What happens to your body on a plane – and how to stay healthy

Follow these tips to avoid emerging bloated, dehydrated and exhausted when travelling by plane

Humans never evolved to be transported through the skies in a long metal tube at more than 500 miles per hour, so it’s perhaps unsurprising that aeroplane travel has some pretty unusual effects on our body.
A recent scientific study in the journal Thorax made headlines after finding that the cabin pressure at cruising altitude appears to lower blood pressure and increase heart rate, even among young, healthy passengers. This is particularly exacerbated by in-flight alcohol consumption, and as a result, the authors recommended either avoiding, or at least minimising the number of alcoholic drinks you have on a flight, especially if you have pre-existing heart problems. 
Dr Stephen Hughes, a senior lecturer at Anglia Ruskin University and an emergency medicine consultant, says that there are a range of both physiological and psychological adaptations which happen to the body as a consequence of being on a plane. 
To begin with, humans are not supposed to be at 35,000 feet, the typical cruising altitude of a commercial plane. At these heights, oxygen levels are so low that without the inbuilt cabin pressurisation system, we would rapidly lose consciousness. Instead, to allow the crew and passengers to breathe comfortably, the cabin simulates the kind of pressure we would experience at around 6,000-8,000 feet, around halfway up Mont Blanc.
Dr Hughes is at pains to emphasise that the vast majority of the time, planes are extremely safe environments and adverse health events are incredibly rare, but these different factors help to explain why they do occasionally happen.
The more we understand the unusual environment of flying, the more we can take steps to make it a better and less stressful experience for our body. So from dehydration to circadian rhythms, this is how flying impacts you and what to do about it. 
As air pressure reduces after take off, it can lead to mild levels of something called hypoxia where there is less oxygen in the blood flow. Studies show it can alter our ability to think clearly, our memory and also our mood, increasing negative emotions such as anxiety and tension.
Some people are also more nervous fliers than others, triggering the release of stress hormones. “It’s an alien environment and you have no control, so people sometimes get anxious and over-breathe,” Dr Hughes explains. “Your heart rate might go up and that can be a risk factor for cardiac trouble. So you’ve got this interplay of the physiological and psychological.”
As we go higher into the atmosphere, the air pressure around us reduces. To equalise the pressure, your ears ‘pop’ to release air from the middle ear and the sinuses.
Because of the drop in humidity, your eyes and nose might feel dry. It can also weaken your sense of smell, which is why we crave salty foods.
Your mouth may feel dry because of the drop in humidity. That combined with low air pressure reduces the sensitivity of our taste buds to salt and sweet by 30 per cent.
As the airplane ascends, most of us will feel gas expansion in the abdomen which may result in feeling bloated or gassy. Because of the lower air pressure, there is a reduced level of oxygen in the body, which can slow down digestion in the body.
Sitting for long periods of time as you do on a flight can increase the risk of a blood clot inside a deep vein, known as deep vein thrombosis (DVT). A WHO study showed that the risk increased two- to three-fold after long-haul flights of more than four hours and similar forms of travel.
The humidity in an aircraft is usually less than 20 per cent, compared to the 30 per cent we’re used to in our homes so your skin might feel drier than usual.
At high altitudes, the air is almost completely devoid of moisture, with humidity levels on a plane often just 10 to 20 per cent, compared to 35 to 65 per cent in a normal everyday environment.
“That means people will breathe out water vapour, so they’re losing total body water and on a longer flight, you get very dehydrated,” says Dr Hughes.
He says that nervous fliers are even more prone to dehydration as a higher heart rate means that you will be breathing out even more water vapour. Drinking alcohol to relax your nerves can also exacerbate dehydration as alcohol is a diuretic, meaning it stimulates the kidneys to flush more fluid out of the blood and produce more urine. Instead of alcohol, Dr Matthew Goldman, a family medicine physician at the Cleveland Clinic, recommends aiming to consume around 250ml of water per hour. 
However, as Dr Rosie Godeseth, an associate medical director at Vitality, explains, there are some simple steps you can take to mitigate the impact of dehydration and lessen any related discomfort. 
“Drink plenty of water,” she says. “[Avoid] caffeinated and high sugar drinks, which can also have a similar effect to alcohol, dehydrating us further. Other easy steps include, wearing glasses instead of contact lenses if you wear them to reduce discomfort to your eyes, and applying a hydrating lotion or lip balm to avoid dry lips and skin.”
Dr Godeseth explains that being in a low humidity environment for a number of hours, actively dries out the nasal passages increasing your chances of catching an infection. 
“This dryness may make us more susceptible to catching a virus or bug, such as the common cold,” she says. “Additionally, like any environment where we are in close proximity to others, germs spread much easier throughout airborne particles or touching contaminated surfaces such as tray tables, seat belts and arm rests.”
To mitigate your risk, Dr Godeseth advises regularly washing your hands on the flight, trying to avoid touching your face with your hands and using hand sanitiser.
One of the problems with becoming too dehydrated during a flight is that your blood becomes a little thicker and more likely to clot, simply because there’s less fluid in it.
Dr Hughes says that being sat in a seat for many hours with your knees bent can cause blood to pool and stagnate in the veins of the lower limbs. “You’re impeding the drainage,” he says. “Having your legs stretched out may improve flow for a bit but not for long. Getting up for a walk now and then is really important because it allows the calf muscles to pump blood back towards the heart.”
Dr Goldman recommends standing up, walking around and stretching once an hour, particularly when you’re on a long-haul flight. “Perform leg exercises such as ankle circles and foot pumps,” he suggests. “And wear compression socks to help maintain circulation and reduce swelling.”
Most airplane food comes with an extra dollop of seasoning, and one of the reasons for this is that a third of your taste buds stop working on high altitude, making the majority of foods seem rather bland. Your appetite is even impacted by the dry air on a plane, as this impacts the nose and appetite is intrinsically connected with our ability to sense the aroma of food.
Dr Godeseth says that you’re also less likely to feel hungry if you’re stressed. “For some people, the fight or flight response can kick in, disrupting hunger-regulating hormones like leptin,” she says. “Taste can also be affected as the cabin environment dulls our senses, with factors such as cabin lighting, stress, and even in-flight entertainment impacting how things taste to us.”
Because of this, she recommends taking light, easily digestible snacks on a plane such as bananas. 
According to Dr Eva Winnebeck, a lecturer in chronobiology at the University of Surrey, there are three main aspects in which plane travel impacts your sleep. The first is the time of the flight – with morning flights which require you to be at the airport early, curtailing your sleep. The second is the flight itself, with noise, light, and uncomfortable seating often impacting your sleep quality.
But while there isn’t always much you can do about the first two, apart from booking an airport hotel to ensure you get a little more shuteye, there are certain steps you can take to mitigate the third – the impact of flying on your circadian rhythms. 
Dr Winnebeck explains that the human body operates to a precise 24-hour clock which regulates all kinds of biological processes. Disruptions to this clock through switching time zones can impact how many enzymes are around to digest our food, how well our metabolism is working, bowel movement patterns, heart rate, cognition and of course sleep. 
She says when we fly west, for example from the UK to the United States, you’re extending the day and so you need to slow down your circadian clock. “That’s usually easier to do, because the clock tends to be a little slow anyway, so people tend to find it easier to stay up for a little longer without too much sleep disruption,” she says.
However, when you fly east, for example from the US back to the UK, or from the UK to Asia, you’re actively speeding up your clock. “That’s much harder which is why people suffer from sleep inertia for a few days after landing where it’s hard to wake up or concentrate,” Dr Winnebeck explains. 
To mitigate this, she advises sticking to the new time schedule as much as possible, getting as much daylight during the day as you can because this helps reset the circadian clock to the new time zone.
Jet lag can also impact digestion for a couple of days, which is why Dr Winnebeck advises eating meals in the new time zone because the circadian rhythms which determine processes in the stomach and the liver react swiftly to the timing of food intake. 
But simply being on a flight can also impact both your digestion and your gut in surprising ways. “The atmospheric pressure changes you find on planes are responsible here,” says Dr Godeseth. “As the plane ascends and descends, the cabin pressure fluctuates, and your digestive system adjusts to this pressure, leading to excess gas in your intestines. This can make you feel bloated and uncomfortable.” 
Dr Goldman warns that the combination of shifts in cabin pressure and dehydration can affect bowel movements, occasionally triggering constipation. “My recommendations are to consider taking probiotics to support your gut microbiota,” he says. “Proper hydration also helps maintain gut motility.”
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