Vitamin A is a fat-soluble micronutrient that serves several important functions in your body. In addition to supporting your vision, skin and reproductive health, immune system and cell specialization, vitamin A is important to bone formation and maintenance. Too much vitamin A, however, may weaken your bones and increase your risk of fractures. The medical condition in which there is too much vitamin A in your body is called hypervitaminosis A.
How Hypervitaminosis A Occurs
Hypervitaminosis A almost always occurs because you've taken too much supplemental vitamin A. Acute hypervitaminosis A describes an accidental or intentional vitamin A overdose that occurs over a short period. Chronic hypervitaminosis A typically occurs when excess supplemental vitamin A accumulates in your body over months to years. Chronic hypervitaminosis A is the form of this condition that may have adverse effects on your bones.
The Evidence
At least two large studies have found a link between excess vitamin A intake and an increased risk of bone fractures. In a January 2002 article published in the "Journal of the American Medical Association," Diane Feskanich, Sc.D., and colleagues reported that among more than 72,300 nurses followed for 18 years, those who consumed the most vitamin A intake had the highest risk of hip fracture. In January 2003, Dr. Karl Michaëlsson and colleagues reported in an article published in the "New England Journal of Medicine" that among more 2,300 men living in Sweden, those with the highest blood vitamin A levels were most likely to suffer a bone fracture.
Bone Effects
At normal levels, vitamin A supports bone formation. If you consume too much vitamin A, however, you may lose bone mass. Based on animal models, biomedical researchers speculate that too much vitamin A overstimulates cells called osteoclasts, which break down bone. Additionally, excess vitamin A may interfere with the bone-supporting activities of vitamin D. The net effect is thin, weak bones that are more susceptible to breakage than normal, healthy bones. Mild to moderate bone loss is termed osteopenia; severe loss of bone mass is called osteoporosis. Several factors may contribute to the development of osteopenia or osteoporosis other than excess vitamin A consumption, including vitamin D and calcium deficiencies, menopause and lack of physical activity.
How Much Is Too Much?
The tolerable upper intake level for vitamin A for adults is 3,000 micrograms, or 10,000 IU, daily. This includes the amount you consume from your diet and any supplements you take. Importantly, if you eat a well-balanced diet with the recommended servings of fruits and vegetables, you are unlikely to need a vitamin A supplement. Symptoms of chronic hypervitaminosis A to be aware of include joint and bone pain, poor appetite, headaches, loss of eyebrow hair, dry eyes, itchiness and dry, rough skin.
References
- National Institutes of Health, Office of Dietary Supplements: Vitamin A and Carotenoids
- National Institute of Arthritis and Musculoskeletal and Skin Diseases: Vitamin A and Bone Health
- "American Journal of Clinical Nutrition"; The Acute and Chronic Toxic Effects of Vitamin A; Kristina L. Penniston, Sherry A. Tanumihardjo; February 2006
- "Bone Health and Osteoporosis: A Report of the Surgeon General"; Lifestyle Approaches to Promote Bone Health; U.S. Department of Health and Human Services, Office of the Surgeon General; 2004
- "The Journal of the American Medical Association"; Vitamin A Intake and Hip Fractures Among Postmenopausal Women; Diane Feskanich, Sc.D., et al.; January 2002
- "The New England Journal of Medicine"; Serum Retinol Levels and the Risk of Fracture; Karl Michaëlsson, M.D., et al.; January 2003
