Tuesday, February 21, 2012

Milk does not increase hip fracture risk.

Background: Milk intake has been linked to higher risk of bone fractures. If we google the following combination of search words "milk bone fractures women", we see several titles stating that milk may cause osteoporosis/bone fractures. Or titles with "myths about milk".

Did past studies truly show that milk increases risk of bone fractures?
The links found by google consistently refer to the same scientific article (1). In a 12-year follow-up study, the group of women with the highest intakes of milk, were said to have had a 45% increased risk of hip fracture.

Table 1:

Table 1 shows effects as described in the full text of the article. We see a Relative Risk (RR) of 1.45 (95% CI = 0.87-2.43; P = 0.22) for women consuming 2 or more glasses milk per day. No association was found with risk of forearm fractures.
An effect can be significant in 2 ways:
  • The 95% CI of the RR excludes the "1". This is not the case, because the lower boundary = 0.87 and the upper boundary = 2.43.
  • The P for trend = < 0.05. This is also not the case. The P for trend = 0.22.
In scientific terms, this means that no significant association was found. Or possibly, that a nonsignificantly increased risk was found.

The nonsignificantly 45% increased risk of hip fractures was found for adult milk consumption. But the article also describes effects from teenage milk consumption (see table 2).

Table 2:

We see that women who drank 3 or more glasses of milk per day, during their teenage years, had a nonsignificantly 47% lower risk of hip fractures (RR = 0.53; 95% CI = 0.25-1.16; P for trend = 0.20).

Women with higher intakes of milk during both their teenage years and their adulthood did not have an increased risk op hip fractures (RR = 0.88; 95% CI = 0.58-1.36; no P-value available).

Newer studies: In 2011, a group of researchers looked at all prospective studies (6 studies for women and 3 studies for men) with available data about the relation between milk intake and hip fracture risk (2). Results from the individual studies were grouped together in a so-called meta-analysis. The pooled results showed no association with risk of hip fractures among women (RR = 0.99; 95% CI = 0.96-1.02 per glass of milk per day), but a nonsignificant protective effect among men (RR = 0.91; 95% CI = 0.81-1.01 per glass of milk per day).

Note: Associations found in the newer studies are based on a small amount of cohorts, including a small amount of hip fractures (3.574 fractures among women and 195 fractures among men). More research is needed before any valid conclusions can be drawn.

1) Feskanich et al. Milk, dietary calcium, and bone fractures in women: a 12-year prospective study. Am J Public Health. 1997 Jun;87(6):992-7. http://www.ncbi.nlm.nih.gov/pubmed/9224182
2) Bischoff-Ferrari HA et al. Milk intake and risk of hip fracture in men and women: a meta-analysis of prospective cohort studies. J Bone Miner Res. 2011 Apr;26(4):833-9. doi: 10.1002/jbmr.279. http://www.ncbi.nlm.nih.gov/pubmed/20949604


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