Water - Nutrition

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What is Water?

Water is a clear, odourless liquid comprised of 2 hydrogen atoms covalently bonded to 1 oxygen atom. Water is essential for plant and animal life, yet is often overlooked as a dietary nutrient. Daily water requirements are in an approximate 1:1 ratio with daily energy requirements for dogs and cats[1].

Why is it Important?

Water has multiple functions: it is the solvent in which the majority of intra- and extracellular processes occur, it facilitates the transport of oxygen and nutrients in blood, it is needed for normal digestive function, thermoregulation and the excretion of waste products. It is the major component of most body tissues accounting for approximately 60% of the total body weight in adult mammals with 2/3 in the Intracellular and 1/3 in the Extracellular compartment. Extracellular water is further dispersed between the interstitium (3/4) and plasma volume (1/4)[2]. Water balance is regulated by antidiuretic hormone (ADH) produced in the hypothalamus and released in response to decrease in blood volume or increases in plasma osmolarity (determined mainly by plasma Na concentrations). Mild dehydration resulting in a loss of 0.5% fluid volume or an increase in osmolality of 1-3% stimulates production of ADH resulting in water reabsorption in the renal tubules and activation of the renin-angiotensin system with angiotensin II stimulating the thirst response[3][4][5].

Regulation of Water Balance

Water Losses:

  1. Urine: Obligatory water loss is the minimal urine output required to eliminate metabolic waste products (including urea and minerals). Diets that provide protein and mineral intake in excess of daily requirements will cause an increase in urine output[6]. In contrast to obligatory water loss, free water does not contain additional solutes. Free water loss through urine is regulated by ADH at the level of the renal tubule and the amount of free water lost each day will depend on water intake, total body water needs, and health status.
  2. Faeces: Water that enters the gastrointestinal tract (either through diet, voluntary intake, or endogenous secretions) is effectively reabsorbed through both the small and large intestines. Compromised function (vomiting, diarrhoea) can result in excess water loss.
  3. Respiratory Epithelium: Respiratory water loss will fluctuate depending on ambient temperature, humidity, and activity. At higher temperatures dogs will increase respiratory rate by 12-20 times and cats by 4.5 times[2] to facilitate cooling, which correlates with an average loss of 469 ml/day for a panting dog versus 41 ml/day for an average cat. Dogs at rest in a thermoneutral environment will have an evaporative loss of <1 ml/kg/hr, while during periods of activity, losses can reach up to 7 ml/kg/hr[7].
  4. Cutaneous Evaporation: Cutaneous evaporation is not considered a significant source of water loss in dogs and cats as sweat glands are limited to the foot pads.

Factors that Affect Water Intake:

  1. Temperature: Water intake will also depend on ambient temperature, with dogs and cats consuming more water during warmer conditions.
  2. Diet: Dogs and cats eating a higher moisture containing diet (greater than 67% water) may be able to meet daily water needs through diet alone and will have a decrease in voluntary water intake[8][9][10]. Greater protein and mineral (especially Na) intakes will result in increased urinary water losses and a compensatory increase in water intake. Similarly an increase in total energy intake causes an increased water intake to accommodate the additional nitrogenous and mineral waste.
  3. Activity: Exercising dogs will have additional respiratory water loss and drive to drink to maintain hydration[11]. Cats have less sustained physical activity and changes in water requirement with exercise are not seen.
  4. Physiologic State/Health Status: Lactating dogs and cats will require a larger water intake to maintain milk production. The ability to maintain water homeostasis requires functioning organ systems (renal and gastrointestinal) and normal production of ADH.

Roles in the Body

Water maintains circulating volume; contributes to thermoregulation by evaporation; is necessary for elimination of waste products through the renal tubules; aids in digestion through secretions of body fluids within the gastrointestinal tract (e.g. saliva, bile fluid, pancreatic fluid), allows for interaction of food constituents and enzymes; forms an aqueous environment for metabolic processes and chemical reactions (e.g. hydrolysis); and is a major component of other body fluids such as joint fluid and cerebrospinal fluid[11]. Increased water intake can also be used as a preventative or therapeutic treatment for cats and dog with lower urinary tract disease.

Consequence of Water Deficiency (Dehydration)

Clinical signs of dehydration include loss of skin turgor, lethargy, decreased appetite, weakness, and constipation. Mild dehydration in exercising dogs can result in decreased work capacity, decreased strength, and hyperthermia[1]. Losses > 10% of total body water volume in any dog or cat can result in hypovolaemia, coma or death[2]. Recognised syndromes influenced by subclinical dehydration in cats include:

  1. Crystalluria and Urolithiasis: Chronic mild dehydration results in more concentrated urine production. Increased urine concentration with excess intake of crystal precursors may lead to an increased risk of crystalluria and subsequent urolithiasis. Crystals commonly encountered include struvite, but calcium oxalate and urate crystals can also be seen in otherwise healthy adult cats.
  2. Feline urologic syndrome (FUS): Also called feline lower urinary tract disease (FLUTD) is believed to be highly influenced by the concentration of urine. High urine concentrations can exacerbate clinical signs of FUS. Increased risk of FUS has been associated with eating dry diets[12]. Therefore, feeding higher moisture diets (e.g. wet foods) is often recommended to reduce the risk of FUS and improve clinical signs of lower urinary tract health in cats[13].

Toxicity

Water toxicity is rare in dogs and cats. Water intake in excess of need is excreted as free water through the renal tubules. Over-consumption can be induced in animals that are offered water ad libitum following prolonged dehydration, exercise, or prolonged heat exposure[1]. Clinical sings of water toxicity include seizures, ataxia, and coma.

Sources of Water

  1. Dietary moisture content can range from 7% as fed in a dry commercial diet to greater than 80% as fed in a wet diet.
  2. Voluntary intake (drinking) is influenced by ambient temperature, the diet type and amount fed, exercise demands, physiologic state, and health status.
  3. Metabolic water production occurs during oxidation of energy containing macronutrients (i.e. protein, fat, and carbohydrates) and constitutes the smallest contribution to the total water intake (10-15%) of an animal[4]. The volume of metabolic water produced will also vary with the macronutrient composition of the food; fat oxidation produces the largest water volume (107 ml/100 g), compared to protein (55 ml/100 g) and carbohydrates (41 ml/100 g)[11].

Diagnosis of Dehydration

Clinical signs of dehydration include loss of skin turgor, lethargy, tachy mucous membranes, decreased food intake, and weakness. Replacement therapy is based on the ability to estimate the degree of dehydration, water maintenance needs and volume of ongoing water losses due to disease (e.g. vomiting, diarrhoea, urination). Accurate estimates of water requirement and deficits are essential for correction of dehydration in the clinical setting.

References

  1. 1.0 1.1 1.2 National Research Council (NRC). Physical Activity and Environment. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.292-296.
  2. 2.0 2.1 2.2 Wellman ML, et al. Applied Physiology of Body Fluid in Dogs and Cats. In Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice. 2012 4th Ed. St. Louis: Elsevier Saunders p.2-22.
  3. Robinson EA and Adolph EF. Pattern of normal drinking in dogs. Am J Physiol 1943;139:39-44.
  4. 4.0 4.1 Anderson RS.Water balance in the dog and cat. J Small Animal Practice 1982;23:588-598.
  5. O’Connon WJ and Potts DJ. The external water exchange of normal laboratory dogs. Q J Exp Physiol Cogn Med Sci 1969;54:244-265.
  6. Cizek LT. Long-term observations on relationship between food and water ingestion in the dog. Am J Physiol 1959;197:324-326.
  7. Caldwell GT. Studies in water metabolism of the cat. Physiol Zool 1931;4:324-359.
  8. Danowski TS, et al. The deleterious effects in dogs of a dry protein ration. J Clin Invest 1944;23:816-823.
  9. Prentiss PG, et al. Hydropenia in cat and dog: ability of the cat to meets its water requirements solely from a diet of fish or meat. Am J Physiol 1959;196:625-632.
  10. O’Connor WJ. Drinking by dogs during and after running. J Physiol 1975;250:247-259.
  11. 11.0 11.1 11.2 Case LP, et al. In Canine and Feline Nutrition: A resource for Companion Animal Professionals. 2011 Third Ed. St. Louis: Mosby p.9-11.
  12. Buffington CA, et al. Clinical evaluation of cats with non-obstructive urinary tract disease. JAVMA 1997;210(1):46-50.
  13. Markwell PJ, et al. Clinical evaluation of commercially available urinary acidifying diets in the management of idiopathic cystitis. JAVMA 1999;214(3):361-365.



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