202 Introduction to Human Nutrition
other studies indicate that heavy metals, such as lead
and mercury, may also contribute to increased blood
pressure.
Effi cient sodium conservation mechanisms mean
that current sodium intakes in many populations are
unnecessarily high and are probably much higher
than the generally lower sodium diets eaten during
the long period of human evolution. Clinical studies
indicate that a high-sodium diet increases calcium
excretion and measures of bone resorption, thereby
suggesting a possible role for high salt intakes in
osteoporosis.
Cross-cultural epidemiology suggests that high salt
intakes are associated with gastric cancer, whereas a
low-salt diet is regarded as having a potentially favor-
able effect in asthma patients.
Genetic diseases
A number of rare genetic disorders have thrown some
light on the pathological mechanisms linking sodium
balance and hypertension (pathologically elevated
blood pressure). A number of candidate genes have
been identifi ed in monogenic forms of low renin salt-
sensitive hypertension. These encode for enzymes
involved in aldosterone biosynthesis or cortisol
metabolism and for the epithelial sodium channel.
These genetic defects decrease the ability of the renal
tubules to excrete sodium. It is possible that similar
genetic mechanisms operate in more common forms
of hypertension such as essential hypertension and
especially salt-sensitive hypertension. Moreover,
molecular mechanisms associated with renin–angio-
tensin–aldosterone are central to the pathophysiology
of this condition. Common essential hypertension,
however, is complex and heterogeneous and has a
genetic heritability of about 30%.
Assessing status
The tight regulation of plasma sodium and, in turn,
chloride ensures that fl uctuations in the plasma
concentration of these ions are minimized and
changes only occur in certain pathological
circumstances. Measurements of plasma sodium,
therefore, are of little consequence as far as nutri-
tional status is concerned. Total body (excluding
bone) sodium, however, is increased in malnutrition
and trauma and this total exchangeable sodium
can be measured, with some technical diffi culty, using
radioisotopes.
Salt intakes are notoriously diffi cult to measure,
and urinary sodium excretion is considered to be a
valid measure of sodium intake under circumstances
where little sodium is lost in sweat. Sodium in urine is
easily measured, but the collection of complete 24 h
urinary samples is diffi cult because of subject com-
pliance, and the completeness of these collections
should be validated using a marker such as para-amino
benzoic acid. Lithium (as carbonate) fused with
sodium chloride can act as a reliable tracer to estimate
discretionary salt (cooking and table) intakes.Requirements and dietary sources
Average requirements for sodium and chloride are
estimated to be about 500 and 750 mg/day, respec-
tively. Normal sodium (mostly from salt) intake
varies from about 2 g/day to 14 g/day, with chloride
(mostly from salt) intakes generally slightly in excess
of sodium (Table 9.6). Snack and processed foods
have more added salt than unprocessed foods. The
amount of discretionary salt added in cooking or at
the table appears to vary greatly among individuals
and among countries. Discretionary salt intakes can
vary from less than 10% to 20–30% of total salt intake
and these fi gures emphasize the major effect of pro-
cessed foods on total salt intakes in most populations
(Table 9.7).Micronutrient interactions
The major interactions between sodium (and chlo-
ride) and other micronutrients are with respect to
potassium and calcium. Data from animals (and some
clinical studies) indicate that dietary potassium and
calcium potentiate increases in blood pressure in salt-
sensitive experimental models. There is evidence to
suggest that the sodium to potassium ratio correlates
more strongly with blood pressure than does either
nutrient alone. As indicated previously, the metabo-
lism of sodium, chloride, and potassium is closely
related, and sodium and calcium ions have a close
metabolic relationship within cells.9.6 Potassium
Potassium, sodium, and chloride make up the princi-
pal electrolytes within the body. In contrast to sodium
and chloride, nutritional concerns with potassium
are mainly concerned with the possibility of
underconsumption.