For such a bland and versatile plant protein, soy has endured its fair share of controversy. Headlines have trumpeted soy as the secret to Japanese longevity, the ultimate cancer-fighting food, the unexpected carcinogen lurking in energy bars and the reason everyone’s growing man boobs.
Now, a new study from Japan, published in Nutrition Journal, says soy-eaters are sound sleepers. Though previous research connected sleep and soy intake in menopausal women, this study is the first to assess the link in the general adult population.
Soybeans are rich in isoflavones, a type of phytoestrogen — a plant-based compound that mimics the effects of estrogen when ingested by humans. During menopause, ovaries ramp down production of hormones including estrogen. To combat insomnia, among other unpleasant side effects of “the change,” women can take hormone replacement therapy (HRT).
But, due to health concerns, some women take soy instead. While some experts feel that soy is more bunk than beneficial, research does support a connection between soy-ing and snoozing. In one 2011 study, for example, postmenopausal women took 80 mg of soy daily for four months and exhibited considerable improvements in sleep efficiency, as measured by polysomnography.
Japanese research teams sought to see if the soy-sleep connection persisted outside postmenopausal women. So, they looked at quality and duration of sleep with respect to isoflavone intake in 1076 adults (827 men and 249 women), aged 20 and 78 years old.
Participants rated how often they ate three popular, isoflavone-heavy foods (natto, tofu and fried tofu) on a scale of “almost never” to “two or more times daily.” Participants also estimated how much sleep they averaged each night and said whether they woke up feeling refreshed. To minimize the influence of other health issues that might affect sleep, researchers also took participants’ blood samples, measured their BMIs and asked them to fill out health questionnaires.
About 13 percent of participants, the study said, reported “normal” sleep duration, defined as seven to eight hours of sleep. Fifty-six percent reported getting sufficient sleep. These healthy sleepers, the study revealed, tended to hit soy hard on a regular basis. And the soy-sleep link persisted across the population, without respect to factors including age, sex, BMI, coffee or vitamin consumption, drinking, smoking, hooking, education level, job, incidence of depression or hypnotic drug use.
The findings, however, only point to a link — it’s not apparent, study authors wrote, whether people who love the soy sleep better or super-sleepers just can’t say no to the flavorless vegan staple. (Although, it’s hard to imagine why a good night’s rest would push someone towards soy.) Also, study authors said they couldn’t “exclude the possibility that sleep duration and sleep quality are affected by other dietary habits that correlate with the habitual intake of isoflavone.”
The biological basis of the sleep-soy link also remains unclear. But, study authors floated some possible explanations. Here’s one: Estrogen may affect levels of the neurotransmitter serotonin, which helps regulate the sleep-wake cycle. Given the structural similarity between isoflavones and estrogen, soy may help serotonin flood brain synapses according to schedule. At this point, however, the mechanism underlying tofu-to-rest remains at large.