: Why vitamin D3 is deficient in children and the elderly? Vitamin D 3 of the most important vitamins that contribute to the growth of the human body. Why this vitamin is deficient and few in children and the elderly?

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Vitamin D is not strictly a vitamin, rather it is the precursor of one
of the hormones involved in the maintenance of calcium homeostasis and
the regulation of cell proliferation and differentiation, where it has
both endocrine and paracrine actions.

It is important to note that the most important source is the endogenous synthesis in the skin by photolysis of 7-dehydrocholesterol compared to dietary sources;
problems of deficiency arise when there is inadequate exposure to sunlight.

Two compounds have the biological activity of vitamin D:
cholecalciferol, which is the compound formed in the skin, and
ergocalciferol, which is synthesized by ultraviolet (UV) irradiation
of ergosterol

There are few foods that are rich sources of vitamin D. It is generally accepted that, for people with inadequate exposure
to sunlight (young children and the house-bound elderly), supplements are
necessary to maintain adequate status.

Unfortunately, dietary sources of vitamin D are few, and most foodstuffs
are devoid of vitamin D. The only significant sources of
vitamin D (D2 or D3) are animal liver, fatty fish (e.g.,
salmon, halibut, cod), egg yolks, and fish oils. Because human milk is an
extremely poor source of vitamin D, breast-fed infants
require a vitamin D supplement. This contributes to the other reason to the inadequacy in the body.

On the other hand, excessively high intakes of vitamin D are associated with hypercalcemia and calcinosis.

References

Nutritional Biochemistry of the Vitamins: Vitamin D (D .A Bender 2nd Ed)
Modern Nutrition in Health and Disease 11thed Ross et al.