Get your hands dirty for mental health?

I was getting worried. My wife is on a gardening kick, fencing off a part of the backyard and tilling and planting and weeding — she’s been coming into the house with disgustingly filthy hands, and has been suggesting that I should get out there and dig in the muck, too. For a moment, I was afraid this article on “Healthy fat hidden in dirt may fend off anxiety disorders” might give her more ammunition in her battle to get me to help out with the weeds. Fortunately, after reading it, I think I can argue it’s irrelevant.

You’ve all heard of the hygiene hypothesis, which suggests that exposure to diverse neutral and pathogenic organisms from an early age might play a vital role in shaping our immune systems. Further, there’s the idea that we might also pick up beneficial organisms from soil that evolution has shaped us to use in regulating our immune systems, so that being away from dirt is throwing our physiology out of balance in subtle ways.

“The idea is that as humans have moved away from farms and an agricultural or hunter-gatherer existence into cities, we have lost contact with organisms that served to regulate our immune system and suppress inappropriate inflammation,” said Lowry, who prefers the phrases ‘old friends hypothesis’ or ‘farm effect.’ “That has put us at higher risk for inflammatory disease and stress-related psychiatric disorders.”

Lowry has published numerous studies demonstrating a link between exposure to healthy bacteria and mental health.

One showed that children raised in a rural environment, surrounded by animals and bacteria-laden dust, grow up to have more stress-resilient immune systems and may be at lower risk of mental illness than pet-free city dwellers.

OK, that sounds plausible, although I’d say that there are so many differences between growing up on a farm vs. in a city that it’s going to be hard to persuade me that exposure to Bacterium X is the crucial variable. The only way to find out is to read the original paper. So I did.

This particular paper does no evolutionary testing. It doesn’t compare farm kids to city kids. It doesn’t look at human stress disorders at all. It tests the effects of a molecule found in cell bacteria on cells from mice isolated in culture. Basically, they synthesized and purified 1,2,3-tri [Z-10-hexadecenoyl] glycerol, 10(Z)-hexadecenoic acid and tested it on cells loaded with receptor and recorder constructs so they could determine its mechanism of action — the bottom line is that this molecule under these conditions seems to have a potent effect in reducing activation of an inflammatory pathway. Here’s their summary of the results:

The free fatty acid form of 1,2,3-tri [Z-10-hexadecenoyl] glycerol, 10(Z)-hexadecenoic acid, decreased lipopolysaccharide-stimulated secretion of the proinflammatory cytokine IL-6 ex vivo. Meanwhile, next generation RNA sequencing revealed that pretreatment with 10(Z)-hexadecenoic acid upregulated genes associated with peroxisome proliferatoractivated receptor alpha (PPARα) signaling in lipopolysaccharide-stimulated macrophages, in association with a broad transcriptional repression of inflammatory markers. We confirmed using luciferase-based transfection assays that 10(Z)-hexadecenoic acid activated PPARα signaling, but not PPARγ, PPARδ, or retinoic acid receptor (RAR) α signaling. The effects of 10(Z)-hexadecenoic acid on lipopolysaccharide-stimulated secretion of IL-6 were prevented by PPARα antagonists and absent in PPARα-deficient mice.

That represents a lot of work, and I think that result sounds reasonable and potentially useful — who wouldn’t want another anti-inflammatory compound? But all that stuff about evolution and mental health and the hygiene effect were extraordinarily hand-wavey, and none of that was tested here at all. Which is a relief if my wife comes to me to say I should do some gardening so I could stock up on 1,2,3-tri [Z-10-hexadecenoyl] glycerol, 10(Z)-hexadecenoic acid and be less stressed and grumpy, because I’ll just tell here that the effective dose in a handful of dirt hasn’t been found, the connection to mental health is speculative, and I am not a mouse.

Smith, D.G., Martinelli, R., Besra, G.S. et al. Psychopharmacology (2019).


  1. Dunc says

    Sure, but gardening is nonetheless very satisfying. Or at least, I find it so… And eating the results is even better.

  2. says

    My family were dirt farmers, although successful ones, my father got away from it as soon as he could and saw no reason to look back, nor do I. No dirt farming for me even for fun.

  3. imback says

    You might think differently if the free fatty acid form of 1,2,3-tri [Z-10-hexadecenoyl] glycerol, 10(Z)-hexadecenoic acid tasted just like steak.

  4. jrkrideau says

    A few years ago I read a paper that pointed out that Mennonite children had fewer allergies than Hutterites. The paper had a lamentably small n size but made the interesting point that Mennonite farms layout encouraged children to play around the barn and animals while the Huttite agricultural model (large dairy stables) did not.

    Mennonites had fewer aleriges than Hutterites.

    Could easily a false positive but interesting.

  5. mountainbob says

    Also speculative, but equally interesting is the notion that direct exposure to nature via hikes, explorations in the woods and streams and mountains is health producing. “It restoreth my soul” comes to mind. Can’t hurt (unless you act with complete disregard for your own ability and preparation).

  6. says

    As a farm kid who grew up on a dairy, with cows, cats, dogs, and pigeons all over the place (and chickens, too, in my earliest days when my grandmother still had a coop behind her house), I’ve found the hygiene hypothesis attractive. Yeah, I was dirty when I was young, and now I’m healthy! But it’s more likely I just won the genetic lottery, with long-lived parents and grandparents who came from robust stock. Besides, I can think of a couple of tweaks that would be nice improvements in my physical condition. I guess being a farm kid doesn’t actually give you a “Get out of ail free” card.

  7. says

    Growing up, I could have been Pigpen’s twin. Still had (and have) massive anxiety issues.


    I got a chuckle out of “get out of ail free” card. Well played.

  8. says

    Speaking of things in the soil, thinking about gardening in this area always reminds me of the public health campaign “be alert in the dirt” about how the Asarco smelter left arsenic and lead in the soil. I think, what’s the point of the ad? Teach your kids to recognize arsenic mud? Maybe it should be don’t trust the fruits of your gardening, PNW earthbabies. I dunno.

  9. Akira MacKenzie says

    Oh please! I grew up in rural America. I played in dirt. We even used to have dirt clod fights. Yet I ended up with depression, anxiety, and ADD. I know I’d be just a ”sample of one” when analyzing this nonsense, but even my broken brain is still calling bullshit!

  10. says

    I find gardening relaxing. I grew up in a household where my grandparents still practised the complementary farming that was typical for this region where the men would go to work in the mines and steel works to bring home money while the women would grow as much food as possible to minimise spending.
    When we lived in a flat I did “balconing”. I don’t need any evolutionary reasons or beneficial bacteria for it to do me good, just like my daily walks through the forests don’t need to resemble “stone age hunting” in order to do me good.

  11. blf says

    I’m one of those people who has a “black thumb”. No matter what I do, or don’t do. plants die. For no apparent reason. I currently have a fine garden of dead houseplants, including one previously-robust planet who somehow managed to ignore me and live for around five years, including the year it got (mostly) ate by the damn flying rats. This year? Deader than a dead dodo. Why? Feck if I know.

    So I wonder if there is a “human hypothesis”, where being around certain people causes plants to croak?