The limited number of long-term clinical trials related to calcium and vitamin D intakes and health outcomes; and
The need to set ULs based on limited data in order to ensure public health protection.
An important question that will undoubtedly be asked given this committee’s report, is: Why is it that so much information about the positive effects of vitamin D on outcomes such as cancer, diabetes, and immunity is said to exist and is reported almost daily in the press, but this committee found no basis to support these causal relationships? The short answer is that a systematic examination of the evidence, using established guidelines for measuring the strength and quality of studies, revealed that the claimed benefits based on the associations of low or high intakes of vitamin D on non-skeletal health outcomes could not be supported by the studies—the evidence was inconsistent and/or conflicting or did not demonstrate causality. In addition, some effects were not related to setting nutritional requirements for vitamin D. This conclusion, however, does not preclude pursuing investigation of causal relationships.
Moreover, a related question that will be asked is: With the advent of newer studies, why is there still so much uncertainty? At least one reason is that most studies were not designed to seek data maximally useful for DRI development, which is well described by others (Yetley et al., 2009). DRI development fundamentally requires elucidation of dose–response relationships and benefits from data of high quality obtained in randomized controlled trials. In making its conclusions about potential indicators other than bone health, the committee noted the findings previously specified by an IOM committee tasked with examining the evolution of evidence for nutrient and disease relationships (IOM, 2002). That committee concluded that evidence about relationships between specific nutrients and a disease or health outcome typically remains elusive for a number of reasons (IOM, 2002). These include the following:
Although preliminary evidence, usually from mechanistic studies, experimental animal studies, and observational studies in humans, can generate exciting new hypotheses about nutrient–health relationships, evidence from these studies has limitations. For instance, even in well-designed, large-scale observational studies, it is difficult to isolate the effects of a single nutrient under investigation from the confounding effects of other nutrients and from non-nutrient factors.
Scientific advances in understanding relationships between specific nutrients and health outcomes do not necessarily emerge