had to eat salad, and their fish and chips weren’t deep fried in oil that’s older than they are, but based on the recommended dietary allowances in Cronometer, our meal tonight provided me (and Hypothetical You!) with:
- 576% of our vitamin A requirement (mostly from the carrots in the slaw)
- 350% of vitamin B12
- 171% of our vitamin K
- 70% of our vitamin C
- 20% of our folate
- 28% of our fibre.
If, instead, we’d had deep fried, battered fish and chips from the shop down the road, a fillet of fish and the same amount of chips would have had roughly the same number of calories – but we would have only taken in:
- 3% of our daily vitamin A requirement
- 42% of our B12
- 36% of our vitamin K
- 6% of our vitamin C
- 10% of our folate
- 16% of our fibre.
We might have had a similar amount of “energy”, but we’d be missing many of the things that our bodies need to function properly. For one meal, or one day, this might not be a huge deal, but for most of us now, most of the time, it’s not just one meal or one day. So next time you really, really want those greasy, salty corner-store fish and chips, remember this post and… at least add a salad?