Week 2 (1) Lecture - Water

Question 1

What nutrient is consumed in the largest amount?

Answer 1

Water

Question 2

Hydration levels of the body

Answer 2

Skin = 72%
Organs = 76%
skeleton = 22%
Blood = 83%
Adipose = 10%
Muscle = 76%

Question 3

lean vs obese water levels

Answer 3

Larger individuals will have more water in their body (Absolute basis). In relative terms (% of the body made up by water) is dictated by adipose mass.
- A very lean athlete will have more of their body made up of water e.g., 70% water whilst someone who is morbidly obese may be 40% water

Question 4

Labelled water

Answer 4

labelled water is a stable isotope of water
- Has heavy hydrogen in the water
- Take a sample of body water (urine, saliva, blood) and measure the concentration of hydrogen in the heavy water.
- Then give the ppt a known dose of the heavy water (deuterium oxide – where the hydrogen has an additional neutron added to make it heavier than normal hydrogen)
- Then take another body water sample, and from the change in the concentration of the heavy hydrogen we can determine the total body water of the individual
- The isotope alone cost £100s

Question 5

Body water content

Answer 5

• Distributed into 2 distinct compartments:
• Intracellular fluid (ICF)- 2/3 of TBW (all of the fluid inside the cells e.g., RBCs and skeletal muscle cells) – largest component of body water
• Extracellular fluid (ECF)- 1/3 of TBW
• ECF distributed between 2 compartments:
• Interstitial fluid (ISF)- ¾ of ECF
• Intravascular fluid (IVF)- ¼ OF ECF (everything that is inside your blood vessels)

Question 6

Fluid intake (ml)

Answer 6

Fluid intake (1600)
Food intake (1000)
Metabolic water (400) - end products of respiration (CO2 and water)

Question 7

Fluid losses (ml)

Answer 7

Urine (1400)
Expired air (320)
Faecal loss (100)
Skin loss (530)
Sweat loss (650)

Question 8

Water loss is separated into 2 aspects:

Answer 8

Sensible water loss = water than is seen and perceived (e.g., urine, sweat…)

Insensible water loss = water loss that is not seen e.g., expired air, Faeces (~100g water a day), Skin loss

Question 9

Euhydration measurements:

Answer 9

• Normal blood chemistry
• Plasma osmolality <290 mosmol.kg-1
• Urine osmolality <700 mosmol.Kg-1
• Urine specific gravity <1.020

Question 10

Euhydration is not one set point – it will fluctuate throughout the day (represent a range of values:

Answer 10

• Fluid loss is continuous – fluid loss happens as soon as the water is in the bladder (you cannot do anything with this water except excrete it)
• 20-30ml of urine per hour will be produced even if you do not drink
• There is an obligate requirement for water loss
• There is a constant oscillating pattern for euyhdration

Question 11

Hypohydration definition

Answer 11

Low hydration water

Question 12

what’s the difference between dehydration and hypohydration?

Answer 12

Dehydration is the process of losing water whilst hypohydration is a state

Question 13

Regulation of plasma osmolality:

Answer 13

• Plasma osmolality is regulated in the range 280-290 mosmol.l-1
• This is regulated by water and solute excretion by the kidneys and intake by the thirst mechanism
• Control centres in hypothalamus/ forebrain (2 ways that sense hydration in the body= osmoreceptors and baroreceptors):
• Respond to changes in blood osmolality, volume and pressure
• Osmoreceptors: 2-3% change in plasma osmolality – sense the concentration of your blood
• Baroreceptors (sense pressure): ~ 10% change in blood volume/ pressure – less sensitive than osmoreceptors (because we experience large fluxes in blood volume all the time).
• 2 ways these are controlled = kidneys and thirst mechanism:
• Renal excretion: kidneys can only reduce losses, cannot correct deficit (kidneys cannot create more water)
• Thirst: most fluid intake is habitual, rather than due to physiological needs. True physiological first is when you have a high plasma osmolality.

Question 14

Composition of body fluids

Answer 14

• Sweat is hypotonic (low tonicity/ low concentration) relative to plasma. Sweat has a lower concertation than blood
• This is an important part of the fluid regulatory response to exercise-induced dehydration
• Isotonic sports drinks are the same concentration as blood

Question 15

Typical values of what sweat contains: main components = sodium & chloride (salt)

Answer 15

Sodium concentration in Sweat = 50 mmol/l
Sodium concentration in Plasma (ECF) = 140 mmol/l
Sodium concentration in ICF = 12 mmol/l

Potassium concentration in Sweat = 4 mmol/l
Potassium concentration in Plasma (ECF) = 4 mmol/l
Potassium concentration in ICF = 150 mmol/l

Chloride concentration in Sweat = 40 mmol/l
Chloride concentration in Plasma (ECF) = 104 mmol/l
Chloride concentration in ICF = 4 mmol/l

Sweat also contains small amounts of calcium and magnesium

Question 16

what encourages an athlete to drink?

Answer 16

The increase in plasma osmolality caused by sweat
When we sweat we lose proportionally more water than we are electrolytes (salts), the slat concentration in the blood goes up and plasma osmolality goes up – athlete becomes more thirsty. This is the physiological stimulus that encourages the athlete to drink

Question 17

Hypohydration:

Answer 17

Increased sweat (which is a lower concentration than blood) causes plasma osmolality to increase

1) Increased Plasma Osmolality: Hypohydration leads to an increase in plasma osmolality.
2) Activation of Osmoreceptors (activated at 2-3% change in osmolality) : The change in osmolality is detected by osmoreceptors, which triggers two responses:
- Increased Thirst: This leads to increased fluid intake.
- Increased AVP (Antidiuretic Hormone/Vasopressin): This decreases urinary excretion.
3) Water Retention: Combined, these responses help retain water in the body.
Reduced Plasma
4) Osmolality: Water retention lowers plasma osmolality, helping restore balance.

2 effects = endocrinological (hormonal- arginine vasopressin decreases water loss in urine at the kidneys to reduce overall loss) or behavioural (drink water)
- This increased intake and reduced loss through urine, causes plasma osmolality to normalise and the osmoreceptors will detect this and reset the circuitry
- Osmoreceptors are located in the hypothalamus

Question 18

Hyperhydration (excess hydration):

Answer 18

1) Decreased Plasma Osmolality: Hyperhydration results in decreased plasma osmolality.
2) Activation of Osmoreceptors: The change in osmolality is detected by osmoreceptors, leading to two responses:
- Decreased Thirst: This reduces fluid intake.
- Decreased AVP: This increases urinary excretion.
3) Water Excretion: These actions lead to increased water excretion from the body.
4) Increased Plasma Osmolality: Excreting water raises plasma osmolality, helping restore balance.

If an athlete is hypo hydrated we will see an increase in the hormone AVP

Question 19

Rehydration process:

Answer 19

1) Mouth (Drink Ingestion): The process begins with the intake of fluid through the mouth.
2) Stomach (Gastric Emptying): The fluid moves to the stomach, where it undergoes gastric emptying, passing gradually into the small intestine.
3) Small Intestine (Intestinal Absorption): In the small intestine, the majority of fluid absorption into the bloodstream takes place.
4) Circulation (Retention): Finally, absorbed fluid enters circulation, where it is retained and distributed throughout the body as needed.

Question 20

Factors affecting the fluid replacement process:

Answer 20

1) Fluid Intake: The initial step, involving the amount and frequency of fluid consumption.
2) Rate of Gastric Emptying: The speed at which fluid leaves the stomach and enters the small intestine.
3) Rate of Intestinal Fluid Absorption: The efficiency of fluid absorption from the intestines into the bloodstream.
4) Renal Water Reabsorption: The kidneys’ role in reabsorbing water to maintain fluid balance.
5) Fluid Distribution: The final step, where absorbed fluids are distributed throughout the body to maintain hydration.

Question 21

what urine osmolality level could indicate a state oh hypohydration?

Answer 21

700-900 mosmol/kg

Question 22

Voluntary rehydration study (Funnell et al 2023)

Answer 22

• Athletes left to do their own thing are not good at recovering hydration
• After exercise athletes did not recover body weight (lost 2% of bodyweight after the exercise)
• Body weight is a good surrogate measure for body water

Question 23

Rate of drinking (Jone et al 2010)

Answer 23

• Looked at slow vs fast drinking
• slow drinking resulted in better fluid balance (lower urine output).
• The practical utility of drinking really slowly is week

Question 24

Drink volume (Shirreffs et al 1996)

Answer 24

If an athletes drinks exactly what they lost they will not become fully hydrated due to urine (fluid loss) – you need to give 150-200% of fluid lost to elicit full fluid balance recovery (after exercise

Question 25

Sodium content (Shirreffs and Maughan 1998)

Answer 25

• 150% rehydration volume with 0, 25, 50, 100 mmol/l Na
• Most sports drinks are about 20-25mmol/l Na
• Inverse Linear relationship found between sodium in the drink and urine production
• The more sodium in a drink= the better the hydration outcome (less sodium = more urine)

Question 26

Carbohydrate content (Clayton et al 2014)

Answer 26

• 150% rehydration volume, with 2% or 10% glucose
• Compared a low and glucose drink
• Only the high CHO drink had a beneficial effect on hydration (the threshold is very high – 10% Carbohydrate drink e.g., coke/ specifically designed sports drink)
• Sports drinks are usually 4-6% carbohydrate

Question 27

Milk protein (James et al 2013)

Answer 27

• This has an effect at a lower value than CHO drinks
• Adding milk protein to a drink improved hydration outcomes

Milk protein improves hydration but whey protein doesn’t - Whey protein responds differently to milk protein - Milk protein in the stomach clots to slow the deliver of the proteins and some of the water within the solution – beneficial effect of hydration

Question 28

2 ways of affecting rehydration:

Answer 28

1) Strategies that influence the restoration of fluid balance: Consume enough drink in the right amount of time (volume and time)
2) Maintain fluid balance- Consume the correct components in the drink (CHO, sodium, protein)

Question 29

Lecture summary:

Answer 29

• Important factors for post-exercise rehydration: Drink palatability, volume, composition and rate of drinking
  1) 1) Slow the rate of delivery of the drink to the circulation, thus influencing AVP response. Slowing the rate gives the body more time to put the water into different tissues in the body, rather than just urinating back out.
• This is why, drinking slowly, having a high CHO content (which will slow gastric emptying and intestinal absorption) and adding milk protein will improve rehydration
  2) Increase drink retention once drink reaches the circulation by influencing AVP response. This is why sodium helps increase hydration (better retained). Sodium and chloride within a drink provide osmolality that stops AVP coming down, resulting in the drink being better retained.