"Results: Poor cognitive test performance, enlargement of ventricles and low brain weight were each significantly and independently associated with higher midlife tofu consumption."
"We hypothesize that regular dietary exposure to soy isoflavones over many years during middle life may be associated with the appearance of accelerated brain aging in later life attributable to chronically sub-optimal neural plasticity."
That's key right there, and it's all you'll ever get from a longitudinal study. You want to make strong claims about the effects of soy like "soy is a terrible idea", you're going to need a controlled trial, not a cohort study.
I would like to see a study like that controlled for cardiovascular exercise.
I am also concerned about the validity of the theory because of the trend stall in figure 2 at ages 80-89 and the stark reversal in the 90-93 age group, where high-high was at almost low-low levels.
It looks to me like there is a good chance of some unaccounted confounding variable, even beyond the possible effects of exercise (which would not explain the 90-93 group).
This study seems to be counteracted by millions of Asian people that are living well into their 80s, 90s, and 100s, and eat a diet that has quite a bit of unfermented (and fermented) soy in it.
Surely that depends on how much of that population falls on the "low" or "high" side of soy intake as measured by the study. I don't know how much of the Asian population falls into the intake levels this study claims might be negatively affected - do you?
http://www.jacn.org/content/19/2/242.full
"Results: Poor cognitive test performance, enlargement of ventricles and low brain weight were each significantly and independently associated with higher midlife tofu consumption."
"We hypothesize that regular dietary exposure to soy isoflavones over many years during middle life may be associated with the appearance of accelerated brain aging in later life attributable to chronically sub-optimal neural plasticity."