Fractures, especially those resulting from osteoporosis are one of the leading causes of morbidity and even death. It is still a global health issue despite the strides made in recent years concerning calcium and vitamin D supplementation, and overall improvements in nutrition levels. Vitamin A though presents a conundrum because of the mixed results seen in many papers linking its intake to fracture risks.
It’s worth just mentioning osteoporosis before we start because it underlies the findings relating to fracture in the recent analyses of papers dealing with vitamin A intake. Osteoporosis is a common condition especially in post-menopausal women. The bones gradually become weaker and less dense. They are more likely to break and fracture. Women are more vulnerable than men but it is still known to occur in both sexes because bone density reduces with age for all.
Back To Vitamin A
A great deal of interest focusses on vitamin A intake. This is characterised by blood retinol concentrations. For some researchers, vitamin A intake is a ‘double-edged sword’ because it appears to have different impacts on the risk of fracture. In any analysis, we have to consider the provitamin for A, beta-carotene, because this is converted to vitamin A in the body.
The level of uncertainty over what is right for vitamin A intakes in relation to fracture (total or otherwise) is still ongoing. Some research suggests that high intake of vitaminA or high serum blood levels of retinol have absolutely no impact on fracture risk whereas some evidence exists to suggest it raises the prospect of fracturing. There is even some research hinting that a higher intake of vitamin A actually lowers total fracture risk. Can we unravel the issue ?
The uncertainty about vitamin A and the association with fracture risk has been around for many years. One meta-analysis looked at eight vitamin A intake studies which included retinol and the provitamin, beta-carotene (Wu et al., 2014). The analysis covered nearly 284,000 people in that group of eight studies. The study also considered four prospective studies looking at blood retinol levels, which covered 8725 participants.
Following statistical analysis, a high intake of vitamin A and retinol increased the risk of hip fracture which is not good news for those supplementing with vitamin A in older age. However, there was no association with beta-carotene intake and an increased risk of fracture. The overall conclusion was that too low or high a level of blood retinol was associated with increased risk of hip fracture. The other suggestion made was that consumption of beta-carotene from vegetable sources was better than obtaining retinol from meat which is directly absorbed into the blood following ingestion. The mechanism for this was not clear at all.
The latest research from another Chinese meta-analysis still finds associations debateable. Researchers from the University of Wuhan conducted a further meta-analysis to determine what effects vitamin A intake and blood retinol levels would have on fracture risk (Zhang et al., 2017). The study looked at 13 research articles covering 11 cohort studies and two case-controlled studies. This involved a total of just over 319,000 participants over the age of 20.
The overall conclusions were that a higher total vitamin A intake might slightly decrease the overall or total risk of fracture but increase the risk of hip fracture. On top of this, a lower blood level of retinol might slightly increase both total and hip fracture risks. One finding suggested that a higher retinol intake could increase bone mineral density.
Discrepancies and Inconsistencies in Research Findings
The Zhang paper (2017) thinks it may have answers to the riddle behind the discrepancies in the research. Part of the explanation is due to retinol intake and how bone mineral density varies in different parts of the bone.
There was one study which showed there was a positive correlation between retinol intake and bone mineral density in all bones except at the femoral neck which is part of the hip. The femoral neck (neck of the femur) is a flattened pyramidal process of bone, connecting the femoral head with the femoral shaft, and forming with the latter a wide angle opening medial wards. If the hip and femoral neck are both sensitive to retinol intake then they assume that either vitamin A intake or blood levels of retinol can result in different degrees of fracture risk. This depends on the bone sites being analysed.
Other factors from the study to be considered are sex, menopausal age, location and extent of nutrition. One study confirmed that women generally have a lower bone strength, mineral content and density than men. This makes them more susceptible to fracture.
Women who were aged between 40 and 45 had a lower bone mineral density than post-menopausal women.
In general terms the best advice from nutritionists is to try and minimise the impacts of osteoporosis. This implies keeping to a healthy, well-balanced diet which is rich in protein, vegetables and calcium. If the calcium intake during childhood and adolescence is kept up, then bone development is enhanced and the risk of fracture is reduced. We all know that vitamin D intake should be maintained and it is also suggested that caffeine content be reduced because it helps calcium to leach out of bone.
Wu, A.M., Huang, C.Q., Lin, Z.K., Tian, N.F., Ni, W.F., Wang, X.Y., Xu, H.Z., Chi, Y.L. (2014) The relationship between vitamin A and risk of fracture: meta-analysis of prospective studies. J. Bone Miner. Res. 29(9) pp. 2032-2039. PMID: 24700407 doi: 10.1002/jbmr.2237
Zhang, X., Zhang, R., Moore, J.B., Wang, Y., Yan,H., Wu, Y., Tan, A., Fu, J., Shen, Z., Qin, G., et al. (2017) The Effect of Vitamin A on Fracture Risk: A Meta-Analysis of Cohort Studies. Int. J. Environ. Res. Public Health. Sep 10; 14(9). Epub 2017 Sep 10.