B vitamin treatment led to a difference in final tHcy concentration of 31.7% compared with the placebo, and was accompanied by a reduction in the rate of brain atrophy of almost 30%. No safety issues were found, so it can be concluded that high doses of B vitamins can be used to reduce the rate of atrophy of the brain in elderly people with MCI.
In agreement with the prior hypothesis, the treatment effect was greatest in those with the highest baseline level of tHcy, with a reduction in atrophy rate of 53% in those in the top quartile of tHcy (>13.0 µmol/L). Notably, there was no effect of treatment on atrophy in those in the bottom quartile (≤9.5 µmol/L). The observation in the placebo group that the rate of atrophy was related to the baseline concentration of tHcy is consistent with, and may explain, a number of cross-sectional studies reporting that regional brain atrophy is related to tHcy 
. The results also fit with our earlier finding that raised tHcy predicts the rate of shrinkage of the medial temporal lobe in patients with Alzheimer's disease 
. In contrast, in the group on active treatment there was no relationship between baseline tHcy and the rate of atrophy. It is tempting to suggest that this finding is consistent with the view that raised homocysteine is a direct cause of the atrophy. However, it does not exclude that tHcy is only a marker for low-normal levels of the vitamins, which are themselves the causal factors.
Our previous observational study on another elderly cohort in OPTIMA showed that low-normal baseline levels of vitamins B12
were associated with a more rapid rate of atrophy, whereas folate levels were unrelated 
. In the present study, we found that an increase in either vitamin B12
status or in folate status was associated with a reduced rate of atrophy. Thus, from the present data we cannot decide which of these two vitamins is the most important. The lack of association of atrophy with the change in cystathionine levels, a marker of vitamin B6
, may indicate that vitamin B6
is less important as a determinant. It is noteworthy that a cross-sectional study found that supplementary intake of vitamin B6
or of vitamin B12
, but not of folate, was associated with greater regional gray matter volumes in an elderly cohort 
Possible therapeutic implications
We have shown that treatment for two years with B vitamins markedly slows the accelerated rate of atrophy in people with MCI. This study was carried out in the UK, where voluntary fortification of foods with folic acid is permitted but where there is no mandatory fortification. The effect of treatment was dependent on baseline tHcy, with those in the upper three quartiles, i.e. >9.5 µmol/L, showing a significant slowing of atrophy upon treatment compared with those in the lowest quartile. In the USA, which has mandatory fortification, 13.6% of those ≥60 years-old had tHcy concentrations >13 µmol/L in 2003-4 
, a level at which we found a >50% reduction in the rate of atrophy upon treatment with high doses of B vitamins. The median tHcy concentration in those ≥60 years-old in the USA is 10.1 µmol/L, suggesting that a substantial proportion of those with MCI could benefit from the intervention. The prevalence of MCI is between 14% and 18% in those over 70 years-old 
, which means that about 5 million people in the USA and 14 million in greater Europe suffer from this condition. Since approximately half of those with MCI convert to Alzheimer's disease or to another form of dementia within 5 years 
, there is an urgent need to identify treatments that will slow down or prevent the conversion 
. The outcome of the VITACOG trial indicates that treatment with homocysteine-lowering B vitamins might be one approach to meeting this challenge,
This study was designed to detect an effect of treatment on the rate of atrophy and was not powered to detect effects of treatment on cognitive test scores. Nonetheless, we consider that the findings are relevant to cognitive decline in people with MCI. First, in studies over longer periods (up to 5 years) it has been found that the rate of whole brain atrophy in MCI is correlated with cognitive decline in several tests, including the MMSE 
. Second, when we looked for significant predictors of the final cognitive test score, the rate of atrophy was one of the three main factors determining the final MMSE and TICS-M scores. Third, two other randomised controlled trials of homocysteine-lowering treatments have shown effects on cognition: a trial in which normal participants with baseline tHcy levels >13 µmol/L were treated with folic acid (0.8 mg/d) for three years showed a beneficial effect on several cognitive tests 
. An 18-month trial of high doses of the same three vitamins used here showed a slowing of cognitive decline in patients with mild Alzheimer's disease, although not in patients with moderate Alzheimer's disease 
. The latter result indicates that homocysteine-lowering may have to be targeted at those with early stages of Alzheimer's disease. Since the rate of brain atrophy is more rapid in subjects with MCI who convert to Alzheimer's disease 
, it is possible that high doses of folic acid, vitamins B6
might slow the conversion from MCI to Alzheimer's disease. Clinical trials to test this hypothesis are warranted.
Strengths and limitations
A particular strength of the study is that we used a highly sensitive and accurate tool for assessment of brain atrophy, i.e., MRI and the SIENA protocol. Notably, the SIENA analysis was carried out on MRI data derived from the average of three T1 volumetric scans at each time point, so providing a high degree of accuracy. Another strength was the measurement of vitamins and their markers, which made it possible to show that the baseline tHcy concentration is a key determinant of the rate of atrophy and of the response to treatment. In our study, brain atrophy was among the strongest determinants of MMSE and TICS-M at the end of the study, supporting the view that assessment of atrophy rate may be a useful tool in cognitive studies. The study has, however, some limitations. First, we used combination of the three B vitamins, so we cannot identify whether they are all required or if one is more important. Second, this trial was powered to detect change in rate of atrophy, not cognition; even so we observed a strong association between atrophy rate and cognition. A subsequent report will describe the cognitive results in more detail. Finally, although some of the findings in subgroup analyses appear quite striking, for example in the stroke or diabetes groups, or those taking aspirin, these results should be interpreted with caution because of the small sample size and lack of biological explanation for the interaction. The importance of these findings will be in relation to future trials. There is every reason to include those who have had a previous stroke or subjects with high creatinine. We obviously need more data on those using aspirin. In subjects already taking oral supplements of B vitamins, or with good plasma tHcy and B vitamin status, the effect may be limited, but it should be investigated whether subgroups could benefit from even higher intakes, as appeared to be the case for those with mild AD and with lower levels of tHcy than in our cohort 
We show that a simple and safe treatment that targets homocysteine can slow down the accelerated rate of brain atrophy found in mild cognitive impairment. Although this is relatively small trial, it has provided useful data for planning future studies, whether those will focus on brain atrophy, cognitive decline, or age-related conditions where brain atrophy will be included as part of the assessment.