Nutrition Science and Everyday Application - beta v 0.1

VITAMIN D: IMPORTANT TO BONE HEALTH AND BEYOND 517
Figure 9.11. Vitamin D can be synthesized in the skin (vitamin D 3 ) or provided in the diet (vitamin
D 2 or D 3 ). It is converted by reactions occurring first in the liver (making calcidiol) and then kidney
(making calcitriol, the active form). Once active, vitamin D works in several ways to ensure blood
calcium homeostasis and enhance the availability of calcium for bone mineralization.
Beyond its role in bone health, vitamin D has many other functions in the body. Cells
throughout the body have vitamin D receptors in their nuclei, and by binding to these
receptors, vitamin D is thought to regulate the expression of hundreds of genes. Specifically,
vitamin D is known to play important roles in regulating cellular differentiation and growth,
immunity, insulin secretion, and blood pressure. Studies have found correlations between
low circulating vitamin D levels and increased risks of chronic diseases, including cancer,
diabetes, cardiovascular disease, and multiple sclerosis. However, it has been difficult to
determine if a lack of vitamin D actually contributes to the cause of these diseases, and
research in this area is ongoing. 1
SUNLIGHT AS A SOURCE OF VITAMIN D
In most people, vitamin D synthesis in the skin provides a significant portion of their body’s
needs, and a little sun exposure can go a long way. Vitamin D researchers suggest that
most people need between 5 and 30 minutes of sun exposure between 10 AM and 3 PM,
at least twice per week, in order to synthesize adequate vitamin D. However, any factor