Vitamin A is an essential oil-soluble micronutrient. Vitamin A is known for many important functions such as vision, maintenance of normal structure of skin and mucous membranes, immunity, growth, anti-aging and cancer.
1. General role of vitamin A
Vitamin A with important functions:
a. Involvement in visual function
This is the most well-understood function of vitamin A. Vitamin A is the biologically active form of retinol. Vitamin A is transferred to rod or cone light-sensing cells in the form of 11-cis retinal, which combines with opsin in the cell membrane to form rodopsin to participate in low-light vision. Therefore, when vitamin A deficiency is present, the first clinical symptoms usually appear in the eyes, in which night blindness (the ability to see is reduced when the light is low) is the earliest clinical symptom.

b. Maintenance of normal structure of skin and mucous membranes, cell differentiation:
Vitamin A helps in the development and regeneration of skin and mucous membranes, the ability to secrete secretions of mucosal cells, and stimulates the development of epithelial layers such as stratum corneum, intestines and respiratory tract. Vitamin A stimulates scar healing.
c. Immune response:
Vitamin A participates in both specific and non-specific immunity of the body:
+ Non-specific immunity: protects the integrity of the skin and mucous membranes, against the penetration of bacteria and pathogens into the body.
+ Specific immunity: helps to maintain and protect the lymphocyte lineage, participates in the T-lymphocyte-mediated immune response.
d. Hematopoiesis:
The mechanism is still unknown, but it has been found that when vitamin A deficiency is closely associated with iron deficiency anemia. Tt is possible that vitamin A deficiency interferes with iron absorption, transport and storage.
e. Growth
Retinoic acid acts as a hormone in regulating the growth and development of tissues in the musculoskeletal system. Because of its important role in human cell development, vitamin A is indispensable for embryonic and child development. Vitamin A also plays a role in bone growth. Vitamin A deficiency makes bones softer and thinner than usual, and causes the calcification process to be disturbed.
f. Anti-aging:
Vitamin A prolongs the ageing process by preventing the development of free radicals. Therefore vitamin A may also fight the growth of some cancers.
2. The roles of vitamin A in bone development
Mechanistically, vitamin A differentially affects stages of osteoclastogenesis by enhancing early osteoblast differentiation and inhibiting bone mineralization through retinoic acid receptor signalling (RAR) and regulation of osteoclast/osteoblast-associated bone peptides.
However, adequate intake of vitamin A through food or supplements has been shown to maintain healthy bones. Meanwhile, provitamin A (carotene and β-cryptoxanthin) may also protect bones.
In vitro evidence suggests that carotene and β-cryptoxanthin may act as precursors to retinoids, particularly all-trans-retinoic acid, which plays a role as a ligand for RAR to promote osteoclastogenesis and inhibits activation of nuclear factor kappa B to inhibit osteoclast differentiation and maturation.
Current evidence provides inconsistent results regarding the positive, negative, and insignificant effects of vitamin A for bone health.
Carotene and β-cryptoxanthin have the potential to influence bone homeostasis by exhibiting osteoblast-stimulating effects and inhibitory effects on osteoclastogenesis and osteoblastogenesis. Higher bone mineral density (BMD) and a lower risk of fracture have been reported in people with higher vitamin A intake.
In contrast, several studies report that dietary vitamin A supplementation in the form of a multivitamin or food supplement is associated with an increased risk of fracture and accelerated age-related bone loss. Several other reports have shown a lack of association between vitamin A intake and fragility fractures. Short-term hypervitaminosis A also induces an increase in cortical bone loss in laboratory animals.
In addition, vitamin A has been shown to be in opposition to the role of vitamin D in increasing calcium absorption and maintaining serum calcium homeostasis. Both retinoic acid and 1,25-hydroxyvitamin D share the same receptor (RXR). Therefore, high vitamin A levels can impair vitamin D function.
Therefore, vitamin A can have both positive and negative effects on bones. Consequently, understanding the mechanism of action of vitamin A is important to determine its skeletal effect. The differences obtained from these studies may be due to variations in study design, population, duration, age, sex and health status of subjects.
However, the impact of vitamin A also depends on a number of other factors such as serum vitamin D levels, physical condition (vitamin A easily causes bone mass loss in overweight and obese people), supplementation with other antioxidants or bone-building agents (such as estrogen).

Getting enough vitamin A from the diet or other sources is necessary to maintain healthy bone growth. The recommended need for vitamin A for Vietnamese adults in Vietnam is 850-900 mcg/day, for non-pregnant women is 650-700 mcg/day. At this dose, vitamin A can protect the skeleton.
However, increasing vitamin A overdose causes bone problems, especially in people who don’t get enough vitamin D. Thus, vitamin A fortification induces low BMD and increases fracture risk because retinoic acid inhibits osteoblast differentiation and mineralization at high doses. The recommended vitamin A intake limit is 2700 mcg/day excluding provitamin A carotenoids.

References:
[1] L. D. T. Pham Van Phu, Basic Nutrition. Medical Publishing House, 2016
[2] M. M. F. Yee, K.-Y. Chin, S. Ima-Nirwana, and S. K. Wong, “Vitamin A and Bone Health: A Review on Current Evidence”, Molecules, vol 26, p.h 6, p. 1757, March 2021, doi: 10.3390/molecules26061757.
[3] Le Danh Tuyen, Recommended nutritional needs for Vietnamese people. National Institute of Nutrition, 2016.
Article source: Nutrition Research and Development Institute (https://inrd.vn/)
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