Water

Water is arguably the most important of all the nutrients we need, not least because around 60% of our body is water.

It also aids many other vitamin functions in the Body.

Most of the water we drink, depending upon its source, also contains numerous electrolytes and minerals including sodium, chloride, magnesium, potassium, fluoride and calcium.

Thus, the water we drink can contribute to our intake of these important nutrients.

Water is a transportation system of the body. Basically it moves everything (nutrients, oxygen, vitamins and minerals) to where they are needed and takes waste products to excretory organs.

Water plays an important role in temperature regulation. Again, at a simple level, it distributes heat around the body from sites where it is produced, such as exercising muscles, to cooler places like the skin.

Water is the environment in which every single chemical reaction that occurs in the body takes place. The water content of each individual cell and indeed the whole body needs to be kept constant between very narrow limits so that metabolism or any other function in the body remains efficient.

The body has two main water “compartments” — intracellular (referring to inside the cell) and extracellular (referring to fluids surrounding the cell).

A human body will suffer a net LOSS of between 2 and 2.5 litres of water per day with little or no Exercise in normal temperature and humidity.

A loss of 2% of body weight as water will seriously compromise performance. A loss of 5% can be fatal.

Water is lost from the body continually throughout the day, by breathing, in sweat and in urine.

Fluid needs during exercise

The goal of drinking fluids during exercise is to maintain plasma volume and electrolytes, prevent abnormal elevation in heart rate and core body temperature, and provide fuel to the working muscles. This, in turn, may prevent or delay fatigue and the fluid imbalances that occur during exercise. Activity, even simply walking around, will increase this water loss. An hour of exercise, depending upon intensity and the weather conditions, could be responsible for a further 1 — 2 litres of water lost.

If we consider for example a person weighing 60kg — 2% of that weight (1.2kg) is water.

As 1 litre of water weighs 1kg, that equates to a fluid loss of 1.2 litres or 1200mls.

Fluid and electrolyte needs for children and adolescents

Research has shown that children are less efficient thermo-regulators than adults, especially when exercising in warm environments. Compared to adults, children acclimatize more slowly, have a higher set point — that is rectal temperature at which sweating begins, and a lower sweat rate, produce more – metabolic heat per kilogram of body weight during exercise, and have greater physical impairment from exercise. Children can maintain hydration during prolonged and intermittent exercise if they drink 4 oz of fluid (120 ml) every 15 — 20 minutes (Bar-Or and Unnithan, 1994).

RECOMMENDED FLUID INTAKE BEFORE AND AFTER EXERCISE IS AS FOLLOWS:

• Before exercise – approximately 4 — 8oz or 120 — 240 ml.
• After exercise – approximately 160z or 0.5L for every pound, 0.5 kg lost.

Children should be encouraged to drink fluids before exercise and, like adults, children can learn to monitor their urine colour as an indicator of fluid balance; the darker and more concentrated the urine, the more dehydrated one is becoming.

Fluids should be flavoured, palatable and cool. Research shows that children prefer flavoured beverages over water. Flavoured drinks containing sodium and carbohydrate promote adequate fluid intake and help maintain fluid balance during exercise in the heat,

Be aware of early signs of dehydration in active children. Look for dry lips and tongue, dark yellow urine, fatigue and apathy, muscle cramps, infrequent urination and sunken eyes.

REMEMBER:- Water losses at this level will slow down metabolism and therefore energy generation so that muscles will feel tired.

Blood pressure will drop and induce a potential lack of blood flow to the brain leading to poor concentration and tiredness.

It is not difficult to see how very active people may have water needs of around 5 litres per day or 8-9 pints. Some of this water will be derived from the food we eat.

Fruit and vegetables for example have high water content in addition to their other attributes. It has been estimated that on average we can probably get around 1 — 1.5litres of fluid in this way. This still means that a considerable amount of fluid needs to be consumed on a daily basis if we are to ensure that we remain fully hydrated.

But how does the body know it needs water? What triggers the thirst response? How does the body know when fluid and electrolyte needs have been met? The body has an intricate way of regulating body water through the stimulation of thirst and through regulation of fluid losses through the kidneys. The body is constantly assessing fluid balance and adjusting both intake and output to maintain total body water at an optimal level.

EFFECTS OF DEHYDRATION

Increase in:

• Gastrointestinal stress
• Heart rate
• Core temperature at which sweating begins
• Core temperature at which blood flow increases to the ski
• Core temperature at a given exercise intensity
• Muscle glycogen use

Decrease in:

• Central blood volume
• Stroke volume
• Cardiac output
• Sweat rate at a given core temperature
• Maximal sweat rate

Resulting in:

• Headaches
• Fatigue
• Insufficient kidney function
• Sub-optimal performance

Excessive water and electrolyte losses impair heat tolerance and exercise performance and can lead to severe dysfunction in the form of heat cramps, heat exhaustion, or heat stroke.

Alcohol, caffeine and many other soft drinks that contain caffeine are diuretics; that is, they cause the body to accelerate water loss from the body, contributing to dehydration rather than rehydration and so should be avoided altogether or taken in moderation.

Dehydration also causes a paradox effect in that a lack of water can actually result in water retention. The kidneys regulate acid-base balance by the secretion of hydrogen into the tubules and the reabsorption of bicarbonate. Both a decrease in body water (dehydration) and an increase in plasma osmolality stimulates ADH (antidiuretic hormone) secretion. ADH causes an increase in water reabsorption in the kidney tubules thus resulting in a negative feedback. Water is returned to the body fluids and the plasma osmotic pressure is decreased to normal levels, thus causing a retention effect throughout the body.

Dehydration also has an effect upon gut function. Fluid acts as a “flusher” within the gut, speeding up the rate of gut function, waste elimination, and helping to maintain an effective and efficient metabolism. As stated before water is held within the digestive tract increasing bulk, thus stimulating the muscles of the tract so that they retain their health and tone, preventing illness, constipation, haemorrhoids and more severe forms of bowel problems such as diverticulitis and even bowel cancer. An adequate intake of water (as well as adequate amounts of fibre) encourages efficient elimination of waste products, and helps to prevent the reabsorption of toxins in the bowel.

The human body is highly adaptable and will learn to cope with a continually lowered intake, but at a cost, that we may come to accept as normal. Changing poor eating habits does require time and effort, but beneficial changes in fluid intake can occur overnight and you can experience the benefits immediately.

However a big increase in fluid intake will initially lead to much more frequent visits to the toilet. This should settle down to an acceptable rate within a few weeks though — so stick with it.

Effects of Dehydration

• Decrease in blood volume (blood pressure) and therefore inadequate distribution of nutrients and oxygen around the body.
• Decreased blood flow to the brain therefore headaches, poor concentration and compromised motor fitness.
• Decrease kidney function because of water retention in the body.
• Decrease in metabolic rate in all cells with consequences for metabolic efficiency and weight management.
• Increased risk of poor digestive processes and constipation.

GASTRIC EMPTYING AND INTESTINAL ABSORPTION

Optimal fluid balance, especially during exercise, depends on the effectiveness of oral rehydration to maintain plasma volume and electrolyte balance; which in turn depends on the rates of fluid ingestion, gastric emptying and intestinal fluid absorption. Since little water or nutrients are absorbed in the stomach, these substances must enter the small intestine, while other nutrients (glucose, fructose and sodium) are absorbed at different rates. Therefore, the rate that water and nutrients empty into the small intestine from the stomach is an important limiting factor to absorption and determines to a great extent the benefit derived from drinking a particular beverage. A good oral rehydration fluid or sports drink should empty rapidly from the stomach, enhance intestinal absorption, and promote fluid retention. A number of factors influence the rate of gastric emptying and the subsequent absorption of fluid from the small intestine.

FACTORS AFFECTING GASTRIC EMPTYING

• EXERCISE:
• Intensity / mode
• NUTRIENT:
• Carbohydrate; concentration / type
• Protein
• Fat
• Particle size
• DRINK:
• Volume / caloric density
• Osmolality
• Temperature
• pH balance

Low concentrations of glucose (sugar) and electrolytes (sodium) dissolved in water can increase the rate of absorption (osmosis) of the fluid from the intestine. Thus, electrolyte solutions can increase fluid and carbohydrate needs during exercise. Water, on its own, is absorbed across the small intestinal mucosa by simple diffusion, while low concentrations of glucose and electrolytes are absorbed by facilitated diffusion.

This can therefore serve a dual purpose:

• Increasing the rate of fluid replacement
• Providing glucose during exercise which may delay the onset of fatigue in endurance exercise.

HYPERTONIC DRINKS: that is those that are more concentrated than the blood. These force water to leave the blood and enter the gut by the process of osmosis, so not a useful rehydrater.

HYPOTONIC DRINKS: such as water, have a much lower concentration than the blood.

ISOTONIC DRINKS: have the same concentration as the blood and are therefore the most rapid rehydrater.

In order to fulfil these criteria any sugary drink should have no more than about 6grams of glucose in every 100mls of fluid. Mast fruit juices contain between 10 and 12 gms/100 mls. Of course this doesn’t mean you have to stop drinking fruit juice; simply dilute half and half with water if fluid intake is a priority.

Be aware that thirst cannot be used as an indicator of fluid status. This is because thirst is a response to dehydration so by the time you feel thirsty it is too late. it is also likely that when you drink your thirst will feel satisfied long before you have fully re-hydrated. By far the best drink you can have to regularly replace fluid loss is water itself. Try to drink regularly throughout the whole day until you have completed your quota.

Alcohol guidelines from the department of health

Official guidelines on alcohol consumption are usually produced by a government department, public health bodies, medical associations or non-governmental organisations such as the World Health Organisation (WHO).

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