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DRI Dietary Reference Intakes Calcium Vitamin D
tions. The distribution of body fat may influence bone mass, such that excess intra-abdominal fat could adversely affect bone remodeling and even contribute to greater fracture risk (Premaor et al., 2010; Sukumar et al., 2011). One possibility is that intra-abdominal adipose tissue is more biologically active than subcutaneous fat, secreting cytokines and adipokines that negatively affect osteoblast and osteoclast activity (Kawai and Rosen, 2010). Moreover, both lean and fat mass contribute to weight-bearing effects. Because obesity is accompanied by increases in both lean mass and fat mass, at least in younger individuals, it is difficult to attribute the effect on bone density to fat mass as opposed to lean mass. Further, body composition changes with age, even in the obese; in turn, there may be less lean body mass in older individuals.
This complicates the ability to clarify how adiposity may affect bone health. As noted, some studies have suggested that adiposity or increased fat mass itself may be a factor in the development rather than the prevention of osteoporosis, particularly in the elderly. Zhao et al. (2007) observed that when the effect of mechanical loading from high body weight on bone density was statistically controlled, fat mass was inversely correlated with bone mineral content. Further investigation by Zhao et al. (2008) suggested that molecular signaling pathways involved in osteoblast differentiation may contribute to the previously identified effect of increased adiposity on decreased bone mineral content, although a mechanism has not been elucidated. However, this science is just emerging and it is premature to speculate on its significance or relevance to bone health and bone density.
At this time, there is the possibility that obesity, at least in older persons, may not be beneficial for bone health and may be demonstrated to be a risk factor, not an advantage, for decreased bone density and, in turn, reduced bone health. There is no evidence that increases in calcium or vitamin D nutriture beyond the requirements specified for non-obese persons can affect this purported outcome.
Persons Living at Upper Latitudes in North America
The question of the impact of latitude on vitamin D nutriture is often a topic of concern or, at least, interest. The issue, however, is set in the context of the inability to specify a safe dose of sunlight that could contribute to vitamin D synthesis while also avoiding the risk of skin cancer. There are also the recognized challenges associated with quantifying the contributions from sun exposure coupled with the limited information on the role of stored vitamin D during seasonal changes. The prevailing assumption about the effect of latitude is that ultraviolet B (UVB) penetration decreases with increasing latitude (i.e., distance from the equator) and