Vitamin A

Vitamin A represents a group of fat-soluble retinoids that includes retinol, retinal, and retinyl esters [11, 12]. This vitamin serves many roles in the body: it is critical for vision, involved in immune function, and is necessary for cellular growth and differentiation [13]. Vitamin A exists in the diet as preformed vitamin A (from animal sources) and as provitamin A carotenoids (sourced from plants). Both sources of vitamin A must be metabolized intracellularly to their active forms (retinal and retinoic acid). The majority of vitamin A is stored in the liver as retinyl esters. When measuring retinol and carotenoid levels, plasma levels are typically sufficient for determining adequacy. A plasma retinol concentration of < 0.70 μmol/L signifies vitamin A inadequacy [13].

In most cases, a balanced diet will supply a healthy amount of vitamin A [14]. The recommended dietary allowance of vitamin A for adults aged ≥ 19 years is 1300 mcg/day (4300 IU [international units]) for U.S. populations. While there is no upper intake level for provitamin A carotenoids, ingestion of very high levels of preformed vitamin A can be toxic. For adults aged ≥ 19 years, the tolerable upper intake level of preformed vitamin A is 10,000 IU [13]. It is therefore important to consider what form of vitamin A is contained in supplements (provitamin A carotenoids or preformed vitamin A) and in what proportion.

As a general rule, consuming too much or over-supplementing vitamin A can cause hair loss [15, 16]. Typically, fat-soluble vitamin A is stored in the liver where its dispersal is tightly regulated by anabolic and catabolic reactions between the inactive and active metabolite. When levels of vitamin A are too high, the capacity of the transport system is exceeded and vitamin A spills over into the circulation [17]. Maintaining homeostasis—and by extension the proper concentration of active metabolite—is important for healthy hair [18].

In one study with the aim to determine the effects of isotretinoin on acne vulgaris in the skin, special care was taken to evaluate changes in the hair and hair growth. Thirty patients were evaluated over a 4- to 7-month treatment period, with examinations carried out using a FotoFinder dermoscope (FotoFinder Systems, Inc., Columbia, MD, USA) with TrichoScan® Professional software. Consistent with other findings, the authors reported a decrease in hair count, density, and percentage of anagen hairs [19].

In a case documented in 1979, a 28-year-old woman undergoing renal dialysis noticed sudden hair loss. Further investigation revealed that she had been taking a daily vitamin A supplement (5000 IU) and that her vitamin A serum levels were well above normal (140 μg/dL). Gentle traction yielded four to five hairs, all of which were in the telogen phase. One month after termination of vitamin A supplementation, hair loss was no longer a problem. The authors concluded that signs of hypervitaminosis A were misinterpreted as symptoms of chronic renal failure. The authors also highlighted the possible “insidious” effects of exogenous vitamin A on dialysis patients [20].

Consumption of vitamin A exceeding the recommended daily limit of approximately 10,000 IU a day can lead to vitamin A toxicity. In a case report, a 60-year-old male who had been taking excess vitamin A supplements experienced non-scarring fronto-central alopecia as well as decreased pubic and axillary hair. The patient also reported dystrophic nail changes and an erythematous rash. Taken together, these changes were concurrent with drug toxicity that aligned with the patient’s over-consumption of vitamin A [21].