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

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Milk and dairy products in human nutrition
4.4.8 Summary
The main dietary factors that affect bone mass are calcium and vitamin D,
while other nutrients such as potassium, zinc, vitamins A, C and K, protein and
energy also contribute. Few other foods naturally contain as much calcium as
milk. Calcium in milk also has high bioavailability. Calcium and vitamin D are
interdependent. The current recommended intake of vitamin D for ranges from 5
to 15 μg/day (FAO and WHO, 2002). FAO and WHO (2002) also recommended
a calcium intake of 1 300 mg/day for adults more than 65 years old in countries
with high animal protein consumption, and 800 mg/day for adults more than
65 years old in countries with low animal protein consumption (20–40 g/person
per day). However, this was based on calcium balance studies of average duration
90 days, not clinical outcomes; the time-scale of such calcium balance studies may
still be too short for true bone balance, and may merely reflect changes in the bone
remodeling transient rather than reflecting long-term calcium balance. The Expert
Consultation also highlighted the “calcium paradox”, i.e. that hip fracture rates are
higher in developed countries where calcium intake is higher than in developing
countries where calcium intake is lower, and suggested that it may be related to protein
intake, vitamin D status or sodium intake. Recognizing that requirements may
vary, for countries with low consumption of animal protein (20–40 g/day), a lower
recommendation of 800 mg/day for adults more than 65 years old was proposed.
A subsequent WHO/FAO expert consultation (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. After considering
the strength of the evidence with fracture as an end point (rather than BMD), the
report recommended a minimum daily intake of 400–500 mg of calcium to prevent
osteoporosis in older people in countries with a high fracture incidence (WHO and
FAO, 2003). The report cautioned that before recommending increased calcium
intake in countries with low fracture incidence, the interaction between calcium
intake and physical activity, sun exposure and intake of other dietary components
and protective phytonutrients needs to be considered.
Many epidemiological studies have found a positive relationship between protein
intake and bone mass or density, and some studies have reported an inverse
association between protein intake and hip fracture. High protein intakes have also
been found to enhance calcium absorption. Protein is a major bulk constituent of
bone and must be regularly supplied by the diet; milk and other dairy foods are
a source of dietary protein. Some dairy products also provide other nutrients that
support bone health, such as potassium, zinc and vitamin A, and if fortified, vitamin
D. Exercise has been shown to help prevent bone loss only if calcium intake
is greater than 1 000 mg/day. However, these conclusions are based on short-term
studies, and there is currently no evidence to support such an interaction in relation
to risk of fractures.
Overall, genetics are believed to control 60–80 percent of differences in bone
mass and environmental factors, such as diet and physical activity, 20–40 percent.
Increased calcium intake suppresses bone resorption relative to bone formation,
resulting in greater calcium balance. The impact of dietary dairy products on bone
health depends on life stage. In adolescents, dietary calcium explains 12–22 percent
of the variation in skeletal calcium acquisition. Dairy product consumption and cal-