Agriculture Reference
In-Depth Information
s o D i u m
Sodium is the most abundant cation in the body, comprising about 93% of the total
cation supply. Approximately 30% is found as a sodium reserve on the surface of
bone crystals within the skeleton. The remainder is found in soft tissues and extra-
cellular fluids. Sodium is involved in maintenance of tissue fluid balance, trans-
mission of nerve impulses, and contraction of muscles. An absorptive pathway in
the small intestine involves cotransport of sodium ions and glucose; thus, as it is
absorbed, sodium also plays a role in providing energy to the tissues. At the brush
border of the enterocyte, Na + and glucose bind to a carrier that shuttles them from
the intestinal lumen to the cell interior. They are released from the carrier, and Na +
is pumped across the enterocyte basolateral membrane by the Na + /K + -ATPase (ade-
nosine triphosphatase) pump, while glucose crosses into the plasma by facilitated
diffusion. A second sodium absorptive pathway in the small intestine and proximal
colon involves electroneutral cotransport of Na + and Cl , with Na + pumped across the
basolateral membrane, as discussed, and Cl diffusing into the plasma. In the colon,
sodium enters the luminal membrane of the enterocyte through Na + -conducting
channels, diffusing inwardly as a consequence of the downhill concentration gradi-
ent. It is accompanied by water and anions and is pumped through the basolateral
membrane into the blood plasma by the Na + /K + -ATPase pump.
Processed foods, such as soups, canned meats, pickled foods, condiments, and
salted snacks, account for nearly 75% of sodium intake in the United States. Naturally
occurring sodium in animal and vegetable products accounts for only about 10%.
Water generally provides less than 10%, and the remainder comes from salt (sodium
chloride) added during cooking or at the table. Estimated adequate sodium intakes
range from 1000 mg per day for 1- to 3-year-old children to 1500 mg per day for
teenagers and men and women to 50 years of age. Estimated adequate intakes then
decline to a low of 1200 mg per day at ages greater than 70. Actual sodium intakes
in the United States are estimated to range from 1800 to 5000 mg per day.
P o t a s s i u m
Potassium constitutes about 0.35% of the body mass of a 70-kg adult human. It is
the major intracellular cation, and about 95-98% of the total supply is found within
body cells. Potassium is involved in the contractility of muscle, the excitability of
nervous tissue, and the maintenance of tissue electrolyte and pH balance. It appears
to be absorbed both in the small intestine and colon. Studies of absorption in the
colon indicate it enters the enterocyte via a K + /H + -ATPase pump. Alternatively, it
may enter the enterocyte via a membrane channel. Passage through the basolateral
membrane into blood plasma appears to be by diffusion through a K + channel down
a concentration gradient.
There are many food sources of potassium, including fruits, vegetables, meats,
whole grains, legumes, and milk. Salt substitutes may contain potassium instead of
sodium. Estimated adequate potassium intakes range from 3000 mg per day for 1- to
3-year-old children to 5100 mg per day for lactating women.
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