Introduction to Human Nutrition

200 Introduction to Human Nutrition

Assessing status, requirements and
dietary sources

Previously, dietary phosphorus recommendations
have been tied to those for calcium, usually on an
equimass or equimolar basis, and this approach was
used in the USA, EU, and UK in establishing recom-
mended dietary allowances, population reference
intakes, and reference nutrient intakes, respectively,
for phosphorus. However, in 1997 the US Food and
Nutrition Board suggested that a calcium–phospho-
rus concept of defi ning phosphorus requirements is
of severely limited value, in that there is little merit in
having the ratio “correct” if the absolute quantities
of both nutrients are insuffi cient to support optimal
growth. Therefore, because the phosphorus intake
directly affects serum inorganic phosphate, and
because both hypophosphatemia and hyperphospha-
temia directly result in dysfunction or disease, the US
Food and Nutrition Board considered that the most
logical indicator of nutritional adequacy of phospho-
rus intake in adults is inorganic phosphate. If serum
inorganic phosphate is above the lower limits of
normal for age, the phosphorus intake may be con-
sidered adequate to meet cellular and bone formation
needs of healthy individuals. Current US RDAs for
phosphorus are infants 100 mg (fi rst 6 months),
275 mg (7–12 months), children 460 mg (1–3 years),
500 mg (4–8 years), 1250 mg (9–18 years), adults
700 mg, pregnant women 1250 mg (<18 years),
700 mg (19–50 years), and lactating women 1250 mg
(<18 years), 700 mg (19–50 years).
Phosphates are found in foods as naturally occur-
ring components of biological molecules and as food
additives in the form of various phosphate salts. The
phosphorus density of cow’s milk and other dairy
produce is higher than that of most other foods in a
typical diet (Table 9.2). The same is true for diets high
in colas and a few other soft drinks that use phos-
phoric acid as an acidulant.

Micronutrient interactions

It has been reported that intakes of polyphosphates,
such as are found in food additives, can interfere with
the absorption of iron, copper, and zinc.

9.5 Sodium and chloride

Sodium is the sixth most abundant element in the
Earth’s crust and salt (sodium chloride) makes up

about 80% of the dissolved matter in seawater.

Although there is a wide variety of sodium salts, many

of which are used as additives in food processing (e.g.,

sodium nitrate and monosodium glutamate), sodium

chloride is the major source of sodium in foods. As

sodium and chloride intakes in humans are so closely

matched, both will be considered together in this

text.

Salt was of major importance in early civilizations

and in prehistory. Humans have special taste and salt

appetite systems, which led to special culinary uses for

salt and made it a much sought-after commodity.

Nowadays, salt is still used widely to modify fl avor, to

alter the texture and consistency of food, and to

control microbial growth (Table 9.5).

Absorption, transport and

tissue distribution

Sodium is the major extracellular electrolyte and

exists as the fully water-soluble cation. Chloride is

also mainly found in ECF and is fully water soluble

as the chloride anion. Both ions are readily absorbed

from the digestive tract. Glucose and anions such as

citrate, propionates, and bicarbonate enhance the

uptake of sodium. The “average” 70 kg male has about

90 g of sodium with up to 75% contained in the

mineral apatite of bone. Plasma sodium is tightly

regulated through a hormone system, which also

regulates water balance, pH, and osmotic pressure.

Table 9.5 Sodium-containing additives used in food processing

Additive Use

Sodium citrate Flavoring, preservative

Sodium chloride Flavoring, texture,

preservative

Sodium nitrate Preservative, color fi xative

Sodium nitrite Preservative, color fi xative

Sodium tripoliphosphate Binder

Sodium benzoate Preservative

Sodium eritrobate Antioxidant

Sodium propionate Preservative

Monosodium glutamate Flavor enhancer

Sodium aluminosilicate Anticaking agent

Sodium aluminum phosphate acidic Acidity regulatory,

emulsifi er

Sodium cyclamate Artifi cial sweetener

Sodium alginate Thickener and vegetable

gum

Sodium caseinate Emulsifi er

Sodium bicarbonate Yeast substitute