Greetings from Seattle, where it's rainy and grey from September through
May!  When mothers DO venture out, they usually bundle their infants deep
inside slings or backpacks.  The following reference was published by a
group of Seattle pediatricians who are known for their support of home
birth, naturopathic/alternative medicine, and breastfeeding:

Author:        Feldman-K-W.  Marcuse-E-K.  Springer-D-A.
Title:         Nutritional rickets.
Source:        Am-Fam-Physician.  1990 Nov.  42(5).  P 1311-8.
Journal Title: AMERICAN FAMILY PHYSICIAN.
Abstract:      Nutritional rickets was diagnosed in 18 infants aged
               eight to 24 months. Clinical features included progressive leg
               bowing, poor linear growth, a diet deficient in vitamin D,
               seizures, and abnormal serum calcium, phosphate and alkaline
               phosphatase levels. Wrist radiographs and serum alkaline
               phosphatase levels were the most useful confirmatory tests.
               Breast milk may not contain enough vitamin D to protect
               infants, particularly dark-skinned children and those living in
               cloudy, northern U.S. cities, from rickets after six
               months of age. As breast feeding becomes more widely practiced,
               care is required to ensure that infants at high risk for
               rickets receive appropriate vitamin D supplementation.

My Medline search uncovered dozens of related articles; do you want me to
post the references?