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Walking an hour a day (more vitamin D) decreased chance of breast cancer by 14 percent – Oct 2013

73,000 women followed for 17 years
7 hours a week of walking: 14% less breast cancer than those who walked three or fewer hours per week

There was no mention in the article of sunshine and vitamin D reducing Breast Cancer
Probably no need to walk 7 hours a week every week of the year
Best to walk during the summer, at noon, with as much skin exposed to the sun as possible (see tan-thru shirts below)

Suspect that 1 hour of walkin once a week, at noon, during the summer, while wearing a tan-thru shirt would provide the 14% benefit.
That would be about 30 hours a year.
Alternately, could take about 10,000 IU of vitamin D once a week to get a similar benefit (annual cost of the vitamin D: about $10)
If you value your time as worth more than 30 cents per hour ($10/30 hours) then you should take the vitamin supplement

Of course, walking more or taking more vitamin D will reduce the chance of breast cancer

See also VitaminDWiki

derived from Grassroots 2013