3.8: Vitamin D Production
- Page ID
- 83979
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Though the production of vitamin D begins in both the epidermis and the dermis, most of it occurs in the epidermis. Skin cells start the process by modifying cholesterol molecules. When the modified molecules are struck by light, they are altered again to form an inactive form of vitamin D. Ultraviolet light seems to be the best type of light for this process, and sunlight is the natural source of UV light for the body. Exposure of the hands and face to only 10 to 15 minutes of summertime sunlight provides enough light for the skin to produce all the vitamin D needed by the body. Inactive vitamin D is carried away by the blood in dermal blood vessels.
The inactive form of vitamin D can also be obtained from foods such as fish and vitamin D-enriched milk. Whether from the skin or from the diet, inactive vitamin D is sent to other parts of the body (i.e., liver and kidneys) for additional modification and final activation. It is then transported throughout the body. Vitamin D influences movement of calcium into and out of bones directly and indirectly. Vitamin D reaching the intestines helps absorb calcium from food.
Calcium performs many essential functions in the body. It is a main building material in bones and teeth and is essential for the contraction of muscles, the passage of impulses in the nervous system, and the clotting of blood. Calcium also controls many chemical reactions in cells.
Age Changes in Vitamin D Production
The ability of the skin to produce inactive vitamin D decreases with age. This seems to result from several factors. For example, there may be a decrease in the delivery of cholesterol‑like molecules to the skin because of the decrease in blood flow in the skin. Also, the skin cells seem slower at converting this material. Furthermore, the process powered by light becomes less efficient. Therefore, an older person must get more exposure to sunlight to produce the same amount of vitamin D. Finally, the slower movement of materials through the skin and the decrease in dermal blood flow may slow the removal of inactive vitamin D from the skin.
The overall result of reduced vitamin D production by the skin is an increased risk of vitamin D deficiency with age. The risk is further increased because the elderly often have less exposure to sunlight because of reduced mobility, social customs, and the higher risk of developing sunburn. There is also a decline in the ability of the kidney to complete vitamin D activation. If a vitamin D deficiency develops, the absorption of calcium will become inadequate. All body functions that depend on calcium will then become abnormal.
Fortunately, many ordinary foods (e.g., bread, milk) have vitamin D added to them. Incorporating such foods into the diet can largely eliminate the risk of vitamin D deficiency. In situations where the diet cannot provide the necessary vitamin D, vitamin supplements such as vitamin pills can do so.