all those asians are so infertile

BBC NEWS | Health | Soy foods ‘reduce sperm numbers’

The Harvard School of Public Health study looked at the diets of 99 men who had attended a fertility clinic with their partners and provided a semen sample.

The men were divided into four groups depending on how much soy they ate, and when the sperm concentration of men eating the most soy was compared with those eating the least, there was a significant difference.

The “normal” sperm concentration for a man is between 80 and 120 million per millilitre, and the average of men who ate on average a portion of soy-based food every other day was 41 million fewer.
Dr Chavarro noticed that overweight or obese men seemed even more prone to this effect, which may reflect the fact that higher levels of body fat can also lead to increased oestrogen production in men.

So what questions immediately come to mind about this study?

  • These are all people attending a high-end fertility clinic. It’s a nice convenient sample, but it’s not normal, and male factor infertility is badly studied but not unknown.
  • 99 is not a terribly large sample, especially in the US, where eating soy products isn’t particularly popular. So if it was only the highest and lowest groups that gave significantly different results (and the average consumption for the high group was one portion every two days, i.e., not a lot at all) then this does not bolster confidence in the results.
  • If the “effect” is more common in overweight and obese men, which way does any possible causation go, since these guys have a bunch of other medical problems that affect sperm count, and are also kinda likely to be trying to cut back on red meat?
  • There’s a bunch of data missing here, even though it is typically, um, collected by any fertility clinic worth its salt. Might be the fault of the reporting, might not. Fer example: sperm morphology and motility (tends to be more important than count), total volume of ejaculate (duh), fallow period before collection (duh). Especially with such a small sample of patients, a few outliers would clobber the results pretty seriously.
  • And, of course, what does this actually mean for fertility? Sure a sperm count of 100 million would be nice, but 60 million isn’t exactly peanuts. Also, Asia.

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