"Given recent findings, it's easy to see why people are so enthusiastic about the potential power of vitamin D, but we must recognize the limitations of any study and exercise caution when making broad public health recommendations," said Mary Frances Picciano, a senior nutrition research scientist in the Office of Dietary Supplements of the federal National Institutes of Health (NIH). "This is a very complex set of issues, and there is still a lot we don't know about how vitamin D levels affect health, especially across different age groups and ethnic populations." The NIH researchers point to the following facts to show the extent to which there is still enormous scientific ambiguity concerning the vitamin: -- Many studies haven't controlled for variables that could muddy research findings, such as diet, baseline vitamin D status, age, disease, sun exposure and physical activity. -- Vitamin D's effects, independent of calcium or other nutrients, have been very little examined. -- Dependable information on foods' vitamin D content is unavailable. -- There is wide variability among current lab tests used to measure vitamin D levels in blood. -- Definitive research has not been done on how much vitamin D is needed to achieve health in people at different life and reproductive stages and in people of color. "It's tempting to think that an essential nutrient is safe at any level - that if some is good, more is better," said Paul M. Coates, director of the Office of Dietary Supplements. "We've learned that this isn't always true, and there are potential harms associated with high levels of many nutrients."