Potassium (K) in the form of K+ is the most essential cation of the
cells. Its high intracellular concentration is regulated by the cell
membrane through the sodium-potassium pump. Most of the total body
potassium is found in muscle tissue. Total body potassium has been used as
a measure of lean body mass, of muscle mass, or (more accurately) of cell
mass. Because of its association with the metabolizing, oxygen-consuming
portion of the body, a decline in total body potassium is usually
interpreted as a loss of muscle mass due to a catabolic condition.
Potassium exists in nature in three isotopes: 39K
(93.26%), 40K (0.0117%) and
41K (6.73%). 40K
is radioactive and responsible for most of the naturally occurring
internal radioactivity in the body. This property enables investigators to
monitor total body potassium values as a function of age and disease.
Deficiencies: With the exception of starvation, low or
declining total body potassium is not a result of insufficient dietary
intake but the outcome of a catabolic, protein wasting condition which
reduces the total cell mass of the body. Hypokalemia (low serum K) is the
result of excessive loss of K in the urine, usually as a result of use of
diuretic agents to treat hypertension. Hypokalemia may result in cardiac
failure.
Dietary recommendations: The Estimated Minimum
Requirement for potassium for adolescents and adults is 2000 mg or 50 mEq/day.
The usual dietary intake for adults is about 100 mEq/day. For hypertension
patients using diuretic medications, it is recommended often to supplement
their diet with orange juice, bananas and vegetables which contain high
amounts of potassium. Increased potassium intake helps maintain normal
plasma levels. However, the blood level of potassium (which is sensitive
to diet) is not indicative of total body potassium which is an index of
cell mass and muscle.
Food sources: Most foods contain potassium. The best
food sources are fruits, vegetables and juices; potassium also is present
in meats and cereals. There is no known food supplement which would
directly increase total body potassium by compensating for a dietary
potassium deficiency.
Toxicity: The fraction of potassium which is present
outside the cells plays an active role in the propagation of electrical
signals between neurons, skeletal muscle function and regulation of blood
pressure. Urinary excretion protects against the accumulation of high
levels of potassium. However, acute hyperkalemia can be lethal by causing
cardiac arrest.
Recent research: Most of the recent research is
related to the importance of total body potassium as an index of cell
mass. The accelerated loss of total body potassium compared to protein
loss in AIDS patients can be used to predict the time of death of the
patient. Total body potassium is depleted with age, a phenomenon
associated with sarcopenia (loss of skeletal muscle mass and function with
age).
