Katherine Mykytka Nutrition 313 03/11/09

Katherine Mykytka Nutrition 313 03/11/09

Vitamin A and Osteoporosis Introduction: Vitamin A is currently of large interest because it has been indicated that excess can stimulate bone resorption and inhibit bone formation. If these effects occur over time, there is the possibility of them leading to bone loss and fracture, which would ultimately be a risk factor for osteoporosis (1). Higher vitamin A intake may account for a component of fracture risk within the general population and supplemental vitamin A may be harmful even within recommended limits. However, there is substantial lack of consensus or evidence (2). It has been suggested that vitamin A stimulates bone resorption in vitro and in vivo. In addition, it has been reported that vitamin A toxicity decreases bone formation, in addition to increasing the already present bone resorption. This uncoupling of bone resorption and formation would be anticipated to produce bone loss. Osteoporosis, a disease of low bone mass and skeletal fragility, naturally becomes a major health problem with advancing age. A very sufficient amount of postmenopausal women and of men will sustain osteoporotic fractures, which substantially play a factor in morbidity, and mortality (3). The action of osteoporosis is controlled by more than one factor; however, nutritional practices that are considered to contribute to bone loss, particularly vitamin a intake, is what is being examined here. Objective: To prove that the association between vitamin a intake and osteoporosis is one that has little to no true correlation in several demographics of people. First, the relationship between biochemical retinol status and fracture in older women was tested. (3) Second, the relation between total vitamin A and retinol intakes and the total risk of incidents and hip fracture in postmenopausal women was examined (2). Finally, healthy men were observed by using short-term vitamin A supplementation at this dosage to show the lack of serum markers of skeletal turnover and that short-term administration of vitamin A would not contribute to the development of osteoporosis (1). Materials and Methods: The first study tested serum retinol, retinyl palmitate, and β -carotene as predictors of hip and other fractures in a large prospective study of British women over the age of 75 years. Fasting blood samples were collected, as well as univariate predictors of incident osteoporotic or hip fracture (3). The second study included women from the Women's Health Initiative Observational Study. The risk of hip and total fractures was determined using Cox proportional hazards models according to different intakes of vitamin A and retinol (2). The third study was a trial in which healthy, male volunteers were randomly assigned to receive daily a tablet containing 7576g of retinol palmitate or a nonmatching placebo consisting primarily of microcrystalline cellulose (1). Results: All three studies came to the relatively same conclusion involving vitamin a and bone loss. The first concluded that serum retinol, retinyl palmitate, and β -carotene were not significant univariate predictors of either hip fracture or any fracture (3). The second study found that the association between vitamin A intake and the risk of fracture was not statistically significant (2). Third, the trial concluded that it is unclear what intake is required to produce skeletal toxicity. The current study demonstrated that supplementation does not alter serum markers of bone turnover in healthy men and that short-term ingestion of that amount is unlikely to affect the skeleton adversely (1). Summary and Discussion: The research involved in these studies was fairly concise in concluding it is unlikely that the action of vitamin A contributes to the development of osteoporosis. Whether long-term vitamin A supplementation could have adverse skeletal effects remains to be determined, but there was little to no evidence to support any skeletal harm associated with increased vitamin A exposure.

References 1.

Kawahara, T. (2002) Short-Term Vitamin A Supplementation Does Not Affect Bone Turnover in Men: J. Nutr. 132: 1169-1172. Barker, M. (2005) Serum Retinoids and β -Carotene as Predictors of Hip and Other Fractures in Elderly Women: J. Bone and Mineral Research. 20: 913-920.

2. 3.

Caire-Juvera, G. (2008) Vitamin A and Retinol Intakes and The Risk of Fractures Among Participants of the Women's Health Initiative Observational Study: J. Clinical Nutrition. 89: 323330.