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

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Milk and dairy products in human nutrition
PBM until menopause in women or old age in men, bone is considered more stable,
although increasing evidence shows bone loss begins much earlier than menopause,
especially in sedentary individuals (Weaver et al., 2001). Bone loss is common in the
aging skeleton. Bone turnover increases with the loss of oestrogen and, as in puberty,
bone formation and bone resorption becomes uncoupled. At this life stage, bone
resorption rates exceed bone formation rates. The magnitude of bone loss is highly
dependent on body weight. Smaller skeletons are more vulnerable to loss, likely
because their bones are less loaded with lower body weight and therefore provide
less mechanical stimulus. Overall, genetics are thought to control 60–80 percent of
bone mass and environmental factors such as diet and physical activity 20–40 percent
(Krall and Dawson-Hughes, 1993; Bonjour and Chevalley, 2007).
4.4.2 Dietary factors that affect bone health
The main dietary factors that affect bone mass are calcium and vitamin D, although
other nutrients such as potassium, zinc, vitamins A, C and K and protein, as well
as energy, also play a role. Calcium, phosphorus and magnesium are the most
important minerals to bone health, of which calcium is the most abundant. More
than 99 percent of the body’s calcium, 85 percent of its phosphorus and 60 percent
of its magnesium are in bone.
Calcium
Calcium balance is determined by the relationship between calcium intake and
calcium absorption and excretion. Dietary intake of calcium has to be large enough
to match obligatory losses, if skeletal damage is to be avoided. About 20–30 percent
of calcium consumed in the diet is absorbed in the gastrointestinal tract (Theobald,
2005). The amount absorbed depends on the form in which calcium is present in
food (e.g. insoluble complexes with phosphate), the amount present, its solubility
and the presence of dietary factors that inhibit or promote absorption (e.g. phytates
and oxalates inhibit absorption by forming insoluble salts). Protein has both positive
and negative effects on calcium balance (see “Protein”, below). Calcium bioavailability
is also influenced by physiological variables such as historical calcium
intakes, vitamin D status (see “Vitamin D”, below) and age (absorption appears to
decline with age), pregnancy and lactation status (calcium absorption is up regulated
during lactation) (FAO and WHO, 2002; Theobald, 2005).
The FAO/WHO expert consultation on vitamin and mineral requirements
(FAO and WHO, 2002) presented recommendations for calcium intakes based on
long-term 21 calcium-balance data for adults in Western countries (Table 4.3). 22 The
consultation noted that mean calcium requirement of adults at present can only
21 The mean duration of the 210 experiments from eight publications used in this report to derive the
recommended intakes was 90 days with a range of 6–480 days. (The four 6-day balance studies in the
series used a non-absorbable marker and are therefore acceptable).
22 The report states that other possible beneficial effects of calcium, such as in the prevention or
treatment of pre-eclampsia, colon cancer or hypertension, have not been considered in making
these recommendations, as experimental results in these regards have been disappointing/inconclusive
or negative.