Vitamin D3 structure
Vitamin D metabolism
The ‘sunshine‘ vitamin that is also a hormone, vitamin D is a fat-soluble vitamin which is known best by its biological effects on calcium absorption, bone mineralization, and bone resorption (degradation). There are multiple forms of vitamin D with the major two forms denoted as D2 and D3. Vitamin D exerts its biological effects when its most active form (1,25-(OH)2-D) binds to vitamin D receptor.
My Ph. D. research with Dr. Kimberly Livingston at North Carolina State University encompassed dietary vitamin D impacts on physiology. One aspect of my research examined how a synthetic form of vitamin D3, called 1-alpha-hydroxycholecalciferol (1α), affected physiological status of growing broiler chicks fed differing levels of calcium. 1α is equal in potency to 1,25-(OH)2-D3, but is cheaper to synthesize and supplement in diets. This work showed how 1α supplementation has beneficial effects in starter diets on calcium absorption (Warren et al. 2020).
Figure 6 from Warren et al. 2020. Hypothetical model comparing how dietary vitamin D3 and 1α are converted to calcitroic acid to be excreted in urine.
My Ph. D. work also explored super-dose impacts of dietary vitamin D3 on plasma and egg yolk vitamin D3 in aged laying hens. My study had two implications: 1) laying hens fed very high dietary levels of vitamin D3 will lay vitamin D-enriched eggs that are value-added for production; 2) menopausal women take vitamin D supplements to reduce their risk of osteoporosis; however, there may be risk of vitamin D toxicity if they accidently overdose. Aged laying hens exhibit increased risk of osteoporosis similar to women in peri- and postmenopausal stages of life which makes the hens a comparable model to investigate vitamin D super-dosage impacts. My findings illustrated a dose-dependent response in which higher levels of dietary vitamin D3 led to increased plasma and egg yolk vitamin D3. Also, hens fed the diets with super-dose vitamin D3 levels had decreased relative expression of kidney 24-hydroxylase (converts 25-vitamin D3 to 24,25-vitamin D3; in which 24,25-vitamin D3 is an inactive form of vitamin D3 (Warren et al. 2024). This work suggests that older animals given high levels of vitamin D supplementation could be a risk of vitamin D toxicity.
Figure 3 from Warren and Livingston 2021. The metabolic pathways of vitamin D3 and the major hydroxylated forms.
I wrote a review paper with that highlighted the significance of the chicken as an important model for vitamin D research for understanding vitamin D metabolism and how such is important for advancing human nutrition (Warren and Livingston 2021).