What’s in it for me? Learn the real science behind metabolism.
There’s a lot of noise around what we eat.
Avoid fats, some say, and you’ll discover health and happiness.
Others advocate going vegan or ditching carbs.
These supposed miracle solutions all boil down to the same thing: starving your body of calories.
That does work – for a while.
But as dieters the world over know all too well, lost pounds don’t stay away for long.
In fact, you usually end up bigger than when you started dieting.
There has to be a better way of looking after your weight and health.
Andrew Jenkinson believes your best bet is to skip the fads and learn how your body’s metabolism really works.
And that’s just what we’ll be helping you do in these blinks.
A chance meeting solved single-celled organisms’ energy problems.
Along the way, you’ll learn how single-celled bacteria shaped our metabolism four billion years ago; why restricting calories primes your body for future weight gain; and how the war on “bad” fats fueled today’s obesity pandemic.
Our story starts around four billion years ago, when our planet was little more than a dark and stormy tropical sea under an oxygenless sky.
Simple, carbon-based chains of chemicals drifted aimlessly in this primordial soup.
Then, by chance, they found a formula to replicate themselves.
At first, these replicants simply integrated free-floating chemicals into their structures.
Later, they split those structures into two chains – a primitive form of DNA – and began creating new carbon copies.
Over time, replicants became increasingly complex until, at last, Earth’s first life-form appeared on the evolutionary stage: the single-cell bacterium.
The key message here is: A chance meeting solved single-celled organisms’ energy problems.
Inside the protective wall of its cell is the bacterium’s DNA code – the formula that allows it to reproduce.
Evolutionarily speaking, it has a single task: to grow and survive long enough to create a new generation of bacteria.
To do that, though, it needs energy.
Early bacteria were highly efficient at converting food into a form of energy that could be used by the various components of their cells.
Because they couldn’t process oxygen, however, there was a hard limit to how much energy they could generate.
That ultimately held back the advancement of more complex life-forms.
Then, around three billion years ago, a new kind of bacterium that could process oxygen emerged.
Existing bacteria were efficient; this bacterium, by contrast, was a powerhouse.
It vacuumed up vast amounts of food and created energy on an industrial scale.
Older bacteria couldn’t compete.
Luckily for them, they didn’t have to.
Instead, they swallowed – but didn’t digest – these newcomers, who now lived inside them.
Both parties benefitted.
The first kind of bacterium offered the second kind protection from predators while profiting from its ability to generate lots of energy.
In short, one cell moved into the other, and both thrived – an arrangement known as endosymbiosis.
Fast forward to the present, and the ancestors of these energetic upstarts are still there, inside the cells of every plant, fish, fungus, and animal.
All these organisms take their energy from these tiny power stations, which are called mitochondria.
Without them, life on Earth as we know it wouldn’t be possible – on their own, our cells simply can’t produce enough energy to keep us alive.
Humans paid for big brains by skimping on other organs.
Snakes, mushrooms, birds, roses, and humans – all living beings are descendants of the same single-celled ancestor.
That means that all these organisms generate energy in the same way.
How?
Well, the mitochondria inside cells convert food into energy, which is then stored inside battery-like molecules called adenosine triphosphate, or ATP.
These molecules are the currency of life.
Even viruses use them, although they hijack the ATP of the cells they invade.
Because all organisms have a finite number of cells, they have to make ends meet and cover all their vital functions within a strict energy budget.
Every species has this budget, but some are more creative accountants than others.
The key message here is: Humans paid for big brains by skimping on other organs.
The first anatomically modern Homo sapiens emerged some 250,000 years ago.
What set them apart from Homo erectus, our earliest non-ape ancestors, was the size of their brains.
In energy terms, the brain is a gas guzzler.
It alone accounts for 25 percent of the calories we consume.
How, then, did we find the resources for such an expensive organ?
The short answer is that we made cuts elsewhere.
Let’s break that down.
Every animal’s energy budget is related to its size.
The larger the animal, the more cells it has, which means more ATP-producing mitochondria.
In mammals, the energy required for survival depends on weight.
A 65-kilogram human, for example, needs a lot more energy than a dog – unless it’s a 65-kilogram Saint Bernard.
In that case, they both need a similar number of calories to fuel vital organs and life-sustaining functions like breathing.
The animals whose energy budget most closely resembles our own are large primates.
Take gorillas, whom we’ll assume have ready access to as much food as they want.
Their organs closely resemble our own in size and weight, and they can generate about as much energy as we can.
The major difference is that our brain is about four times larger than these primates’.
Where do we find the extra energy?
The answer lies in our guts, which are significantly shorter – and thus less energy-intensive – than gorillas’.
Gastrointestinal tracts are just as important for both animals, but ours are shorter because we can do something gorillas can’t: we can cook.
In the next blink, we’ll see how that ability made us into what we are today.
The discovery of fire made us who we are, culturally and biologically.
Prehumans evolved larger brains than other primates long before Homo sapiens appeared on the scene some 250,000 years ago.
Homo erectus, for example, outsmarted chimps a million years ago.
Archaeological evidence tells us what they did with that extra brainpower: they crafted rudimentary tools and perfected the art of hunting.
Slowly but surely, they abandoned the virtually vegetarian diet of their primate cousins.
Unlike other species, humans didn’t develop sharper teeth and stronger jaws as they evolved into carnivorous predators – in fact, the size of both decreased over time.
The more meat prehumans consumed, the more human they became.
To understand why, we need to look not just at what they ate, but how they ate it.
The key message here is: The discovery of fire made us who we are, culturally and biologically.
At the foot of a grassy hill in South Africa’s Northern Cape province, there’s a massive cave complex known as the Wonderwerk.
For a long time, humans lived in these caves.
Before them, it was prehumans like Homo erectus.
Before them, it was apes.
All in all, it’s been inhabited by large mammals for some two million years.
In 2012, archaeologists discovered evidence that Homo erectus started cooking with fire in these caves a million years ago.
Fire, it turns out, was discovered much earlier than previously thought.
But why is that significant?
First off, it tells us that cephalization – the evolution of larger-than-average brains – coincided with the advent of fire cooking.
That in turn helps us explain why the human body was able to transfer energy resources from the gut to the brain.
Cutting up meat and cooking it over fire makes it much easier to chew and swallow.
Our ancestors didn’t develop sharp teeth and bigger jaws because they didn’t need to – cultural and technological innovation had already done the heavy lifting.
Using fire also makes it easier to digest things like roots and tubers.
Unlike raw foods, which require lots of energy to be processed into fuel, cooked foods have already been “predigested.
” Cooking, after all, breaks down molecular bonds and makes hard-to-access nutrients more readily available.
Gorillas have longer gastrointestinal tracts than humans because they spend their days eating raw foods, which are harder to digest.
Cooked meals are a much more efficient food-to-fuel pipeline.
As a result, our guts became smaller, freeing up energy for an ever-larger brain.
The human body’s survival depends on self-correcting dangerous deviations.
So far, we’ve followed the evolutionary path that led from single-celled bacteria to big-brained, meat-eating Homo sapiens.
Our guiding thread was energy.
Before we can continue our exploration into how we generate and use energy, however, we need to take a detour and talk about negative feedback systems.
All systems require negative feedback.
Think of an office.
It has a set way of working – it’s open from nine to five, for example.
When there’s a deviation from this rule, the system self-corrects.
That’s what a manager is doing when she reprimands a worker for being late: her negative feedback realigns the employee with the system.
Our bodies also work like this.
The key message here is: The human body’s survival depends on self-correcting dangerous deviations.
A negative feedback system needs two components: a sensor and a switch.
The sensor detects deviations and triggers the switch, which changes the system back to how it’s supposed to be.
In our office example, the manager was the sensor.
She detected an employee arriving late and issued a verbal warning – a switch to alter that worker’s behavior.
Our bodies also have negative feedback systems.
Take hydration.
We’re 70 percent water, and we can only function if we keep our water levels at that mark.
Dehydration leads to dizziness, weakness, and, eventually, death.
Overhydration, which can cause fatal seizures, is just as dangerous.
Timely self-correction, then, is essential to survival.
The kidney is our hydration sensor.
When it detects that we’ve consumed too much or too little water, it uses a hormone called renin to send messages to two switches.
The first controls thirst and, by extension, how much water we take in.
The second regulates how much water we let out in our urine.
Say you don’t drink anything for half a day.
The kidney senses this and triggers the first switch.
The brain receives a thirst signal, and all you can think about is finding some water.
At the same time, the kidney turns the second switch off.
That’s why your urine is darker and more concentrated when you’re dehydrated – your kidney is trying to conserve body water.
Drink too much, by contrast, and it will deactivate the thirst switch and increase urine production.
As we’ll soon see, the human body doesn’t just regulate hydration through negative feedback – it uses the same system to control energy consumption, use, and storage.
Overeating raises the rate at which we burn fuel.
Let’s recap.
Negative feedback regulates our water levels by correcting two potentially fatal deviations: overhydration and dehydration.
Of course, water isn’t the only thing we need to survive – our bodies also require energy to keep cells and organs working.
For most of our species’ history, food was scarce.
With famine an ever-present threat, accurately predicting how much energy needed to be stored for the future was a huge evolutionary advantage.
The body can’t hoard energy indefinitely, though.
Trying to do so would make us so fat that we’d barely be able to move – a distinct evolutionary disadvantage.
So how do you split the difference?
Well, you create a negative feedback system just like the system that regulates water intake and output.
The key message here is: Overeating raises the rate at which we burn fuel.
In 1976, an American scientist called Ethan Sims conducted an experiment with volunteers drawn from the inmates of a state prison in Burlington, Vermont.
Sims was interested in obesity.
What would happen, he wondered, if people deliberately overate for three months to increase their body weight by 25 percent?
Sims upped the prisoners’ daily calorie intake from 2,200 to 4,000 calories – almost double the calories an adult male needs.
As expected, they quickly gained weight.
But then something strange happened.
Despite their calorie-rich diet, their weight flatlined.
Sims increased their daily rations to 10,000 calories.
Astonishingly, lots of participants still failed to gain additional weight.
What was going on?
The answer emerged when Sims measured the prisoners’ metabolic rate – that is, how quickly their bodies were burning energy.
He found that every volunteer’s metabolism had increased: they were burning off calories at an above-average rate.
Later research shows that they weren’t an anomaly.
In 2006, researchers at the Mayo Clinic in Rochester, Minnesota, looked at 21 overfeeding experiments and confirmed that, on average, overfeeding raises the metabolic rate by 10 percent.
This data suggests that our bodies try to protect us against runaway weight gain in the same way that our kidneys expel excess water to prevent overhydration.
But if we’ve discovered another negative feedback system, shouldn’t it also self-correct in the other direction – that is, shouldn’t it also protect us against weight loss?
And if so, wouldn’t that explain why so many diets fail?
Let’s find out!
Calorie restriction lowers your metabolic rate.
The first rule of metabolism takes the form of an equation: energy in minus energy out equals energy stored.
Let’s rephrase that rule to make it a little less abstract.
Food contains energy, which the body converts into heat, movement, and thought.
These activities in turn burn off energy.
If you eat more calories than you expend, you’ll store unused energy in fat cells.
According to this equation, weight loss should be simple: all you have to do is burn more energy than you consume.
And, indeed, that’s the advice of pretty much every diet out there.
In reality, it’s not that simple.
After initial success losing weight, dieters usually end up flatlining.
Later, they often gain weight.
To see why this is, we need to look at the physiology of starvation.
The key message here is: Calorie restriction lowers your metabolic rate.
In 1944, Ancel Keys, an up-and-coming nutritional scientist at Minnesota University, set up a study to see what happens to people’s metabolism during starvation.
Volunteers were allowed to eat a diet suited to their occupation as manual laborers for 12 weeks.
During this period, they were consuming 3,200 calories a day.
After three months, that was reduced to just 1,500 calories a day – an amount Keys described as “semi-starvation.
” After another three months, participants’ metabolic rates had plummeted.
That wasn’t a surprise.
As a general rule, larger people have faster metabolisms than smaller people, so weight loss usually results in a slower metabolism.
But Keys anticipated a 25 percent fall – not the massive 50 percent reduction he was now observing.
When he examined volunteers, he also found that their heartbeats were sluggish and that they were breathing more slowly.
Even their body temperatures had fallen.
In short, their bodies were shutting down.
That was surprising.
It also had long-term effects.
When participants returned to their normal diets, their weight rose alarmingly quickly – a development Keys attributed to their depressed metabolisms.
Every subject ended the experiment heavier than he’d started it, and lost muscle mass was mostly replaced by fat deposits.
Reducing calorie consumption is like reducing water intake: it triggers negative feedback.
Faced with starvation, the body tries to save as much energy as possible by lowering its metabolic rate.
We can now see why diets fail.
The body can’t tell if calorie restriction is the result of free choice or famine, so it takes a safety-first approach and presses the energy-conserving switch that maximizes its chance of survival.
Hormonal messengers control our weight.
The body can’t tell we’re consciously dieting.
From its perspective, calorie restriction looks like a famine and a threat to our survival.
That’s why it reduces its metabolic rate: it’s trying to conserve energy.
Our metabolism doesn’t return to its previous level once this period of calorie restriction is over, either.
We continue burning fuel more slowly and storing energy in fat cells – the body’s safety buffer to get us through the next famine.
All this explains why diets don’t work.
Our biology rebels against starvation.
But it also tries to prevent us gaining too much weight.
As we’ve seen, overfeeding raises the metabolic rate.
How exactly does this negative feedback system work?
It’s time to take a closer look at our fat cells.
The key message here is: Hormonal messengers control our weight.
In 1994, the American molecular geneticist Jeffrey Friedman discovered a hormone called leptin.
His breakthrough meant scientists could explain the workings of the negative metabolic feedback system.
Remember how the kidney uses a hormone, renin, to send messages?
Well, fat cells work in a similar way – they use leptin to communicate with the hypothalamus, the brain’s weight-control center.
Leptin tells it how much energy the body has stored.
The brain then uses that information to operate two switches.
One turns hunger on or off; the second raises or lowers metabolism.
When we overeat and store more energy in fat cells, those cells release leptin into the bloodstream.
The hypothalamus reads this message, realizes that the body has enough energy, and triggers both switches.
Appetite sinks and metabolism speeds up; this constrains energy intake and depletes existing energy stores more rapidly, thus preventing runaway weight gain.
Leptin also constrains weight loss.
When we restrict calories, the number of fat cells in our bodies decreases, lowering the amount of leptin in our blood.
The hypothalamus takes this as a cue to increase appetite and reduce metabolism.
Triggering these switches slows weight loss and leads to rapid weight gain once food becomes freely available again.
All of this suggests that our bodies should be keeping us in good shape.
Just as the hydration system we explored earlier stops us from consuming too much water, this metabolic system should prevent us from eating too much.
That isn’t the case, of course – obesity is a global health epidemic, especially in the Western world.
Why is this?
In the final blink, we’ll try to explain this puzzling phenomenon.
The modern Western food environment is a recipe for obesity.
Our ancestors were hunter-gatherers.
Meat and starchy tubers like yams and sweet potatoes made up the bulk of their diet.
They prized fatty offal like liver and bone marrow.
Natural vegetables, fruits, seeds, nuts, and herbs rounded out their meals.
Our bodies still resemble those of early Homo sapiens, but the energy we use to fuel them couldn’t be more different.
Take a 2016 survey analyzing the eating habits of over 9,000 Americans.
Almost 60 percent of their daily calorie intake was from highly processed foods.
Data from other Western countries paints a similar picture.
We eat a lot less natural fat and a lot more sugar and industrially produced vegetable oils than humans ever have in our species’ history.
That’s sabotaging the negative metabolic feedback system that’s designed to protect us against obesity.
The key message here is: The modern Western food environment is a recipe for obesity.
In the 1970s, heart diseases were on the rise.
Facing a looming public health crisis, governments looked to science for answers.
The scientists they called upon weren’t objective truth-tellers, however – in fact, lots of influential research was funded by industrial lobbies.
The biggest lobbyist of all was the sugar industry.
Research backed by “big sugar” pinned the health crisis on fats, especially saturated fats found in red meat, dairy, and butter.
The health risks of sugar consumption, which had been rising exponentially for over a century, weren’t explored.
In the 1980s, governments launched an offensive against these fats.
Today’s obesity epidemic is partly down to that decision.
Here’s why.
The consumption of vegetable oils like sunflower, canola, and soybean oils has tripled since governments began urging us to abandon saturated fats.
These processed oils contain high levels of omega-6 fat – a type of polyunsaturated fat found in nuts and seeds.
Omega-6 does two things.
First, it stabilizes fats, making them less prone to spoilage.
That’s handy for manufacturers.
Second, it dulls the effectiveness of leptin – the hormone produced by fat cells – which keeps us thin.
That’s bad for our health.
Government guidelines to reduce saturated fats in food products posed a dilemma for manufacturers.
How can you make addictively tasty goods if you can’t load them with butter?
Answer: you use a lot of sugar.
Since 1980, there’s been a 20 percent increase in the amount of sugar consumed by the general population.
That wreaks havoc on our blood sugar levels.
When they spike, we produce too much insulin, which in turn causes cells to suck more sugar from the blood.
That lowers blood sugar levels and leaves us craving even more sugar – a powerful impulse to overeat.
Biologically, our bodies are hardwired to prevent obesity.
Our modern food environment, however, often overrides this programming.
Final summary
The key message in these blinks is that: Life on Earth became possible when organisms evolved ways of producing more energy from food.
Human evolution is also a question of energy.
Our brains became bigger, and our guts smaller, when we found a technical hack to unlock extra fuel in food: cooking.
We also evolved a negative feedback system to regulate our use of energy.
In times of plenty, our bodies increase their metabolic rates.
In times of famine, they conserve energy by slowing our metabolism.
That should regulate our weight, preventing us from becoming too fat or too thin, but our modern Western food environment all too often overrides this biological system.