Long-Term Soy Consumption Makes Monkeys Aggressive Loners, Study Finds

Findings could have serious implications for humans

Image Credits: Himanshu Sharma/NurPhoto via Getty Images.

If long-term consumption of soy isoflavones can make monkeys into aggressive loners, as a 2004 scientific study suggests, what about humans?

Soybean oil is the most widely consumed oil in the United States and is a ubiquitous part of processed foods. 

A few days ago we reported on a shocking recent study which showed that soybean oil caused serious genetic dysfunction in mice, leading to weight gain and serious neurological problems. These results should be cause for alarm for one simple reason: soybean oil is the most widely consumed oil in the United States. Indeed, there has been a 100-fold increase in soybean oil consumption during the 20th century. Soybean oil is everywhere, especially in the processed foods which make up a significant portion, even a majority, of the diets of most people in America and much of the rest of the developed world (see for instance this study from Brazil).

It is becoming increasingly clear that vegetable oils in general, including soybean oil, are seriously bad not just for mice but people too. Recently we dubbed vegetable oil ‘one of the worst things you can eat’, and also included vegetable-oil-laden processed food as one of the main foods that make you ugly. Processed food has been under intense scrutiny, as a result of a new documentary that aired on the BBC last week. For a period of a month, a British doctor conducted a self-experiment by eating a diet composed of 80% processed food, a diet consumed by as much as two-thirds of the adult population in the UK. 

Some of the results from a 2019 study on consumption of ultra-processed food. The study showed a clear association between eating processed foods and weight gain. Participants who consumed processed foods reliably consumed more calories to satisfy their hunger.

The fruits of the doctor’s televised experiment included serious weight gain, piles, anxiety, sleeplessness, loss of libido and, most shockingly of all, changes to the structure of his brain considered to be typical of drug addicts. Weeks after the experiment ended, scans revealed that the neurological changes had not been reversed. The doctor is now quite literally hard-wired to want to eat processed food.

In light of this focus on not just the physical but the mental and emotional effects of consuming vegetable-oil laden processed food, one aspect of the soybean oil study that has generated discussion in recent weeks is the finding that soybean oil consumption caused dysregulation of the mice’s oxytocin system. Only some of the implications of this were discussed in the study, leaving readers to enlarge upon them. As well as being involved in the regulation of weight gain, oxytocin also has an important role to play in the expression of empathy and social bonding.

Discussing the soybean study on Twitter, some speculated that this dysregulation of oxytocin could be responsible for the decline of trust and various other social problems. Carnivore Aurelius (@ketoaurelius) wrote: 

Proving a direct connection between soybean oil consumption and social strife would be a tall order; after all, human societies are complicated things, with a tremendous number of variables involved. Even so, speculating about the relationship between changing dietary patterns and changing patterns of behaviour is not otiose. Dietary patterns really have changed in recent decades – in fact, far more than we might possibly think – and many of the problems we are witnessing do appear to be novel ones. 

Increasing exposure to xenoestrogens, for instance, industrial chemicals that mimic the effects of estrogen in the body, has had widespread effects on expression of sexual characteristics and behaviour over the last half-century, as well as fertility. 

And, interestingly enough, there is some evidence that regular soy consumption can have drastic effects on the social behaviour of some of our nearest evolutionary cousins. A study in the journal Hormones and Behaviour from 2004 claims that long-term soy consumption can make monkeys more aggressive and isolated from their fellow primates.

The authors note the important role of aromatization of male hormones on aggressive behaviour and the mediating role of estrogen in this process.

Estrogen produced by aromatization of gonadal androgen has an important facilitative role in male-typical aggressive behavior that is mediated through its interaction with estrogen receptors (ER) in the brain. Isoflavones found in soybeans and soy-based dietary supplements bind ER and have dose- and tissue-dependent effects on estrogen-mediated responses.’

They note that, although this relationship is well known, studies had yet to be carried out on the effects of soy-rich diets on aggressive behaviour.

Their study took place over a period of 15 months, and involved feeding different diets to groups of adult male macaques living in nine stable social groups. The diets differed only in terms of the protein source the monkeys received: casein and lactalbumin (no isoflavones), soy protein isolate containing 0.94 mg isoflavones/g protein, and soy protein isolate containing 1.88 mg isoflavones/g protein.

The results of the experiment were striking.

‘In the monkeys fed the higher amount of isoflavones, frequencies of intense aggressive (67% higher) and submissive (203% higher) behavior were elevated relative to monkeys fed the control diet (P‘s < 0.05). In addition, the proportion of time spent by these monkeys in physical contact with other monkeys was reduced by 68%, time spent in proximity to other monkeys was reduced 50%, and time spent alone was increased 30% (P‘s < 0.02).’

This led the authors to conclude that ‘long-term consumption of a diet rich in soy isoflavones can have marked influences on patterns of aggressive and social behavior.’

It’s worth noting that the mechanism of action for the soy isoflavones appears to be different than for the soybean oil in the more recent study. The former works through aromatisation of androgens, while the latter appears to work by causing up- and down-regulation of particular genes, including those relating to the production of oxytocin.

Either way, though, it hardly amounts to a ringing endorsement of soy-based foods. Whichever part of them you choose to consume – the protein or the oil – you could looking at serious negative physical and mental changes. 

In an earlier article, we cautioned you to avoid soy products – not just because they’re terrible for you, but also because they’re terrible for the environment – and we’ll happily re-issue that caution in light of this new discussion. As well as choosing not to eat foods that obviously contain soy, if you want to be sure you are not consuming any soy, you should also cut out all processed food from your diet altogether, to avoid hidden soy.

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