Fragility fractures contribute significantly to the morbidity and mortality of older individuals, and in a growing segment of the senior population a marked increase in these fractures is expected in the next 20 years [1, 2]. The most rapidly growing segment of the senior population consists of men and women aged 85 years and older [2]. This translates in a rising number of all fragility fractures, especially those of the hip. Hip fractures are the most serious and most frequent fractures occurring among seniors aged 75 years and older [3, 4], and an estimated 1 in 3 women, and 1 in 6 men will have sustained a hip fracture by their 90th decade [5]. Muscle weakness [6] and falling [7] are closely related to fragility fractures, and are critical in understanding them. Both muscle weakness and falling have been linked to the broad prevalence of vitamin D deficiency among the senior population. Severe vitamin D deficiency (serum concentrations < 10 ng/ml or < 25 nmol/l 25-hydroxyvitamin D) in the senior population causes secondary hyperparathyroidism, osteoporosis, and osteomalacia [8]. Histological osteomalacia, characterized by the accumulation of unmineralized matrix or osteoid in the skeleton, has been found to be common in several hip fracture case studies (12–44 %) [9–14]. At the same time it is well recognized that severe vitamin D deficiency is prevalent in about 60 % of hip fracture patients [15, 16] and that vitamin D supplementation reduces the risk of hip fracture by 30 % [17]. Thus, it is conceivable that a significant number of hip fractures occurring in seniors are explained by osteomalacic changes that soften the bone. Additionally, as a primary clinical sign of osteomalacia, muscle weakness may contribute to the risk of fracture [18], and vitamin D supplementation may not only mineralize bone, but has been shown to reduce the risk of falling by up to 34 % [19, 20]. Apart from hip fractures, the two other most common fragility fractures at non-vertebral sites are distal forearm and proximal humerus fractures, and, similar to hip fractures, distal forearm and proximal humerus fractures show a steep increase with age [3]. Notably, the circumstances of these fractures are strikingly different. Hip fractures tend to occur in less active individuals falling indoors from a standing height with little forward momentum, and they tend to fall sideways or straight down on their hip [21–23]. However, distal forearm or humerus fractures tend to occur among more active older individuals who are, correspondingly, more likely to be outdoors and have a greater forward momentum when they fall [24–26]. This may also explain why hip fracture incidence shows little to no seasonal change, while the winter/summer seasonal swing is pronounced in distal forearm and humerus fractures, and more so in men than in women [27]. Men aged 65 years and older are at a 51 % greater risk of sustaining a distal forearm fracture and a 23 % greater risk of sustaining a proximal humerus fracture in the winter compared with during the summer [27]. Women aged 65 years and older are at a 15 % greater risk of sustaining a distal forearm fracture and a 19 % greater risk of sustaining a proximal humerus fracture in the winter compared with during the summer [27]. In the same study, in winter, total snowfall was associated with a reduced risk of hip fracture (−5 % per 20 inch), but an increased risk of distal forearm and proximal humerus fractures (6–12 %; p<0.05 at all sites) [27]. H. Bischoff-Ferrari (*) Centre on Aging and Mobility, University of Zurich and City Hospital Waid, Gloriastrasse 25, 8091 Zurich, Switzerland e-mail: Heike.Bischoff@usz.ch