Page 119 - JSOM Spring 2021

Vitamin A and Bone Fractures

1
Joseph J. Knapik, ScD *; Sally S. Hoedebecke, MS, RD 2

ABSTRACT
Vitamin A is a generic term describing compounds that have the inflammatory processes, and 4) assists in the growth and
the same biological activity as retinol. Dietary vitamin A can be activation of lymphocytes. 1–3
obtained from “provitamin A” carotenoids (e.g., β- carotene)
found in plant foods such as carrots, cantaloupes, and sweet Vitamin A can be obtained only in the diet because the body
peppers, or as “preformed vitamin A” found in many dietary cannot produce it. While consumption of an adequate amount
supplements, animal livers, and vitamin A–fortified foods, of vitamin A is important for physiological functioning, sev-
such as breakfast cereals, milk, cheese, and yogurt. Low con- eral observational studies suggest that consumption of high
sumption of vitamin A can cause night blindness, reduce im- levels of vitamin A is associated with a higher incidence of
mune function, and have detrimental developmental effects. bone fractures. This suggests there could be a U-shape re-
4-6
Several lines of evidence suggest that excessive dietary intake lationship between vitamin A intake and fracture risk, as de-
of vitamin A might be associated with an increased risk of picted in Figure 1. That is, too little dietary vitamin A will
bone fractures. Meta-analysis of observational human studies increase fracture risk, a moderate amount will reduce fracture
that have examined vitamin A and fractures suggests that di- risk, and too much will again increase fracture risk. The pur-
etary consumption of large amounts of vitamin A in the form pose of this article is to review the effects of vitamin A on me-
of β-carotene likely has a protective effect, reducing the risk tabolism with an emphasis on whether excessive dietary intake
of fractures. On the other hand, meta-analyses that have spe- of vitamin A increases the risk of fractures.
cifically examined hip fractures have shown that total vitamin
A (all types) or retinol consumption may increase the risk of FIGURE 1 Suggested U-shape relationship between vitamin A
hip fractures. Until more information is available, it is advis- intake.
able to consume vitamin A primarily from plant sources, avoid
excessive consumption from dietary supplements and animal
sources, and lower consumption from fortified foods.
Keywords: β-carotene; retinol; meta-analysis; hip fracture

Introduction
Vitamin A is a generic term describing compounds that have
the same biological activity as retinol. These compounds are
important for vision, gene expression, immune function, re-
production, and growth. Vitamin A is required by the eye to
produce visual pigments in the rods and cones that allow the
transduction of light into neural signals that are interpreted in Types and Sources of Vitamin A
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the visual cortex of the brain. Vitamin A regulates genes that
encode for structural protein, enzymes, and extracellular ma- Dietary vitamin A can be obtained from “provitamin A”
trix proteins. In embryonic development, vitamin A is involved carotenoids, which include α-carotene, β-carotene, and β-
in development of the hindbrain, limbs, heart, eyes, and ears. cryptoxanthin. These are found in plant foods such as car-
In immune function, vitamin A 1) assists in maintaining an rots, cantaloupes, sweet peppers, mangoes, apricots, broccoli,
adequate level of natural killer cells that have antiviral and an- and squash. Another way dietary vitamin A can be obtained
titumor activity, 2) increases the phagocytic activity of macro- is from “preformed vitamin A,” which is retinol or retinyl es-
phages, 3) increases the production of cytokines that mediate ters. These are found in dietary supplements, animal livers,

*Correspondence to joseph.j.knapik.ctr@mail.mil
1 MAJ (Ret) Knapik served in the US military as a wheel vehicle mechanic, medic, Medical Service Corps officer, and Department of Defense
civilian. He is currently a senior epidemiologist/research physiologist with the Henry M. Jackson Foundation (Washington, DC) and an adjunct
professor at Uniformed Services University (Bethesda, MD) and Bond University (Robina, Australia). COL (Ret) Hoedebecke served 30 years as
2
a dietitian in the US Army. She had a variety of assignments, including deployment during Operation Desert Storm. She completed a fellowship
in health care policy at George Washington University and has specialized in fitness, nutrition, and policy.
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