Introduction to Human Nutrition

Minerals and Trace Elements 191

However, the effi ciency of intestinal calcium absorp-
tion is infl uenced by a variety of physiological factors
(Table 9.1). Calcium absorption may also be infl u-
enced by a number of dietary factors (Table 9.1).

Metabolic function and essentiality

Calcium is required for normal growth and develop-
ment of the skeleton. During skeletal growth and
maturation, i.e., until the early twenties in humans,
calcium accumulates in the skeleton at an average rate
of 150 mg/day. During maturity, the body, and there-
fore the skeleton, is more or less in calcium equilib-
rium. From the age of about 50 years in men and from
the menopause in women, bone balance becomes
negative and bone is lost from all skeletal sites. This
bone loss is associated with a marked rise in frac-
ture rates in both sexes, but particularly in women.
Adequate calcium intake is critical to achieving
optimal peak bone mass (PBM) and modifi es the rate
of bone loss associated with aging. Extraskeletal
calcium (representing around 1% of total body
calcium) plays a role in mediating vascular contrac-
tion and vasodilatation, muscle contraction, nerve
transmission, glandular secretion, and as an impor-
tant second messenger molecule.

Defi ciency symptoms

Because of the small metabolic pool of calcium (less
than 0.1% in the ECF compartment) relative to the
large skeletal reserve, for all practical purposes meta-

bolic calcium defi ciency probably never exists, at least

not as a nutritional disorder. An inadequate intake

or poor intestinal absorption of calcium causes the

circulating ionized calcium concentration to decline

acutely, which triggers an increase in PTH synthesis

and release. PTH acts on three target organs (either

directly or indirectly) to restore the circulating calcium

ATP

ADP

Ca2+

Lumen

Blood

CaT1

Tight

junction

protein

complex

Mucosal cell

1,25 (OH) 2 D 3

Nuclear

receptor

Calbindin D9k

mRNA

Protein synthesis

Nucleus

Na+

Ca-ATPase

K+

Na-K-ATPase

TRPV6/CaT1

ATP

ADP

ParacellularCa transport

TranscellularCa transport

N N

N N

Figure 9.3 Calcium transport across the intestinal mucosal lining: paracellular calcium transport (between mucosal cells) and transcellular calcium
transport (across the mucosal cell).

Table 9.1 Factors affecting calcium absorption

Increased absorption Decreased absorption

Physiological factors

Vitamin D adequacy Vitamin D defi ciency

Increased mucosal mass Decreased mucosal mass

Calcium defi ciency Menopause

Phosphorus defi ciency Old age

Pregnancy Decreased gastric acid (without

a meal)

Lactation Rapid intestinal transit time

Disease states (e.g.,

hyperparathyroidism,

sarcoidosis, idiopathic

hypercalciuria)

Disease states (e.g.,

malabsorption syndrome,

celiac disease, Crohn’s

disease, chronic renal failure,

diabetes, hypoparathyroidism,

primary biliary cirrhosis)

Dietary factors

Lactose (in infants) Phytate

Casein phosphopeptides (?)a Oxalate

Nondigestible oligosaccharides Large calcium load

Small calcium load High habitual calcium intake

Low habitual calcium intake Ingestion without a meal

Ingestion with a meal

a Confl icting data in the literature.