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(CNN) -- Flying not only disrupts your body clock, it also messes up your digestive system.
So it's good to know what you are tucking into the next time you choose chicken or fish on an airplane.
If you are worried about airline food and its nutritional, salt and fat content, you can do something about it.
Or at least be aware of what in-flight meals are doing to you -- from expanding your stomach gasses to dehydration.
There are also concerns if you have nut allergies or other food constraints.
CNN spoke to nutritionist Hannah Theobald from the British Nutrition Foundation on the issues surrounding airplane food.
What are the key issues in terms of the nutritional values of food served on airlines?
The nutritional value of food served on airlines varies depending on what the meal is and where you are in the world.
One of the key things when considering airline food is that many passengers will be seated for the whole flight, and therefore the energy requirements are not that high compared to normal everyday life.
However, if you look at airline foods, many of them are very high in energy, very high in fat and also very high in salt -- and these are a concern.
What advice would you give to people who are worried about too much fat or salt in their airline food?
Many passengers are not aware that you can pre-order meals that are low in energy, and low in fat or salt before you board the plane.
Normally, you need at least 24 hours to do this. So this is a useful trick to employ if you are worried about the nutritional content of the food.
Do you think airline food provides the necessary nutrition?
There have not really been any surveys on this so it is hard to say. Many airlines will prepare food that is low in salt, energy or fat for people who are concerned or have any specific dietary requirements.
You need to order these foods at least 24 hours in advance to ensure that you will get the food, and these form a good choice for people who are concerned about what they are eating. Alternatively you can take your own food on board.
Why do airlines add extra salt to their food?
Many airline caterers add a lot of salt to their food because it is said that our taste buds do not work efficiently at altitude. However, there are consequences of having too much salt in your food. Too much salt leads to dehydration, which may increase the risk of deep vein thrombosis (DVT).
It can also make you feel uncomfortable. For example, your ankles might swell because there is a lot of salt in your body, so there is a bit of discomfort there. And also a lot of salt in the diet increases the risk of high blood pressure, which is a factor for heart disease and stroke.
Are airline caterers aware of these health risks?
Awareness of nutrition among airline caterers is improving and there are steps they can make to improve the nutrient content of food. They can help reduce the salt content by swapping the amount of salt in food with pepper or herbs and spices, which will help add a bit more flavor. But it is important they have a look at the salt, fat and energy content of their foods.
A recent survey conducted on a U.S. carrier showed that an airline meal provides 950 calories -- to put this in context, this is almost half the average female adult's intake. Also looking at the fat content, it was nearly 50 grams, of which half was saturated fat, so there is certainly room for improvement there.
Where are the possible sources of fat and salt content in an airline meal?
Most airlines provide three course meals, at least on long-haul flights, so if you consider the whole meal there are many opportunities for adding salt and fat to the food. For example, if you have a roll then most people have salted butter with it or margarine, so that is one example. Or if you have a dessert, they tend to be quite high in fat compared to a fruit salad, so it is possible to have a healthy option if you are concerned about this.
What about the freshness of the food? Does the food keep its nutritional value?
There are certain nutrients that are quite prone to being lost over a period of time or during heating, for example vitamin C and B12, which are found in fruit salads -- so this is a concern that the nutrient content of such foods will diminish over a period of time. Storing the foods in a fridge, or adding some antioxidant compound to the food can reduce this. So its possible to extend the nutrient content for longer, but most nutrients are quite stable.
Bearing in mind that your digestion slows down on a plane, what are the best food types to eat?
Flying not only disrupts your body clock, it also messes up your digestive system as well. So I would recommend that people eat light meals before they fly and also try and have light meals on the plane. So for example, just having small meals or perhaps requesting a low energy meal, and then if they are still hungry, having snacks such as bananas and apples, which all travel very well and this will keep your digestive system healthy and help reset your body clock as well.
How does flying affect the body's organs?
At altitude stomach gases can expand by as much as 30 percent, so we might get swollen stomachs. This can be a problem for some individuals, so having small light meals will help reduce these symptoms. Trying to keep the fiber content as small as possible will also help. I think the biggest issue when flying is that people eat too many calories.
My advice to anyone who is about to fly is to have a light snack before they board the plane and then continue to eat light snacks and meals throughout the flight. You should also try to drink a lot of fluid because dehydration is an issue. Rather than having high-caffeine drinks such as tea, coffee and colas, you should drink water and fruit juices.
I would recommend that individuals do not drink too much alcohol because this causes dehydration and can increase the risk of deep vein thrombosis. If you want to drink on the plane, alternate between a soft drink and an alcoholic beverage. It is certainly not advisable to get drunk.
How about people with allergies?
Allergy can be a real problem for some passengers. Some airlines do not really cater for people with nut allergies. However, it is always worth asking because many actually do cater for them. Some airlines are prepared to take peanuts off flights if individuals give them enough warning and they can prepare an alternative snack. Some airlines have nut-free policies.
If given enough warning airlines can provide you with a gluten-free meal, which is useful for people with celiac disease or maybe just a fresh fruit salad if you are particularly concerned about the food that you are getting. Alternatively it might be worthwhile preparing your own foods and taking on board.
Peanuts are a real problem for airline travel. Some individuals actually react to inhaling the proteins when people open packets of peanuts. No deaths have been reported, but it can cause discomfort, for example wheezing and running eyes. This is not pleasant when you are at altitude.
I am aware of one individual who went into a coma after eating peanuts and the plane had to be diverted.
What are the best places to get information about which airlines do and do not serve peanuts?
Information on allergy policies can be obtained from airlines, from their customer services departments. Alternatively it might be worth checking with the UK-based charity "the anaphylaxis campaign." They can provide some information on this, but I would stress that the information needs to be checked again before you fly to make sure the policies are up to date.
There is room for improvement as far as the nutrient content and profile of airline meals are concerned. The food nutritionally tends to improve as you go through the different classes on the airline, but not everyone is lucky enough to be able to fly first class and choose off the a la carte menu.
Do you think airlines do enough for people with special food needs?
The majority of airlines offer a good range of medical meals for individuals with allergies and special dietary requirements. Though cross-contamination is a real issue when catering because large companies are producing a whole number of meals with lots of different ingredients.
You cannot guarantee that trace amount of a certain food is not going to find its way into another meal. The only way to ensure this is not going to happen is to have a dedicated facility that does not handle nuts or seeds for example.
What is the best way to keep healthy on an airplane, as some people almost live on planes?
If you are a frequent traveler I would advise getting a lot of oily fish in your diet because we know that omega-3 fatty acids can help prevent deep vein thrombosis, because they stop the platelets sticking together to some degree.
I would recommend that people who travel frequently look at the dietary options before they fly. Try and pre-order or at least choose healthy options on the flight, and make sure they are eating lots of snacks such as fruit to try and keep the digestion ticking over.
I would also recommend that frequent travelers order the healthy option on board or pre-order if they can. Or if they are flying business class or first class and there is an a la carte option, try and tailor it to their requirements. It is important that they snack on things like fruit because it is quite hard to get adequate intake of this when you are on the go.
What should airline caterers do to improve the nutrition of the food?
Airline caterers should take a look at the energy, fat and salt content of food. It is really important that passengers are eating a balanced diet when flying. This means a diet based on starchy carbohydrates such as breads, potatoes, pasta or rice for example, ensuring that there is a good supply of fruit and vegetables in there and that some protein is included, be it fish, meat or a vegetarian sources such as tofu. It is quite important as well to reduce the amount of sugar in foods and to ensure that there is some dairy food in there as well.
It is really important to stay hydrated on a flight because altitude does cause dehydration. One way of getting around this may be to take a bottle of water on board with you and drink this at regular intervals. Also take glasses of water or fruit juice when it is offered, and if you are still feeling thirsty ask for some water or again a glass of juice.
However, if people are going to drink more I think airlines should provide more toilet facilities because some people actually do not drink on flights, because they are worried they may need to queue for a while and do not like the toilet facilities provided.