Page 32 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Physiology of Body Fluids in Dogs and Cats 21

TABLE 1-12 Metabolic Water per requirement has been variously calculated from the
1,5
Gram of Nutrient following formulas:
Grams Metabolic Water ðaÞ Basal energy requirement ¼ 70 body weight
Nutrient per Gram of Nutrient ðkgÞ 0:73
ðbÞ Basal energy requirement ¼ 97 body weight
Carbohydrate 0.6 0:655
Protein 0.41 ðkgÞ
Fat 1.07
There has been considerable debate over the most appro-
Data from Davidson S, Passmore R, Brock JR, et al. Water and electrolytes. priate exponent to use to relate body weight to metabolic
In: Davidson S, editor. Human nutrition and dietetics. Edinburgh: size in the dog. 1,27,44 We prefer the exponent 0.655 and
Churchill Livingstone, 1979: 81–89.
use formula (b), which has been supported in the veteri-
nary literature. 27,44
Maintenance energy requirements are higher than
WATER REQUIREMENTS basal needs primarily to provide calories for the normal
activity of a healthy dog. Based on formula (b), a 10-kg
MAINTENANCE dog has a basal energy requirement of 44 kcal/kg/day
and a maintenance requirement of 88 kcal/kg/day.
Water balance is complex, and there is no single mainte- Assuming 1 mL of water required per kilocalorie of
nance water requirement for each animal. In healthy, sed- energy need, the maintenance water requirement for this
entary dogs and cats in a thermoneutral environment, dog would be approximately 88 mL/kg/day. A 50-kg
water intake is largely dependent on diet. Water require- dog would require 25 mL/kg/day for basal water needs
ment is a function of the renal solute load in the diet and and 50 mL/kg/day for maintenance. If basal water
the associated obligatory renal water losses for urinary requirements were estimated from formula (a), a 10-kg
solute excretion. In clinical practice, maintenance fluid dog would have a basal daily water requirement
needs in small animal patients are often empirically of 38 mL/kg and a maintenance requirement of
defined as 60 mL/kg/day for smaller dogs and 76 mL/kg.
40 mL/kg/day for larger dogs. 36 Alternatively, mainte- Estimates of maintenance water needs based on caloric
nance needs have been assessed on the basis of caloric requirements are similar to the empirical values for main-
needs: 1 mL of water per kilocalorie (kcal) of energy tenance needs used by some clinicians. However, it is
required. 18,19 Early studies of water balance in healthy, important to remember that caloric needs are a logarith-
caged dogs documented that mean water intake was mic function of body weight, and larger dogs require
approximately 1 mL/kcal ingested. 2 Normal mainte- less fluid per kilogram of body weight than smaller dogs.
nance energy requirement is defined as the number of The physiologic reason for the correlation between
calories required to sustain the basal metabolic rate; to caloric and water needs is not well documented. The rela-
provide energy for digestion, absorption, and assimilation tionship may, in fact, be indirect. Water requirements and
of nutrients (thermal effect of feeding); to maintain body caloric needs may be related because water intake is in part
temperature in a nonthermoneutral environment; and for a function of renal solute load, which is related to diet—
normal activity. 25 Maintenance energy expenditure may both to the quantity of food ingested and to the composi-
be calculated from the following formula: 5 tion of the food. However, the renal solute load per calorie
in the diet varies with the composition of the diet. Diets
0:73
140 kcal body weight varyinwater content(dryvs.canned)andinnutrientcom-
position, and hence in renal solute load per calorie. Fats
A 10-kg dog would require 750 kcal of energy per day or provide more kilocalories per gram (9 kcal/g) than do
75 kcal/kg/day. Following the rule of 1 mL/kcal, the carbohydrates or proteins (4 kcal/g). Fats provide more
water requirement would be 75 mL/kg/day. milliliters of water per gram (1.07) than do carbohydrates
3
Opinions vary on the formula for calculating mainte- (0.56) or proteins (0.40). High-protein diets increase
nance caloric needs. 38 Basal energy requirements may renal solute load, whereas fats and carbohydrates do
be calculated and then multiplied by a factor of approxi- not contribute to it. The mineral content of diets also
mately two 38 to obtain maintenance needs. The basal varies. Therefore, the animal’s water requirement may
energy requirement is defined as the caloric need of a rest- be viewed more accurately as a function of total water
ing, healthy dog in a postabsorptive state (i.e., renal content and renal solute load of the diet rather than strictly
solute load has been excreted) about 18 hours after feed- as a function of calories ingested. Thus, the relationship
ing 25 and in a thermoneutral environment. Basal energy 1 mL of water per 1 kcal of energy may be fortuitous.