Why is MS more common in women?

Last month, I attended ‘MS At the Limits’, and the final talk by Dr Riley Bove (University of California, San Francisco) was truly fascinating. I know I am a bit delayed in posting this but I thought I would share some of the key points…

As most of us know, MS affects more women than men, but you might not know that women tend to have more relapses and that the ratio of men:women affected by MS is on the rise. The most obvious culprit here is hormones, but it is also possible that the higher prevalence/relapse rate in women is related to nutrition or behaviour. For example, women tend to attend the doctors far more frequently than men, so are they more likely to report relapses or symptoms?

When we look at the lifetime of humans, we are very unique in that we are the only species to have a period of puberty and a period of post-fertility (primates are the only other species to have a period of adolescence, and only two species of whale live post-menopause). These life phases are very much dictated by hormones, could it be coincidence that we are the only species to go through all these hormonal changes and the only species to develop MS?

Early puberty is a risk factor for women developing MS, and the earlier puberty occurs, the earlier they tend to develop MS. So surely, this is direct evidence of a hormonal cause? However, it could be that puberty and MS have common risk factors, such as genetic mutations or BMI (increased weight is associated with both increased risk of MS and earlier onset of puberty). Also, if hormones are the main culprit here, you would expect a correlation between disease onset/progression and oral contraceptives wouldn’t you? But studies so far suggest only a slight effect.

However, the more we look, the more links we see to hormonal influence, although most findings show weak correlations or slight effects.

Later age at birth of the first child may be linked with MS onset. So, having children earlier in life appears to be protective. Pregnancy also has a short-term protective effect, reducing relapses but increasing risk of relapse in the short-term, after birth. Breast-feeding appears to be protective, so may help reduce the short-term increased risk of relapse post-birth.

As for the menopause and MS, EDSS scores appear to progress faster post-menopause. Also, early menopause through surgery is linked with increased cognitive decline and dementia (not yet studies in pwMS specifically). Could this be linked to hormone replacement therapy (HRT), which women take after surgical menopause? Further studies are required to find out.

Let’s not forget about the men…  lower testosterone in men has been found to correlate with higher EDSS scores in early MS, suggesting a faster rate of disease progression in early disease. So, could testosterone be protective in MS? Several studies have shown that it might be, but little attention has been paid to this topic in recent years.

So, can we blame hormones for the higher incidence of MS in women? Possibly, but proceed with caution!