ber 4-2³ in Taylor, 2008), because it is not entirely consistent with the statistical approach based on distributions of requirements that underpin the DRIs. Guidelines for the use of AIs would be helpful, both those that exist for other nutrients at this time as well as those that might be specified in the future.

As highlighted in Chapter 3, excess adiposity or obesity—defined as a body mass index (BMI) measure of 30 mg/m² or higher—is associated with lower serum 25OHD concentrations (and higher parathyroid hormone levels) than found in non-obese counterparts. This would appear to be due to sequestration of 25OHD by adipose tissue, given that supplementation of obese and lean persons with vitamin D appears to result in no significant difference in response between the two groups (Jones, 2008). Moreover, a few studies of modest weight loss have found circulating 25OHD levels to increase despite no increased intake of vitamin D from diet or sun exposure (Riedt et al., 2005; Reinehr et al., 2007; Zittermann et al., 2009; Tzotzas et al., 2010), suggesting release from adipose stores with adipose depletion. Further, neither season nor ethnicity influences these biochemical parameters (Alemzadeh et al., 2008).

An important concern is whether the lower serum 25OHD levels associated with obesity have meaningful consequences for the DRI indicator of bone health. Evidence for effects of obesity on bone density is mixed. The combined influence of increased weight-bearing activity and endogenous synthesis of estrogen due to outcomes of increased adiposity has long been associated with higher bone density (Reid, 2008). In a population-based study in Finland of perimenopausal and early postmenopausal women, Pesonen et al. (2005) found that increased body weight was a strong predictor of high bone density. Likewise, Morin and Leslie (2009), in a retrospective cohort study, found a strong correlation between higher BMI category and high bone density in postmenopausal women.

Although these and other studies have suggested that total body mass contributes to bone density and would appear to support the role of increased weight-bearing activity as a factor positively influencing bone density (Prentice et al., 1991; Khosla et al., 1996; Wortsman et al., 2000; Finkelstein et al., 2002, 2008), more recent studies lead to further ques-