Chemistry Reference
In-Depth Information
capacity to concentrate solids is limited and the renal solute load exerts a
major effect on water balance. Renal solute load is determined mainly by
sodium, chloride, potassium, phosphorus and protein. Bovine milk has a
much higher potential renal solute load (
300 mOsmol/l) than human milk
(
93 mOsmol/l) (Ziegler and Fomon, 1989). The high renal solute load
resulting from the ingestion of bovine milk may be of relatively little signifi-
cance in healthy growing infants without increased evaporative water losses,
because the kidney excretes a more concentrated urine. However, this reduces
the margin of safety against dehydration that can occur in conditions of
diarrhea, fever or low water intake. For this reason, it is recommended that
the upper limit of potential renal solute load in formulae for young infants
should be about 220 mOsmol/l (Ziegler and Fomon, 1989).
Potassium plays many roles in the body (Committee on Minerals and
Toxic Substances in Diets and Water for Animals, 2005; Preuss, 2006),
including acid-base balance, maintenance of osmotic pressure and blood
pressure, cellular uptake of amino acids and as a co-factor or activator in
many enzyme systems. Potassium has a critical role in membrane transport
and carbohydrate and energy metabolism. Cellular membrane polarization
depends upon the internal and external concentrations of potassium. As a
result, the major clinical disturbances of severe abnormal potassium status
usually are associated with an altered membrane function, especially in
neuromuscular and cardiac conduction systems. Both deficient (hypokale-
mia) and excess (hyperkalemia) circulating potassium results in disorders in
cardiac, muscle and neurological function (Preuss, 2006). Adverse effects of
hypokalemia include cardiac arrhythmias, muscle weakness and glucose
intolerance. Adverse effects of moderate potassium deficiency without hypo-
kalemia include increased blood pressure, salt sensitivity and bone turnover.
Cardiac arrest caused by abnormal electrical conduction is the most serious
clinical manifestation of hyperkalemia. Neuromuscular symptoms of potas-
sium excess include tingling, paresthesia, weakness and flaccid paralysis.
The FNB determined that an EAR could not be established for potas-
sium, so a RDA could not be derived (Food and Nutrition Board: Institute of
Medicine, 2001). The FNB also stated that the health effects of potassium
intake in infants and children are uncertain. Thus, only an AI was set that
reflected a calculated mean potassium intake through human milk (age 0-6
months) or a combination of human milk and complementary foods (age
7-12 months). The AI for children was derived by extrapolating from the
adult AI of 4.7 g (120 mmol) potassium/d (Table 10.2).
The adult human body contains about 45 mmol K/kg body weight or
about 3150 mmol (1230 g) for a 70 kg adult (Preuss, 2006). Intracellular
potassium accounts for 98% of the potassium and for 75% of total intracellular
cations. Because extracellular fluid contains only 2% of the potassium in the
