Introduction to Human Nutrition

204 Introduction to Human Nutrition

The concentration difference of potassium and
sodium across cell membranes is maintained by the
Na+/K+-ATPase pump and is critical for nerve trans-
mission and muscle function. The physiological
importance of potassium in the body covers many
systems including cardiovascular, respiratory, diges-
tive, renal, and endocrine. In addition, potassium is a
cofactor for enzymes involved in inter alia energy
metabolism, glycogenesis, and cellular growth and
division.

Defi ciency symptoms

The low concentration of potassium in plasma is
tightly regulated. Hypokalemia, however, can result
from either excessive uptake of potassium by cells or
potassium depletion from the body. Insulin excess,
catecholamine increases, Cushing’s disease (excess
steroids), diuretics that enhance potassium loss,
chronic renal disease, diarrhea, vomiting, and laxative
abuse can result in hypokalemia. Low potassium
intakes are unlikely to lead to clinical potassium
depletion and hypokalemia except during starvation
and anorexia nervosa.
The activity of nerves and muscles is affected in
potassium depletion, and other clinical sequelae
involve cardiac (including cardiac arrest), renal, and

metabolic alterations. Potassium supplementation

may have a role to play in treating chronic heart failure,

and increased potassium intakes can decrease blood

pressure via antagonistic metabolic interactions with

sodium, resulting in increased sodium excretion, and

also via a direct vasodilatory effect. Oral administra-

tion of potassium salts has been shown to improve

calcium and phosphorus balance, reduce bone resorp-

tion and increase the rate of bone formation.

Toxicity

Hyperkalemia, as a result of either a shift of potas-

sium from cells to the ECF or excessive potassium

retention, can be caused by major trauma and infec-

tion, metabolic acidosis, Addison’s disease (aldoste-

rone insuffi ciency) and chronic renal failure. Overuse

of potassium supplements can also result in potas-

sium excess. As with potassium depletion, the most

important clinical consequence of potassium excess is

cardiac arrest.

Assessing status

The plasma concentration of potassium is not a

reliable index of whole-body potassium status. Total

body potassium can be measured by^42 K dilution or

by whole body counting of the naturally abundant^40 K

to determine the amount of lean body tissue. More

direct measures of tissue potassium can be obtained

by muscle biopsies.

Requirement and dietary sources

Adult requirements for potassium are estimated to be

about 2 g/day. Because of potential benefi cial antago-

nistic effects against high salt intakes, higher intakes

(around 3.5 g/day) of potassium are considered to be

optimal, although chronic intakes above 5.9 g/day

may be dangerous for individuals with impaired renal

function. Potassium, like sodium and chloride, is

naturally widely distributed in foods (Table 9.8).

Food processing (through leaching) may decrease

potassium content as well as increasing salt content.

Legumes, nuts, dried fruit, and fresh fruit, especially

bananas, melons, avocados, and kiwi fruit, are rich

sources of potassium. Major vegetable sources of

potassium are potatoes and spinach, although cereal

and dairy products, which have a lower potassium

content but are consumed in large quantities, are also

important dietary sources. In addition, meat and

fi sh contain appreciable quantities of potassium.

People who eat large quantities of fruit and vegetables

Table 9.7 Salting (mg/100 g fresh weight) of foods in Western
societies

Na K Ca Mg

Maize-based products
Corn 4 284 55 41
Tortilla, rural 11 192 177 65
Breakfast cereals 866 101 3 11
Processed snacks 838 197 102 56

Wheat-based products
Natural cereals 39 1166 94 343
Tortillas, wheat 622 73 11 17
Breakfast cereals 855 869 81 236
Processed bread (urban) 573 126 47 31
Salted bread, made locally (rural) 410 92 10 74
Sweet bread, made locally (rural) 97 93 87 18
Processed bread (rural) 344 79 213 18
Processed biscuits 582 80 16 17

Pulses
Unprocessed, cooked 53 373 50 41
Processed, canned 354 371 27 79

Reproduced from Sánchez-Castillo and James in Sadler et al. Encyclo-
pedia of Human Nutrition, copyright 1999 with permission of
Elsevier.