Intended for healthcare professionals

Editorials

Milk and bones

BMJ 1994; 308 doi: https://doi.org/10.1136/bmj.308.6934.930 (Published 09 April 1994) Cite this as: BMJ 1994;308:930
  1. R Lindsay,
  2. J Nieves

    “You are what you eat” has been an popular aphorism in the many branches of medicine in which nutritional aberrations are suspected to have a role in disease. In the pathophysiology of osteoporosis the nutritional questions have always centred on calcium. The importance of osteoporosis is clear, with the current epidemic of hip fractures increasing as populations worldwide gradually age: the global load of hip fracture is expected to treble to over six million cases a year by 2050.1

    In this week's journal Sean Murphy and colleagues confirm the benefit of a high calcium intake (as milk) on bone mineral density (p 939).2 In a community based survey of older women (aged 44-74) they found their milk consumption before the age of 25 correlated positively with current bone mineral density. This relation persisted after numerous potential confounding factors were controlled for statistically. A similar, but weaker, association was found between calcium intake during adulthood and bone mass.

    Although peak bone mass is primarily under genetic control, it seems logical that to achieve adequate skeletal maturation during growth requires a plentiful supply of the building blocks of the skeleton, of which calcium is one of the most important. During peak growth in early teenage years the calcium requirement for the skeleton can be as high as 400 mg/day. To provide this amount of calcium, even if adolescents absorb calcium more than adults (and this is disputed), would require an intake of at least 1500 mg/day.3 Dairy products provide the main source of dietary calcium in developed countries; the study's findings are therefore unsurprising and support the importance of calcium nutrition in early life.

    Milk is a cheap source of calcium and one of the most bioavailable sources. Past milk consumption is easy to study because people can usually recall the relevant details. But milk is a complex food, providing other nutrients, and the observed effects on bone mineral density may result from an overall better diet and healthier lifestyle. In fact, separating out the skeletal effects of other nutrients found in high amounts in milk, such as protein and phosphorus, from those of calcium is difficult. In addition, milk contains lactose, which increases the absorption of calcium. Of the nutrients supplied by milk, dietary calcium is the single nutrient otherwise limited in a typical diet - bone mineral density associated with greater milk consumption therefore seems most likely to result from its calcium content.

    A previous study evaluated a wider range of milk consumption and found that it correlated strongly with skeletal status.4 The current study evaluated relatively low intakes. The maximal calcium intake of someone drinking a 227 ml glass of milk each day is only 650 mg/day (about 300 mg from the milk and the remainder from non-dairy sources). This may still be below the presumed “threshold” of calcium intake - that level of intake above which most people would be calcium replete and the relation between skeletal status and calcium intake would be lost.5 This threshold, estimated at 1500 mg for adolescents,3 may vary at different stages of life and lower at later ages.6

    For adolescents there seems to be a critical time during which bone mineral density may be increased by optimising the intake of calcium. Only one controlled clinical study of calcium supplementation has been published in growing children.7 In that study prepubertal children given a 1000 mg calcium supplement increased their bone mass faster than the controls (their monozygotic twin). For those twins who were past puberty supplementation produced no significant effects. In addition, the effects on bone mineral density were lost when supplementation was stopped, which suggests the need to maintain calcium intake throughout adolescence and adulthood.8 Calcium intake needs to be sufficient to dampen down activity at skeletal remodelling sites - that is, to avoid skeletal calcium being used to keep the serum calcium concentration within its tightly controlled limits.

    Calcium intake may have more impact on cortical rather than cancellous bone: a meta-analysis found little impact of calcium on spine bone mineral density.9 Murphy and colleagues also found that bone mineral density at the hip, where more cortical bone is present, seems more dependent on calcium consumption in youth, but this was less the case for the spine, after adjustment for covariates. Our ancestors had calcium intakes in the range of 2000 mg a day or more,10 whereas our current diets average appreciably less - about 600 mg for adults and not much more for teenage women. With intestinal absorption averaging 30%, the average diet therefore provides only 100-200 mg calcium a day. This is barely enough to maintain calcium balance and clearly insufficient for skeletal accretion during the teenage years.

    Doctors should remember that calcium will not prevent the bone loss due to other factors - for example, the loss of oestrogen at the menopause - and should advise their patients accordingly. None the less, milk is a bioavailable, relatively inexpensive source of calcium for those who can ingest it. Fortifying foods such as bread and orange juice may also help to increase the calcium intake, as would using calcium supplements for those who cannot otherwise improve their diet.

    Suggestions for an increase in intake of milk or alternative sources of calcium during youth to maximise skeletal potential should not be the subject of controversy. Rather they should be a matter for public health promotion, particularly as calcium intake in the ranges found to be effective bears little or no risk to the consumer and may even afford some protection against several other diseases, such as colonic cancer and hypertension,11,12 as well as osteoporosis.

    References

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