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Hydration for divers

How many of you really know if your body is fully hydrated and ready for a dive? Fluid intake is one of the most important (but neglected) aspects of diving physiology and is something that you, the diver, have full control over. Hydration is one of the major players in the prevention of both DCI and soft tissue injury, so it is in your interest to understand what your body needs to perform to the max. There are few, if any references in the training manuals (entry level, advanced or technical) about fluid regulation and unless you have prior knowledge or a particular interest in fitness many divers remain blissfully ignorant about the importance of being properly watered. So here it is, the complete definitive guide to optimum hydration.

Physiology and water weight

Seven tenths doesn’t only apply to the water covering our planet, we have coincidentally the same ratio of fluid in our body. In fact between 69% and 72% of our total body mass comes in the form of fluids. Physiologically we have two types of fluid compartments, intracellular (inside cells) and extracellular (blood plasma, transcellular and interstitial fluid). Much of the water in our body is found in lean muscle mass. Interestingly, nitrogen appears to have a much lower affinity for water saturated tissue than for fat. Fat tissue holds five times more nitrogen than water permeable ‘aqueous’ tissue (such as muscles, ligaments, tendons and connective tissue).
We lose fluid through sweat, respiration, urination, diuretic action, stomach upsets and if you are female, menstruation.

In sports physiology, sweat loss is considered to be the most important aspect of fluid regulation. This is a factor that can be misunderstood and underestimated by divers. Our body fluid levels are intrinsically linked with our ability to sweat. Sweat is excreted as a result of heat production. Heat is a by product of muscular activity with about 75% of the energy expended during exercise being converted into heat. This extra heat has to be dissipated in order to keep our core body temperature a constant 37 – 38° C. Sweat is produced  by some 3½ million sweat glands, when water from our body is carried via our blood capillary network to the skin, where cooling takes place as sweat evaporates.

We perspire in two different ways. Sensibly and insensibly. Sensible perspiration is sweat that is excreted in large amounts and is noticeably moist on the skin.  Insensible perspiration is sweat that evaporates from the skin before it becomes moisture and we are therefore unaware of it. Normally, we start sweating on our foreheads first, then our face followed by the rest of our body with palms and soles of the feet last. Conversely, emotional sweat occurs in the palms, soles and armpits first. Incidentally, emotional sweat is relied upon in lie detector tests as even the smallest amount of perspiration contains sodium chloride which acts as an electrical conductor and therefore lowers the electric resistance of the skin. Both types of perspiration are important to a diver.

A ½ litre of water equates to about 1lb or ½ kg of body weight. It is possible to lose as much as 2lbs (about 1kg) of body weight through sweat in one hour performing normal aerobic type exercise, which in fluid terms amounts to about a litre. During high impact workouts or if you happen to be a person who sweats profusely, as much as 2 litres of sweat per hour can be lost.

Diving can be aerobic, anaerobic, stressful and emotional. Sweat production varies greatly during a dive depending on air and water temperatures, exposure suit, anxiety levels, currents, exertion, distance covered, whether you are male or female, individual fitness, experience and so on. Add to this the fact that divers breathe very dry air, are usually unable to replace fluids during a dive (unlike other sports), often dive in tropical destinations with high humidity, blast themselves on the sun-decks of liveaboards and it’s easy to see how dehydration sets in. If you are a cold water diver don’t think for a minute that you are immune ….. the morning after the night before, strong black coffee, cans of coke, mugs of tea and the limitations of peeing in a drysuit all conspire against you.

Most serious athletes know that a loss of only 2% of their body weight through sweating can reduce their aerobic performance by as much as 10%. In diving terms this fluid loss means less efficiency at off-gassing absorbed  nitrogen. Blood becomes thicker and more viscous when it’s volume decreases through dehydration. Consequently blood flow is slowed which means the speed at which we eliminate nitrogen is reduced. The obvious danger becomes an increased risk of developing DCI. Other signs of dehydration include an increased susceptibility to headaches, fatigued muscles, dry or congested sinuses, gritty eyes, light headedness and nausea. 

Dehydration – The facts

The first thing to understand about dehydration is that it is cumulative. How much or little fluid you take in over the days preceding a dive will have a dramatic effect on your hydration at the start of that dive and subsequent physiological performance. This means that a diver will be exponentially increasing their risk of DCI and soft tissue injury, through lack of fluids, over a consecutive series of dives, if this goes unchecked.

Without exception, we all wake up from a nights sleep in a state of dehydration. If you want to work out specifics, then weigh yourself last thing before going to bed (after consuming your last beverage) and again first thing in the morning (before drinking or eating). The weight loss is primarily fluid lost through overnight sweat and respiration (and any nightly visits to the toliet). Most of us will start the day about 1- 1½ litres down on our normal body hydration (known as euhydration). If you are planning an early morning dive, the chances are you will be doing the dive in a state of dehydration as it takes time and effort to fully rehydrate. Add to this the fact that you might be starting your diving soon after a flight and you are even further  at a  disadvantage  as   the atmosphere on aircraft is dry and of low humidity and is severely dehydrating.

So how do you know if you are dehydrated? Well, the easiest way is to check the colour of your urine. The stronger the yellow and the more potent the smell, the more dehydrated your body is. Ideally, urine should be plentiful, very pale yellow or almost colourless and odourless. However, there are exceptions to the rule and diuretic activity is one.

Diuretic Action

To understand how diuretics affect you as a diver it is worth knowing a little bit more about their nature. A diuretic is simply an agent, chemical or natural, that increases the rate of urine formation. The two most common diuretics that influence a diver are caffeine (found in coffee, tea cola drinks and chocolate) and alcohol. These two naturally occurring diuretics act on the body in an entirely different way but with the same end result – increased urine output (diuresis). Alcohol inhibits the secretion of Antidiuretic Hormone (ADH). Caffeine on the other hand inhibits sodium reabsorption. The diuretic action of these two substances is such that the net quantity of urine produced is greater than the actual amount of liquid consumed. Diuretics are therefore dehydrating. The resultant effect has a negative action on thermoregulation and cardiovascular function.

In terms of urine output and colour, the danger here is that urine colour can become diluted by the extra fluid sucked out of hydrated tissue, and visits to the toilet increase, leading to a false sense of security. That beer is roughly 95% water is of no consequence as this is more than offset by the alcohol content, so don’t kid yourself.

To counterbalance fluid lost through diuretic action a useful guideline is to additionally drink water, 1½ times the quantity of diuretic fluid either at the same time, or half an hour either side of intake.

Exposing the truth

In most other sports the use of ‘sweat suits’ is considered positively dangerous. These tight restrictive suits increase the rate of fluid loss by preventing sweat evaporation. Body heat increases as the cooling effect of sweat evaporation is restricted and this in turn causes an increase in sweat production.

In many ways our protective exposure suits work in a similar fashion to ‘sweat suits’ and we lose far more fluid during a dive than we realise (even though we may eventually chill) – it is just much less obvious as sweat is either washed away with water flushing through our wetsuits or is wicked away by drysuit undergarments.


Although it might be comfortable and convenient to hang out between dives in your exposure suit, you could quite innocently be sweating away valuable water so be aware. Do whatever you can to minimise fluid loss, protect yourself from overheating and drink more to compensate.

To Pee or not to Pee

The ‘P’ phenomenon (a.k.a. immersion diuresis), irritating as it is, affects every diver. When we immerse ourselves in innerspace and become weightless, the hydrostatic gradients (pressure exerted by the weight of fluid) in our circulatory system become equal to that of the surrounding ambient atmospheric pressure of water. The resulting effect physiologically is that the fluid which normally pools in the extremities (gravity assisted) floods back into the body increasing central blood volume. In order to control fluid volume the heart responds to this increase and signals the kidneys to start dumping fluid and so we feel the need to pee. No amount of intentional dehydration can stop this occurrence.

Breathing cold compressed air, cold-water immersion or becoming cold during a dive will have a similar effect. Vasoconstriction (decrease in the diameter of blood vessels) shifts blood away from the arms and legs towards the centre of the body to help maintain core body temperature. The extra fluid filtered through the kidneys is similarly expelled.

In view of the fact that whether you want to or not, you will, on most dives, feel the need to pee, you might as well drink lots of fluid. If you are correctly watered, your urine will be so dilute that it really shouldn’t affect the smell of your wetsuit any more than salt water will. If you are dry suit diving, there are the unattractive but functional incontinence undies, nappy, ‘P’ bag options or for shorter dives adjusting the timing of fluid intake might just get you through.

The air that you breathe

Less of a factor if you are on a rebreather unit as the circulating air is warm and moist, but for a diver on an open circuit rig the air you breathe is very dry. There are numerous references to this air being as dry as desert air, which it may well be. However, there is a big difference here between warm and cold water diving.

In warm water the lungs are not having to warm the compressed air all that much as the tank and air will become unified with the ambient water temperature. In effect we will be expending less energy, and there is only a finite amount of moisture particles able to be expelled on exhalation anyway (dry air that has in effect been humidified by moisture in our lungs). The estimated fluid loss from breathing this dry air in warm water is not nearly as significant as that lost through sweat or urine. On a two tank day this moisture loss is estimated at less than half a cupful, a remarkably small amount of fluid. In the scheme of dehydrating factors this is way down the list but added to the other determining elements of dehydration, it certainly counts and is still about double the amount of moisture normally exhaled.

In cold water the difference is that we breathe much colder air and so we work harder to warm the air before exhaling therefore using up more energy and consequently more fluid.

One for the girls

Although current research about menstruation and DCI incidence is inconclusive, there are a few key changes concerning fluid that affect the female diver and, if added to the other elements contributing to poor hydration, might just tip the balance.

During menstruation and for a few days prior to a period, a fluid shift takes place influenced by hormonal, mineral and dietary changes. Blood volume reduces as fluid moves from the capillary network into body tissue causing swelling and bloating. This fluid shift creates a similar condition to that of dehydration.

A further important change is the loss of iron during the menstrual cycle which ranges between 5mg and 45mg. Iron is crucial for the transportation and utilisation of oxygen in the blood. If your blood volume is low from being both dehydrated and menstruating and if your iron levels have dropped significantly, the combined effect becomes a slowed rate of off-gassing nitrogen and a reduction in the oxygen carrying capacity of the blood. Not a good combination for a diver.

With this in mind a number of precautionary measures would be well advised. To start with, the use of Nitrox (as rich a blend as possible for the dive) would reduce nitrogen loading. Include some iron rich foods in your diet (red meat, spinach and watercress, shellfish) or consider supplementing iron (5mg – 10mg daily should be adequate). Be particularly vigilance with water intake and do some light exercise to help reduce congestion in the pelvic area.


Slow and gradual hydration is the best approach as too much water taken in at any one time just gets flushed through your system. Thirst is actually a poor indicator of hydration levels because by the time you become aware of your need for fluid it’s too late, you are already dehydrated. Prevention is a far better and safer option and in the same way that dehydration is cumulative so is hydration.

The most effective approach is to start early, a few days before a dive trip. Drinking about 500mls of water as soon as you wake is advantageous and a good habit to acquire (and is necessary to begin offsetting overnight dehydration). On non-diving, inactive days you should be looking at drinking a further 2 litres during the course of the day. Small amounts often is recommended. At the same time diuretic intake should be reduced to maximise the absorption of water into the intracellular tissue of your muscles and to increase blood volume. If you are exercising or losing fluids through atmospheric influences then drink more to compensate.

The day before a dive increase fluid intake by a further 500mls – 1ltr, especially if the diving is technical, deep, long or strenuous or if you are planning multiple dives. If possible, consume no diuretics at all. Active or aggressive hydration is being championed to some extent in the technical diving arena but really, we should all be following these principles.

The day of diving, and this is the difficult one, it’s all in the timing. If you are a warm water diver, using a wet suit, then you have no excuse – drink and pee to your hearts content. Take into account the logistics of the whole day, number of dives, weather, your own fitness and so on. Add a minimum of  ½ltr – 1ltr of extra water per average 1 tank dive on top of the normal daily fluid allowance you are now hopefully consuming (2½ – 3½ litres). Spread this out evenly through the day making sure you have at least 250mls immediately before and after each dive. This should offset some of the sweat and respiratory fluid loss.

If you are drysuit diving and not using a pee bag of some sort, then following the hydration principles of athletes in competition is a possible option. A minimum fluid intake of 500mls about 2hrs before a dive is a suggestion. This allows plenty of time for absorption and any excretion of excess fluid. Immediately before the dive a further intake of 250mls to see you through the dive. Hypotonic sports drinks would be good here as the sodium content will encourage the body to hold onto water and the glucose will provide energy. Immediately after the dive, in order to restore fluid loss, you should be drinking a further ½ltr – 1ltr. If you want to be obsessive about it and have worked out your individual fluid loss using the weight method, then you should be aiming at 1.5 times the fluid lost during the previous dive.

During the normal course of a day, assuming you are eating three regular meals, it would not usually be necessary to supplement your fluid intake with anything extra. However, there are a number of additional aids that benefit active hydration and might be worth considering if you are planning multiple dives over consecutive days or anything extreme with long deco stops. These include sports or electrolyte drinks (the sodium content stimulates the thirst mechanism urging us to drink even more), mineral rich foods such as bananas, watercress, and mushrooms which are all high in potassium, and finally glycerol which is used by endurance athletes to achieve a state of hyperhydration. 

Glycerol works by pulling water osmotically into both the intra and extracellular tissue increasing total body fluid and blood volume as a result. So far there have only been a handful of studies published on the effects of glycerol and hydration. Information is still emerging and to date there is nothing available that has encompassed diving. Early indications are looking positive elsewhere in the sports industry and trials are continuing. What has emerged though, is if too much glycerol is used too quickly, side effects can occur and include headaches, blurred vision and changes in bowel movement.

Electrolytes and Rehydration

Diarrhoea and vomiting both cause major fluid loss but of equal importance is the subsequent change in electrolyte balance. Electrolytes are mineral salts such as sodium, potassium, magnesium, calcium and chloride. These salts need to be continually replenished which normally happens through our daily diet. The concentration of these salts helps to regulate the distribution, balance and movement of fluid between the blood and different body compartments (such as the cells in muscles, tendons and ligaments). Water movement is affected by the electrolyte concentration on either side of a cell’s membrane. For example, sodium and potassium both draw water osmotically through a cell membrane, from an area of low concentration to an area of high concentration. Therefore a high concentration of these salts inside a cell will increase it’s water content as water is needed to dilute the salt concentration.

Apart from the victims of stomach bugs, divers prone to sea sickness also need to be particularly aware of their hydration levels and electrolyte balance. It’s worthwhile including a few sachets of rehydration salts such in your medi-kit, just in case. If possible, allow about 48hrs to repair the damage once you have recovered and especially if you have been hit hard and are planning deep, long or multiple dives. 

There are primarily three different types of electrolyte drinks – hypertonic, hypotonic and isotonic. Hypertonic drinks are the most concentrated and are therefore absorbed more slowly than water. Hypotonic are the least concentrated and are absorbed faster than water. Isotonic drinks are balanced to suit the osmotic pressure of a solution in the body, and so absorb at the same rate as or faster than water. Fluid replacement drinks are either hypotonic or isotonic. They contain dilute solutions of sugar (carbohydrate) which encourages absorption of fluid into the blood via the small intestine. A hypotonic drink that works well as a rehydration remedy is 250mls fruit juice, 750mls water and ¼ teaspoon salt. A recipe for an isotonic drink would be a 50/50 mix of water and fruit juice with an optional ¼ teaspoon salt per litre. Both hypotonic and isotonic drinks would be suitable for diving and contrary to popular belief, they are not dehydrating.

In-water Hydration

Still relatively unfamiliar within recreational diving circles, in-water hydration has been used by technical divers, cave divers and on deep wreck expeditions with enormous success. With an increasing number of divers qualifying in the use of technical gases, and extended range diving becoming more popular and accessible, we are spending longer and longer under water. The need for rehydration during this type of activity is without doubt of immense value. Advocates of in-water hydration are more than convinced that it contributes positively to reducing the risk of DCI.

There are a number of in-water hydration systems beginning to emerge onto the market which look promising and would be suitable for recreational diving. An example of one such system is the Sea Pump which is designed for use with open circuit kit and supplies a diver with 250ml of fluid during the course of a dive – more than adequate to offset moisture lost through respiration and would contribute towards topping up body fluid levels. It is just as easy though to rig up a DIYW (do it your way) system using any Cordura type drinking vessel with a one way push-sealing nozzle (as found in outdoor pursuit shops). For technical divers there is the SCUDA – Self Contained Underwater Drinking Apparatus or the  Weezle Rehydration System which is capable of supplying a generous 1.8 litres of fluid.

Extreme, technical, extended range, obsessive and very serious divers should know as accurately as possible what their fluid requirements are. This has to be one of the most important considerations of safety. You need to be acutely aware of your physiological response to dehydration and should be doing everything possible to minimise fluid loss and maximise consumption. If you are planning an extreme dive consider taking a combination of plain water, fruit juice diluted with water, cold soups and hypotonic drinks with you.

Fluid for thought

Be pro-active about drinking water. It is your responsibility to look after your body. Optimum hydration is fundamental to your physiological wellbeing and is often grossly neglected by divers. Plan your hydration just as you would plan your dive. It is amazing how many hours we spend talking about and preparing for a dive trip. Calculating gas blends, decompression schedules, boat and tidal logistics, safety considerations, kit configuration and dive site details are all part and parcel of the passion of diving. Hydration is no less important. If you are a diver who deliberately reduces fluid intake or uses diuretics (chemical or natural) before a dive, take heed. Drink to dive another day.



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