A collaborative study between the University of Helsinki, Natural Resources Institute Finland and Tampere University, has demonstrated that just one month of indoor gardening enhances immune regulation, can increase the bacterial diversity of the skin and is associated with higher levels of anti-inflammatory molecules in the blood.
It has previously been shown that contact with natural microbially-rich soil media alters the human microbiota, that is the 2 kg or so of microorganisms within each of us. Beneficial microbial exposure has decreased among people living in urban areas and they have distinct microbiota compositions compared to their rural counterparts.
Evidence suggests that home surroundings play a key role in determining microbiota-related health outcomes:
- Surroundings of forest and agricultural land during the first year of life are inversely associated with the risk of atopic sensitization and type 1 diabetes.
- Biodiverse and green living areas correlate with better respiratory health.
- Diverse garden vegetation and outdoor nature-related activities have been linked to health-related changes in the gut microbiota composition.
- Living in rural or farm-like conditions is directly associated with a diverse microbiota and a well-functioning immune system.
Participants in this research were recruited from the cities of Lahti (population > 120 000) and Hyvinkää (population > 45 000) in Finland. They had to be broadly healthy, with for example certain immune-mediated diseases excluded. They were randomised to either an intervention or placebo group and were asked to fill out questionnaires assessing their living conditions and lifestyle, including medication, food supplements, diet, outdoor activities and hobbies, and any contact with soil. Hand washing habits were also recorded. Samples from thirteen placebo and fifteen intervention group participants were used in the analyses. The participants’ ages ranged from 29 to 72.
Participants were provided with a planter, seeds, plants, and growing medium. The placebo group used horticultural peat, chosen because of its low bacterial diversity. The intervention group used the same horticultural peat mixed with a microbiologically rich compost-based mixture, containing composted materials such as tree bark, mulch, dung, deciduous leaf litter, peat, agricultural sludge, and Sphagnum moss.
Both groups were provided with the same seven crop species: lettuce, white mustard, radish, garlic, ginger, pea and fava bean. They were instructed to sow their seeds and daily tasks included both soil and dietary exposure: monitoring moisture levels by inserting a bare finger deep into the growing medium, watering, harvesting, and resowing. Blood samples, and skin swab samples for bacterial analyses, were collected at the beginning of the trial and at one month.
After one month, plasma anti-inflammatory cytokine IL-10 and skin bacterial diversity both increased in the intervention group. Since the diversity of bacteria increased in the intervention but not in the placebo group, the results suggest that handling a microbially-rich growing medium enriches the skin microbiota. Immune modulation was also enhanced in the intervention but not in the placebo group.
“The findings are significant, as urbanisation has led to a considerable increase in immune-mediated diseases, such as allergies, asthma and autoimmune diseases, generating high healthcare costs. We live too ‘cleanly’ in cities,” says Doctoral Researcher Mika Saarenpää. Microbial exposure can be increased easily and safely at home throughout the year. The space and financial investment required is minor. In the study, the gardening took place in regular flower boxes, while the plants cultivated, such as peas, beans, mustards and salads, came from the shop shelf. Changes were observed in only a month, but as the research subjects enjoyed the gardening, many of them announced that they would continue the activity and switch to outdoor gardening in the summer.
Noteworthy for gardeners is that this research suggests that peat, the most widely used growing medium in the world, does not appear to bring the health benefits associated with a medium mimicking diverse forest soil.
(Urban indoor gardening enhances immune regulation and diversifies skin microbiota — A placebo-controlled double-blinded intervention study. Environment International, May 2024.)