Forest soil is hot, but why?
The Netherlands is going to copy Finland. Finland is already doing it, sending children into the forest. Dutch researchers would like to see children come into more contact with the soil (earth), to train their immune system at an early age and prevent asthma later on. In turn, the Finns looked at the simple lifestyle of Russians and noticed that Russian children had less asthma than Finnish children. The Finns quickly made a decision and changed the environment in which children live, especially in schools: playing outside, in the woods. The Finns are now leading Europe in terms of low asthma prevalence among children. The NOS news website (15 March 2025) reports on new large-scale research in the Netherlands into how we can enrich children’s environment, bacteriologically speaking. Forest land in and around the house, in the schoolyard, the possibility of coming into contact (at a young age) with bacteria and fungi. Because that is what it is all about.
David Strachan also knew this. Strachan (2000) discovered differences in asthma and allergies in children from large families. It was not the first child, but younger children who suffered less from asthma. Young brothers and sisters were immunologically trained by the presence of other, older children, who infected them with dust, spit and other dirt. Young children learn about the world partly through their mouths; including the bacteria in their environment. Strachan (2000) developed the hygiene hypothesis for this, popularly worded as: ‘a little dirt does not hurt’. You need ‘dirt’, read bacteria and fungi, to train and shape your immune system.
The Germans knew it too, they talk about ‘Dreck reinigt den Magen’ (Dirt cleanses the stomach). After the fall of the Berlin Wall in 1989, it appears that the former East German environment led to less asthma. It may not have been ideal politically and ideologically, but immunologically it was a lot better for young children. A few years later, the differences between East and West in terms of asthma had disappeared. Unfortunately for the East German children, because they ended up just as bad off as the West German children after they had adopted the eating and living habits of Western society (Heinrich et al., 2002).
Weston A. Price also realised this at the end of his global search in the 1930s. He saw that the dietary habits of naturally living peoples were the key to reducing tooth decay, to natural births without Caesarean sections, to children without allergies and asthma; in short, a lifestyle for producing a healthy next generation. What he saw as the common denominator among the 15 peoples studied worldwide was the consumption of raw and raw-fermented products. There was also an awareness of the important immunological time window around pregnancy and birth with numerous high-fat speciality regional products (such as fish roe, raw Alpine butter high in omega-3 fatty acids). Finally, there are the farm children, particularly those who grew up on a dairy farm. They also have a much lower incidence of asthma and allergies after they come into contact with farmyard dirt, urine-enriched stable air and drink raw milk. There are very few allergic problems among Amish children in particular, probably because the protective factors are stacked up (large families, working on the farm, coming into contact with dirt, pregnant mothers working and of course the raw cow’s milk). We have previously described how the heat-sensitive whey proteins play a protective role in this. The results correlate very well with the unchanged beta-lactoglobulin (holo-BLG), which is secreted in both urine and cow’s milk. Other animals on the farm, particularly dogs, provide additional protection, which can also be explained by the structural similarities between whey protein BLG and allergens from dog hair, the lipocalin family.
Seeding a healthy gut microbiome
Immunologically healthy farm children have a more ‘mature’ intestinal flora than children who develop allergies and asthma. This intestinal flora initially develops through the sugars in breast milk (human oligosaccharides), to which the intestinal microbiome is seeded and grows like an ecosystem (Stewart et al., 2018). There are numerous factors that give children a false start in life, such as a caesarean section or (multiple) antibiotics in the 1st year of life. However, there are a range of possibilities that can put young children on the right track to build a rich, balanced, protective intestinal flora. Researchers all claim their contribution and often present their findings into the best solution. For example, Chinese researchers have indeed shown that when you feed asthma mice soil, the intestinal flora changes and the allergic reaction decreases (Li et al., 2024). In our own research at Utrecht University, we were able to demonstrate that mice with asthma benefited from raw milk, not heated milk (Abbring et al., 2017). Earth and raw milk apparently have nothing to do with each other at first glance, but the contribution that both make to directing the immune response is probably essential.
Ultimately, it is about the measures working together to calm the immune system and ensure that there are no overactive immune responses in the form of red skin, difficulty breathing or anaphylactic shock. Often there are cumulative positive effects.
Combining insight into a one-health approach
Attention is being paid to a more natural lifestyle and more preventative measures to enrich the young intestinal microbiome of growing toddlers and preschoolers, so that they do not develop allergies and asthma later in life. However, it would be nice if a study approach also combined insight and stacked effects, as is the idea behind the ‘one-health approach’. It may be the case that contact with soil and playing with sand and earth contributes to the development of the immune system, and after Finland, the Netherlands will now probably also spend at least 10 years of research on this. Research leader Penders of the University of Leiden told the Dutch newspaper Trouw that ‘forest soil can contain pathogens and parasites, so you have to keep a close eye on it…’ Perhaps in 10 years we will have certified forest soil suitable for playgrounds. However, Penders then goes on to talk about raw milk:‘… another example is raw milk. There are indications that it reduces the risk of allergies, but it can also make you sick. That is why I (read: Penders) would certainly not want to implement drinking raw milk as a measure…’. Unfortunately, Penders overlooks a few things. Firstly, the Netherlands Food and Consumer Product Safety Authority (NVWA) has made the consumption of raw milk possible from 1 January 2025, in particular by introducing regular checks for zoonoses. Just like in Germany, where Vorzugsmilch has been consumed without any problems for many decades, also by children. Furthermore, you can also ferment raw milk and offer it as raw kefir. If you do it properly, then most of the dangers that are always mentioned in connection with raw milk consumption are gone in one go. Moreover, you are offering a soured product that contains more than 1 billion bacteria per ml, plus many fungi. A nice addition to forest soil.
Raw and raw fermented products also contribute to immune development, but adults also use them to improve problems with bowel movements, digestion and skin (Baars et al., 2019). When it comes to enriching the intestinal flora and at the same time wanting to allay fears about consuming raw milk, it makes sense to shift the focus to raw fermented milk products such as kefir. We have already been able to show that raw kefir, just like raw milk, suppresses allergic reactions (Baars et al., 2025).
Conclusion
‘One-health’ is about moving away from an immunologically pathogenic Western lifestyle, eating differently – probably more traditionally, growing up differently, and learning to deal with ‘dirt’ again. Whether it is important to know exactly whether sauerkraut, soil, the cowshed or raw (fermented) milk plays a role is less interesting in practice. It seems more important not to wait with the implementation and to pick up the insights from the various epidemiological studies and simply look at what people used to do in the past, based on experience to survive over generations.
Literature
- Abbring, S., Verheijden, K. A., Diks, M. A., Leusink-Muis, A., Hols, G., Baars, T., … & van Esch, B. C. (2017). Raw cow’s milk prevents the development of airway inflammation in a murine house dust mite-induced asthma model. Frontiers in immunology, 8, 1045.
- Baars, T., Berge, C., Garssen, J., & Verster, J. (2019). The impact of raw fermented milk products on perceived health and mood among Dutch adults. Nutrition & Food Science, 49(6), 1195-1206.
- Baars, T., van Esch, B., Diks, M., van Ooijen, L., Zhang, Z., Dekker, P., … & Kort, R. (2025). Higher bacterial diversity, bioactive peptides, and enhanced immunomodulatory effects in raw milk kefir made with defined starter cultures compared to backslopping methods. International Dairy Journal, 106202.
- Heinrich, J., Hoelscher, B., Frye, C., Meyer, I., Wjst, M., & Wichmann, H. E. (2002). Trends in prevalence of atopic diseases and allergic sensitization in children in Eastern Germany. European Respiratory Journal, 19(6), 1040-1046.
- Li, M., Li, N., Dong, Y., Zhang, H., Bai, Z., Zhang, R., … & Zhou, D. (2024). Soil intake modifies the gut microbiota and alleviates Th2-type immune response in an ovalbumin-induced asthma mouse model. World Allergy Organization Journal, 17(4), 100897.
- Roth-Walter, F., Afify, S. M., Pacios, L. F., Blokhuis, B. R., Redegeld, F., Regner, A., … & Jensen-Jarolim, E. (2021). Cow’s milk protein β-lactoglobulin confers resilience against allergy by targeting complexed iron into immune cells. Journal of Allergy and Clinical Immunology, 147(1), 321-334.
- Stewart, C. J., Ajami, N. J., O’Brien, J. L., Hutchinson, D. S., Smith, D. P., Wong, M. C., … & Petrosino, J. F. (2018). Temporal development of the gut microbiome in early childhood from the TEDDY study. Nature, 562(7728), 583-588.
- Strachan, D. P. (2000). Family size, infection and atopy: the first decade of the’hygiene hypothesis’. Thorax, 55(Suppl 1), S2.
News Website NOS: In Finland, the number of asthma patients is plummeting, as getting your hands dirty helps (15 March 2025) (only in Dutch)
