Homocysteine and Fracture Prevention

Osteoporotic fractures are a major health problem in Western society and are associated with increased morbidity and mortality and substantial economic costs.¹ Because the number of fractures will increase throughout the world as the population ages, prevention of fractures is becoming increasingly important. Recently, studies have identified a new and potentially modifiable risk factor for osteoporotic fracture—a mildly elevated circulating homocysteine level.^{2 - 3} These epidemiological studies showed that a relatively high homocysteine level predicts a higher fracture risk but they did not establish a causal relationship. The question remained whether the increase in fracture risk was due to homocysteine itself, or to other covarying factors.

To establish a causal relationship between elevated homocysteine concentrations and osteoporosis, data from 2 types of studies are needed. One consists of randomized placebo-controlled trials studying the effect of lowering homocysteine levels on the incidence of fractures, the most important clinical end point of osteoporosis. In addition, cellular and molecular studies are required to elucidate the biological mechanism linking high homocysteine concentrations with factors that predispose to or cause fracture.

In this issue of JAMA, Sato and colleagues⁴ present the first evidence that an elevated homocysteine level might indeed cause more brittle bones. In this randomized double-blind study, Japanese patients following stroke who were treated with folate and vitamin B₁₂ had a 5 times lower risk for hip fracture over a follow-up period of 2 years compared with the placebo group. This is a very large risk reduction, but care should be taken in interpreting these results. As the authors note, the generalizability of the results is limited. The control patients had an unusually high incidence of hip fracture (4.3% per year) compared with the incidence in the average Japanese population of the same age (0.3% for women and 0.6% for men).⁵ In addition, the high mean levels of circulating homocysteine in the studied population (19.9 μmol/L) could explain the effectiveness of the therapy. Earlier studies suggested that there is a threshold above which homocysteine predicts increased fracture risk^{2 - 3} ; the greater the percentage of the population above the threshold, the greater would be the expected effectiveness of treatment with a homocysteine-lowering therapy. The results of the study by Sato et al are also limited due to a relatively low power, with only 33 hip fractures during the study period of 2 years. This suggests that the 80% risk reduction reported in this study might not be detected in a larger study or with other (high-risk) populations, and the true relative risk might be as high as 0.5, which would translate to a 2-fold risk reduction.

A wide spectrum of diseases is already associated with elevated circulating homocysteine concentrations, such as cardiovascular disease (including stroke) and cognitive impairment.^{6 - 7} Because these diseases are also associated with a high fracture risk, they could be confounding factors. However, Sato et al adjusted their risk estimates for cardiovascular events and presence of dementia without an effect on the study outcome. More important, the fall frequency was similar in both groups, which means that with the same number of falls, the placebo group fractured more easily. Because the recording of the falls was well-validated with a “fall calendar” in combination with a monthly visit to the clinic, cognitive impairment without dementia, which could affect recall of the falls, is an unlikely confounder. Nevertheless, similar intervention studies in patients other than those who have had a stroke will be necessary to show generalizability of the conclusions.

Apart from showing that homocysteine levels were effectively reduced, Sato et al showed that vitamin B₁₂ and folate levels were increased at the end of the study. It is possible that the observed effects on fracture are caused by increasing the levels of vitamin B₁₂ rather than by lowering homocysteine. Vitamin B₁₂ deficiency is common in the elderly population, ranging from 10% to 40%, depending on the diagnostic criteria.⁸ Vitamin B₁₂ has been linked to bone health by a limited but growing number of studies. Patients with vitamin B₁₂ deficiency (pernicious anemia) have a higher risk for fracture,⁹ and recent population-based studies suggest that vitamin B₁₂ status is important for maintenance of bone mineral density (BMD).^{10 - 11} In addition, vitamin B₁₂ has been found to affect osteoblast activity and bone formation.^{12 - 13} Because vitamin B₁₂ plays an important role as a cofactor in metabolizing homocysteine, the levels of each are highly associated, making it difficult to separate the effects of vitamin B₁₂ and homocysteine. This question can only be addressed by clarifying the biological mechanism linking homocysteine, vitamin B₁₂, or both to fractures.

The reduced fracture risk observed by Sato et al could not be explained by BMD, which suggests that bone quality rather than bone quantity explains the difference in fracture risk. Previous studies did not find a relationship between homocysteine levels and femoral neck BMD,² supporting the present findings. A mechanism underlying the deleterious effect of homocysteine on bone quality might involve inhibition of collagen cross-linking by high homocysteine concentrations.¹⁴ This hypothesis is based on observations in patients with homocystinuria, a rare autosomal recessive disease characterized by very high homocysteine levels and early generalized osteoporosis. In vivo evidence for this hypothesis is limited,¹⁵ and it remains to be determined whether disturbed collagen cross-linking is also involved when homocysteine levels are only mildly elevated.

Another way to prove a causal relationship between increased homocysteine and fracture risk is by studying mendelian randomization¹⁶ (ie, performing genetic association studies with polymorphisms known to increase homocysteine levels). This approach was recently successfully applied to examine the relationship between homocysteine and stroke.¹⁷ The most frequently studied related polymorphism is Ala222Val, a common functional polymorphism in the gene encoding for methylenetetrahydrofolate reductase, which has been found to cause higher homocysteine levels.¹⁸ A number of studies have found a relationship between this polymorphism and BMD^{19 - 20} and fracture.²¹ However, definitive answers about the relationship with fracture will require larger studies and possibly meta-analysis.

Despite the potential limitations of the study by Sato et al, the results show that—at least in patients following stroke—folate and vitamin B₁₂ supplementation is effective in preventing hip fracture. Whether a similar effect can also be obtained in other (high fracture risk) patients can only be answered by other intervention studies. After the initial observation of association between circulating homocysteine levels and fracture risk less than 1 year ago, these results now support a causal link. However, final proof of causality will have to come from elucidation of the biological mechanism underlying this relationship.