What is dehydration? How does dehydration affect my body? How much should I drink each day? What is dehydration? Dehydration is a shortage of water in the body and the chemicals dissolved in it. It can be sudden, usually as part of an acute illness, or so gradual we may not be aware of it. Adult humans are 50 to 75% water, varying with body type • is carried inside the cells of the body • is outside, in the spaces between the cells, circulating with the blood and in the bowel. In the average adult about 10 litres of fluid passes through the bowel every day although we drink only a small proportion of that and excrete even less. • 2-3 litres are ingested as food and drink • 7-8 litres enters the bowel from tissues in the body • 8-9 litres are reabsorbed from the small intestine back into the tissues, leaving approximately 1-2 litres reaching the colon. • Further reabsorption leaves a residue of only 200ml, which is excreted with the stool. • The bowel has a large spare reabsorption capacity, to cope with variations. With such large volumes moving in and out of the bowel small disturbances to the balance from a lack of intake or a problem with the movement of fluid can result in a major loss of water. Fluid is also lost when breathing and sweating. • The water lost in the breath is approximately 0.5 litre • In temperate climates, adults unaware of ‘sweating’ will also lose around 0.5 litre • In hot countries, or when body temperatures rises with exercise or illness, up to 15 litres a day can be lost although this loss is usually transient • High humidity reduces the ability to sweat and thus • • • control temperature by evaporation The organs most at risk from dehydration are the brain, the kidneys and the heart. The loss of water is what we can see from the outside, but it is the imbalance of the dissolved chemicals (especially sodium and potassium), that help control the transport of fluid in the body, which determines the clinical severity of dehydration and often causes the damage to internal organs. Those most at risk are: • The young – body mass is low proportional to surface area • The old – compensatory mechanisms may not respond quickly • The ill •• Inflammation of any part of the gastro-intestinal • •tract disturbs normal mechanisms •• Feverish illness increases water loss •• Decrease of the sensation of thirst, and the ability • •to tolerate water •• Athletes – can sustain sudden and severe fluid • •and chemical losses Water is also lost in the urine of course, and the mechanism for this is designed to maintain the balance. It is controlled by the anti-diuretic hormone (ADH) secreted in the brain (also known as vasopressin). This works through the kidneys to slow fluid loss and is produced by changes in blood volume and levels of sodium in the blood. • If sodium level falls, ADH secretion falls, kidneys • • • • produce dilute urine retaining sodium, so that level rises • If sodium levels rise, ADH secretion increases, kidneys • retain fluid until sodium level falls Dehydration results from a failure of the ADH control mechanism, often because the signals received by the gland producing it are conflicting. Examples of this are: • Sodium and blood volume both fall, due to sudden fluid loss •• Common in children, athletes, trauma • Fluids from the cells and the spaces around, including the blood, are lost out of proportion to each other •• There is usually more potassium inside cells and • •more sodium outside, so a loss of one fluid more • •than another will produce an imbalance. •• Haemorrhage, sudden heat loss, gastro-enteritis• •• • • ••(especially in children) •• Inappropriate rehydration, especially with large • •volume of water without sufficient chemicals, pools • •in the gut and encourages further sodium loss by • • •• • •osmolarity •• Both potassium and sodium imbalances are important, • •• abnormal levels of potassium causing cardiac • •• • •• • ••irregularities. How does dehydration affect my body? Water loss for a ten stone adult: Water loss Quantity Result 2% 1 litre Thirst 5% 2 litres Performance affected 10% 5 litres Possible syncope (fainting) 12% 6 litres Medical emergency 25% 12 litres Death Symptoms of dehydration: • Thirst • Dry mouth • Decreased urinary output • Reduced tear production • Sunken eyes • Cognitive impairment • Lethargy • Coma Signs of dehydration: • Poor skin tugor •• Skin loses natural elasticity, if pulled is slow to return • Delayed capillary refill •• A finger pushed into skin causes a ring of blanching, •• •• which is slow to refill on release • Low blood pressure • Pulse rate increases • Shock and collapse ‘Mild Dehydration’ There is debate about the extent of the medical consequences of mild dehydration – meaningful studies are hard to carry out but there is research that shows: • Mild dehydration has been linked to chronic headaches, and a loss of mental alertness • There is some evidence that drinking with a meal increases satiety and can assist weight loss • The formation of renal stones and urinary tract infections may be reduced by increasing fluid intake • A daily fluid intake of as little as 1.2 litres per day may reduce the incidence of heart attacks and coronary artery disease • Cancer of the bladder is significantly reduced in those drinking 2.5 litres a day, compared to those drinking less than 1.5 litres Research has shown an association between reduced water intake and cardiovascular disease. Dehydration increases resting heart rate when standing or lying down, makes it more difficult to maintain blood pressure and induces fainting in individuals susceptible to postural fainting • Colorectal cancer and colonic polyp formation may be reduced by increasing fluid intake (from as little as 1.2 litres per day) • Dehydration is also considered to be a risk for deep vein thrombosis (DVT), including DVT during long distance travel The authors of all these studies are cautious in reporting their findings, and admit that the correlations are not causal – that is there are many other variables that may have significant effects. By Dr Paul Stillman 10 March 2005. How much should I drink each day? In normal circumstances, each person should drink two litres a day. This is not merely some form of urban myth: it is soundly backed by scientific research. The average adult loses over 2.5 litres of fluid per day. Daily fluid loss Litres Perspiration through skin 0.85 Respiration through mouth and nose 0.35 Urine 1.25 Faeses 0.10 Total 2.55 The water content of our food adds up to around 0.5 litres per day, leaving around 2 litres to be consumed in liquid form.