Milk-and-Dairy-Products-in-Human-Nutrition-FAO

Chapter 4 – Milk and dairy products as part of the diet 123
be determined by balance studies conducted with sufficient care, and over a sufficiently
long period of time to ensure reasonable accuracy and then corrected for
insensible losses. The calcium requirement was reported to change depending on
other nutrients present in the diet, two such nutrients being sodium (presumably
competing with calcium for reabsorption in the renal tubules) and animal protein
(see “Protein”, below, for possible mechanisms), both of which increase urinary
calcium and were therefore presumed to increase calcium requirement. Vitamin D
also plays a role in calcium homeostasis and calcium absorption. The expert consultation
also highlighted the “calcium paradox”, that hip fracture rates are higher
in developed countries, where calcium intake is high, than in developing countries,
where calcium intake is lower, 23 and suggested that this may be related to protein
intake and vitamin D status in these countries, or both, with sodium intake being
another possible reason. Hence, the expert consultation provided different recommendations
for countries with low consumption of animal protein (20–40 g/day
rather than the 60–80 g/day typical of developed countries) (Table 4.3).
A subsequent WHO/FAO expert consultation on diet, nutrition and the prevention
of chronic diseases (WHO and FAO, 2003) concluded that there is convincing
evidence that sufficient intake of vitamin D and calcium together reduces the
risk of osteoporotic fracture in older people. Based on the findings of FAO and
WHO (2002), WHO and FAO (2003) recommended a minimum daily intake of
400–500 mg of calcium in countries with a high incidence of fracture to prevent
osteoporosis (WHO and FAO, 2003). This recommendation was made after considering
the strength of the evidence with fracture as an end point (rather than BMD;
see Section 4.4.5 for limitations of studies using BMD as an end point), and appears
to relate to older people (>50-60 yr). This is considerably lower than the amounts
recommended by the previous expert consultation (Table 4.3). The experts further
stated that recommendations for calcium intake in countries with low fracture
incidence should take into account the interaction between calcium intake, physical
activity, sun exposure and intake of other dietary components (e.g. vitamin D,
vitamin K, sodium, protein) and protective phytonutrients (e.g. soy compounds).
Vitamin D
Calcium and vitamin D interact in the human body: when the level of ionized calcium
in the blood falls, parathyroid hormone is secreted by the parathyroid gland,
stimulating the conversion of vitamin D to its active form, calcitriol (1,25-dihydroxyvitamin
D) and thus depleting vitamin D status (measured by the amount of
the inactive form). Vitamin D, as calcitriol, influences calcium absorption across the
intestine, and inadequate vitamin D status is associated with reduced absorption of
calcium from the diet. Vitamin D can either be made in the skin from a cholesterollike
precursor by exposure to sunlight or can be provided preformed in the diet;
from a nutritional perspective, the two forms are metabolized similarly in humans,
are equal in potency and can be considered equivalent (FAO and WHO, 2002).
23 Average total calcium intakes in Africa, Latin America, the Near East and the Far East are less than
500 mg/day; the average total calcium intake for all developing countries is only 344 mg/day (FAO/
WHO, 2002).