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

Chapter 4 – Milk and dairy products as part of the diet 125
protein intake and hip fracture (Kerstetter, Kenny and Insogna, 2011). However,
there is much debate on the effect of protein on calcium absorption and status
(Theobald, 2005). Studies using purified protein or protein hydrolysates have consistently
shown a 1 mg rise in urinary calcium excretion for each 1 g of ingested
protein (Weaver, Proulx and Heaney, 1999; Rafferty and Heaney, 2008). Proposed
mechanisms include the effect of the acid load contained in animal proteins (which
may be neutralized by the body drawing calcium from the bones) and complexing
of calcium in the renal tubules by sulphates and phosphates released by protein
metabolism (see FAO and WHO, 2002 and references therein). However, when
protein is ingested as meat and/or dairy, the urinary loss of calcium has been
reported to be less pronounced (Kerstetter and Allen, 1989). It has been suggested
that the effect of protein intake on urinary calcium levels may be countered by
the hypocalciuric effect (decreasing of urinary calcium losses) of phosphorus and
potassium present in meat and dairy foods (Whiting et al., 1997, and Heaney and
Recker, 1982, both cited in Rafferty and Heaney, 2008). A summary by Roughead
(2003) on the topic stresses the importance of this distinction between purified and
common dietary protein sources, because the latter contain a substantial amount of
phosphorus, which blunts the calciuric effect observed with purified proteins.
Despite the effects on urinary calcium losses, high protein intakes have been
found to enhance calcium absorption, especially when the calcium content of the
diet was limiting (600–800 mg/day) (Kerstetter, O’Brien and Insogna, 1998; Kerstetter,
Kenny and Insogna, 2011). Dawson-Hughes (2003) reported that the impact of
dietary protein on the skeleton appears to be favourable in older subjects who are
meeting their dietary calcium requirements but not in those with lower calcium
intakes. Other authors have highlighted that it is important to consider these effects
in all stages of the life cycle and not just in the elderly population (Roughead,
2003; Spence and Weaver, 2003). In a recent review of the topic Kerstetter, Kenny
and Insogna (2011) state that “Recent epidemiological, isotopic and meta-analysis
studies suggest that dietary protein works synergistically with calcium to improve
calcium retention and bone metabolism. The recommendation to intentionally
restrict dietary protein to improve bone health is unwarranted, and potentially even
dangerous to those individuals who consume inadequate protein”.
Exercise can boost the benefits of good nutrition to growing bone, especially
during growth (Bass et al., 2007; Specker and Vukovich, 2007; Welch et al., 2008;
Nikander et al., 2010): bone strength is increased with exercise, but sufficient calcium
is necessary for increasing bone mass. Exercise helps to prevent bone loss only
if calcium intake is greater than 1 000 mg/day, i.e. when there is sufficient calcium
intake (Specker and Vukovich, 2007).
4.4.3 Milk and dairy foods and bone health
The mineral profiles in milk and bones have much in common. With the exception
of small fish that are eaten whole, including the bones, few foods naturally contain
as much calcium as milk (Weaver, Proulx and Heaney, 1999; Theobald, 2005). Calcium
in milk has a high bioavailability, similar to calcium carbonate, which is readily
absorbed (Theobald, 2005). Although many green leafy vegetables such as spinach
are rich in calcium, they also contain oxalate, which reduces the calcium availability.
Calcium availability is greater in plant foods such as broccoli, sweet potatoes, kale