by Dean Burnett
Room for pudding?
(The brain’s complex and confusing control of diet and eating)
Food is fuel. When your body needs energy, you eat. When it doesn’t, you don’t. It should be so simple when you think about it, but that’s exactly the problem: us big smart humans can and do think about it, which introduces all manner of problems and neuroses.
The brain exerts a level of control over our eating and appetite that might surprise most people.* You’d think it’s all controlled by the stomach or intestines, perhaps with input from the liver or fat reserves, the places where digested matter is processed and/or stored. And indeed, they do have their part to play, but they aren’t as dominant as you might think.
Take the stomach; most people say they feel ‘full’ when they’ve eaten enough. This is the first major space in the body in which consumed food ends up. The stomach expands as you fill it, and the nerves in the stomach send signals to the brain to suppress appetite and stop eating, which makes perfect sense. This is the mechanism exploited by those weight-loss milkshakes you drink instead of eating meals.5 The milkshakes contain dense stuff that fills the stomach quickly, expanding it and sending the ‘I’m full’ messages to the brain without you having to pack it with cake and pies.
They are, however, a short-term solution. Many people report feeling hungry less than 20 minutes after drinking one of these shakes, and that’s largely because the stomach expansion signals are just one small part of the diet and appetite control. They’re the bottom rung of a long ladder that goes all the way up to the more complex elements of the brain. And the ladder occasionally zigzags or even goes through loops on the way up.6
It’s not just the stomach nerves that influences our appetite; there are also hormones that play a role. Leptin is a hormone, secreted by fat cells, that decreases appetite. Ghrelin is released by the stomach, and increases appetite. If you have more fat stores, you secrete more appetite-suppressing hormone; if your stomach is noticing a persistent emptiness, it secretes hormone to increase appetite. Simple, right? Unfortunately, no. People may have increased levels of these hormones depending on their food requirements, but the brain can quickly grow used to them and effectively ignores them if they persist too long. One of the brain’s more prominent skills is the ability to ignore anything that becomes too predictable, no matter how important it may be (this is why soldiers can still get some sleep in war zones).
Have you noticed how you always have ‘room for dessert’? You might have just eaten the best part of a cow, or enough cheesy pasta to sink a gondola, but you can manage that fudge brownie or triple-scoop ice-cream sundae. Why? How? If your stomach is full, how is eating more even physically possible? It’s largely because your brain makes an executive decision and decides that, no, you still have room. The sweetness of desserts is a palpable reward that the brain recognises and wants (see Chapter 8) so it overrules the stomach, saying, ‘No room in here.’ Unlike the situation with motion sickness, here the neocortex overrules the reptile brain.
Exactly why this is the case is uncertain. It may be that humans need quite a complex diet in order to remain in tip-top condition, so rather than just relying on our basic metabolic systems to eat whatever is available, the brain steps in and tries to regulate our diet better. And this would be fine if that was all the brain does. But it doesn’t. So it isn’t.
Learned associations are incredibly powerful when it comes to eating. You may be a big fan of something like, say, cake. You can be eating cake for years without any bother, then one day you eat some cake that makes you sick. Could be some of the cream in it has gone sour; it might contain an ingredient you’re allergic to; or (and here’s the annoying one) it could be that something else entirely made you sick shortly after eating cake. But, from then on, your brain has made the connection and considers cake out of bounds; if you even look at it again it can trigger the nausea response. The disgust association is a particularly powerful one, evolved to stop us eating poison or diseased things, and it can be a hard one to break. No matter that your body has consumed it dozens of times with no problem; the brain says, No! And there’s little you can do about it.
But it doesn’t have to be anything as extreme as being sick. The brain interferes with almost every food-based decision. You may have heard that the first bite is with the eye? Much of our brain, as much as 65 per cent of it, is associated with vision rather than taste.7 While the nature and function of the connections is staggeringly varied, it does reveal that vision is clearly the go-to sensory information for the human brain. By contrast, taste is almost embarrassingly feeble, as we shall see in Chapter 5. If blindfolded while wearing nose plugs, your typical person can often mistake potato for apple.8 Clearly, the eyes have a much greater influence over what we perceive than the tongue, so how food looks is going to influence strongly how we enjoy it, hence all the effort on presentation in the fancy eateries.
Routine can also drastically influence your eating habits. To demonstrate this, consider the phrase ‘lunchtime’. When is lunchtime? Most will say between 12 p.m. and 2 p.m. Why? If food is needed for energy, why would everyone in a population, from hard physical workers like labourers and lumberjacks to sedentary people like writers and programmers, eat lunch at the same time? It’s because we all agreed long ago that this was lunchtime and people rarely question it. Once you fall into this pattern, your brain quickly expects it to be maintained, and you’ll get hungry because it’s time to eat, rather than knowing it’s time to eat because you’re hungry. The brain apparently thinks logic is a precious resource to be used only sparingly.
Habits are a big part of our eating regime, and once our brain starts to expect things, our body quickly follows suit. It’s all very well saying to someone who’s overweight that they just need to be more disciplined and eat less, but it’s not that easy. How you ended up overeating in the first place can be due to many factors, such as comfort eating. If you’re sad or depressed, your brain is sending signals to the body that you’re tired and exhausted. And if you’re tired and exhausted, what do you need? Energy. And where do you get energy? Food! High-calorie food can also trigger the reward and pleasure circuits in our brains.9 This is also why you rarely ever hear of a ‘comfort salad’.
But once your brain and body adapts to a certain caloric intake, it can be very hard to reduce it. You’ve seen sprinters or marathon runners after a race, doubled up and gasping for breath? Do you ever consider them a glutton for oxygen? You never see anyone tell them they’re lacking in discipline and are just being lazy or greedy. It’s a similar effect (albeit a less healthy one) with eating, in that the body changes to expect the increased food intake, and as a result it becomes harder to stop. The exact reasons why someone ends up eating more than they need in the first place and becoming accustomed to it are impossible to determine as there are so many possibilities, but you could argue that it’s an inevitability when you make endless amounts of food available to a species that has evolved to take whatever food it can get whenever it can get it.
And if you need any further proof that the brain controls eating, consider the existence of eating disorders such as anorexia or bulimia. The brain manages to convince the body that body image is more important than food, so it doesn’t need food! This is akin to you convincing a car that it doesn’t need petrol. It’s neither logical nor safe, and yet it happens worryingly regularly. Moving and eating, two basic requirements, are made needlessly complex due to our brains interfering with the process. However, eating is one of life’s great pleasures, and if we were to treat it as if we were just shovelling coal into a furnace, maybe our lives would be a lot duller. Maybe the brain knows what it’s doing after all.
To sleep, perchance to dream … or spasm, or suffocate, or sleepwalk
(The brain and the complicated properties of sleep)
Sleep involves doing literally nothing, lying down and not being conscious. How complicated could it possibly be?
Very.
Sleep, the actual workings of sleep, how it happens and what’s going on during it, is something people don’t really think about that often. Logically, it’s very hard to think about sleep while it’s happening, what with the whole ‘being unconscious’ thing. This is a shame because it’s baffled many scientists, and if more people thought about it we might be able to figure it out faster.
To clarify; we still don’t know the purpose of sleep! It’s been observed (if you adopt a fairly loose definition) in almost every other type of animal, even the simplest kinds like nematodes, a basic and common parasitic flatworm.10 Some animals, such as jellyfish and sponges, don’t show any sign of sleeping, but they don’t even have brains so you can’t trust them to do much of anything. But sleep, or at least some regular period of inactivity, is seen in a wide variety of radically different species. Clearly it’s important, with deep evolutionary origins. Aquatic mammals have evolved methods of sleeping with only half the brain at a time because if they slept fully they’d stop swimming, sink and drown. Sleep is so important it outranks ‘not drowning’, and yet we don’t know why.
There are many existing theories, such as healing. Rats deprived of sleep have been shown to recover much more slowly from wounds and generally don’t live nearly as long as rats that get sufficient sleep.11 An alternative theory is that sleep reduces the signal strength of weak neurological connections to make them easier to remove.12 Another is sleep facilitates reduction of negative emotions.13
One of the more bizarre theories is that sleep evolved a means of preserving us from predators.14 A lot of predators are active at night, and humans don’t need 24 hours of activity to sustain themselves, so sleep provides prolonged periods where people are essentially inert, and not giving off the signs and cues that a nocturnal predator could use to find them.
Some may scoff at the cluelessness of modern scientists. Sleep is for rest, where we give our body and brain time to recover and recharge after a day’s exertions. And, yes, if we’ve been doing something particularly exhausting, a prolonged period of inactivity is helpful for letting our systems recover and replenish/rebuild where necessary.
But if sleep is all about resting, why do we almost always sleep for the same length of time whether we’ve spent the day hauling bricks or sitting in our pyjamas watching cartoons? Surely, both activities don’t require equivalent recuperation time. And metabolic activity of the body during sleep lowers by only 5 per cent to 10 per cent. This is only slightly ‘relaxing’ – like dropping from 50 mph to 45 mph while driving because there’s smoke coming from the engine is only slightly helpful.
Exhaustion doesn’t dictate our sleep patterns, which is why people seldom just fall asleep while running a marathon. Rather, the timing and duration of sleep is determined by our body’s circadian rhythms, set by specific internal mechanisms. There’s the pineal gland in the brain that regulates our sleep pattern via secretion of the hormone known as melatonin, which makes us relaxed and sleepy. The pineal gland responds to light levels. The retinas in our eyes detect light and send signals to the pineal gland, and the more signals it receives the less melatonin it releases (although it does still produce it at lower levels). The melatonin levels in our body rise gradually throughout the day, and increase more rapidly when the sun goes down, hence our circadian rhythms are linked to daylight hours so we’re usually alert in the morning and tired at night.
This is the mechanism behind jet-lag. Travelling to another time zone means you are experiencing a completely different schedule of daylight, so you may be experiencing 11 a.m. levels of daylight when your brain thinks it’s 8 p.m. Our sleep cycles are very precisely attuned, and this throwing off of our melatonin levels disrupts them. And it’s harder to ‘catch up’ on sleep than you’d think; your brain and body are tied to the circadian rhythm, so it’s difficult to force sleep at a time when it’s not expected (although not impossible). A few days of the new light schedule and the rhythms are effectively reset.
You might wonder, if our sleep cycle is so sensitive to light levels, why doesn’t artificial light affect them? Well, it does. People’s sleep patterns now have apparently changed wildly in the last few centuries since artificial light became commonplace, and sleep patterns differ depending on culture.15 Cultures with less access to artificial light or different daylight patterns (for example, at higher latitudes) have sleep patterns that have adapted to their circumstances.
Our core body temperature also changes according to similar rhythms, varying between 37°C and 36°C (which is a big variation for a mammal). It’s highest in the afternoon, then drops as evening approaches. At midway between the highest and lowest points is when we typically go to bed, so we’re asleep when it’s at its lowest, which may explain the human tendency to insulate ourselves with blankets while we sleep; we’re colder then than when we’re awake.
To challenge further the assumption that sleep is all about rest and conserving energy, sleep has been observed in hibernating animals.16 That is, in animals that are already unconscious. Hibernation isn’t the same as sleep; the metabolism and body temperature drops much lower; it lasts longer; it’s closer to a coma really. But hibernating animals regularly enter a sleep state, so they use more energy in order to fall asleep! This idea that sleep is about rest is clearly not the whole story.
This is especially true of the brain, which demonstrates complicated behaviours during sleep. Briefly, there are currently four stages of sleep: rapid-eye-movement sleep (REM) and three non-rapid-eye-movement (NREM) stages (NREM Stage 1, NREM Stage 2 and NREM Stage 3, in a rare example of neuroscientists keeping things simple for the lay person). The three NREM stages are differentiated by the type of activity the brain displays during each.
Often the different areas in the brain synchronise their patterns of activity, resulting in what you might call ‘brainwaves’. If other people’s brains start synchronising too, this is called a ‘Mexican brainwave’.† There are several types of brainwaves, and each NREM stage has specific ones that occur.
In NREM Stage 1 the brain displays largely ‘alpha’ waves; NREM Stage 2 has weird patterns called ‘spindles’, and NREM Stage 3 is predominately ‘delta’ waves. There is a gradual reduction in brain activity as we progress through the sleep stages, and the further you progress the harder you are to wake up. During NREM Stage 3 sleep – ‘deep’ sleep – an individual is far less responsive to external stimulus such as someone yelling, ‘Wake up! The house is on fire!’, than at Stage 1. But the brain never shuts down completely, partly because it has several roles in maintaining the sleep state, but mostly because if it did shut down completely we’d be dead.
Then we have REM sleep, where the brain is as active, if not more so, as when we’re awake and alert. One interesting (or sometimes terrifying) feature of REM sleep is REM atonia. This is where the brain’s ability to control movement via motor neurons is essentially switched off, leaving us unable to move. Exactly how this happens is unclear; it could be that specific neurons inhibit activity in the motor cortex, or the sensitivity of the motor control areas is reduced, making it much harder to trigger movements. Regardless of how it occurs, it does.
And that’s a good thing, too. REM sleep is when dreaming occurs, so if the motor system was left fully operational people would be physically acting out what they’re doing in their dreams. If you can remember anything you’ve done in your dreams, you can probably see why this would be something you’d want to avoid. Thrashing and flailing while asleep and unaware of your surroundings is potentially very dangerous, for you and any unfortunate person sleeping nearby. Of course, the brain isn’t 100 per cent reliable, so there are cases of REM behavioural disorders, where the motor paralysis isn’t effective and people do in fact act out their dreams. And it’s as hazardous as I’ve suggested, resulting in phenomena such as sleepwalking, which we’ll get to shortly.
There are also more subtle glitches which will probably be more familiar to the everyday person. There’s the hypn
ic jerk, where you twitch suddenly and unexpectedly while falling asleep. It feels as if you’re falling suddenly, resulting in spasm while in bed. This occurs more in children and gradually declines as we age. The occurrence of hypnic jerks has been associated with anxiety, stress, sleep disorders and so on, but overall they seem to be largely random. Some theories state it’s the brain mistaking falling asleep for ‘dying’, so it tries urgently to wake us up. But this makes little sense as the brain needs to be complicit in us falling asleep. Another theory is that it’s an evolutionary holdover from a time when we slept in trees and sudden tilting or tipping sensations meant we were about to fall out, so the brain panics and wakes us. It could even be something else entirely. The reason it occurs more in children is likely to be due to the brain still being in the developing stages, where connections are still being wired up and processes and functions are being ironed out. In many ways we never truly get rid of all the glitches and kinks in such complicated systems as those used by our brains, so hypnic jerks persist into adulthood. Overall it’s just a bit odd, if essentially harmless.17
What’s also mostly harmless, but doesn’t feel like it, is sleep paralysis. For some reason, the brain sometimes forgets to switch the motor system back on when we regain consciousness. Exactly how and why this happens hasn’t been confirmed, but the dominant theories link it to disruption of the neat organisation of the sleep states. Each stage of sleep is regulated by different types of neuronal activity, and these are regulated by different sets of neurons. It can happen that the differing activity doesn’t alter smoothly, so the neuronal signals that reactivate the motor system are too weak, or the ones that shut it down are too strong or last too long, and as such we regain consciousness without regaining motor control. Whatever it is that shuts down movement during REM sleep is still in place when we become fully alert, so we’re unable to move.18 This typically doesn’t last long as once we wake up the rest of the brain activity resumes normal conscious levels and overrides the sleep system signals, but while it does it can be terrifying.
