Susana Delgado (Oviedo, 50 years old) gets “goosebumps” from expressions like microbial flora —“Flora is vegetation,” she emphasizes— or germs —“It’s an archaic term that people associate with pathogens, when most of the microorganisms living with us are good,” she explains—. The senior scientist at the Institute of Dairy Products of Asturias (IPLA-CSIC) has been studying the microbiota (bacteria, viruses, fungi, and other microorganisms that inhabit our body) for two decades, which is increasingly linked to many aspects of health.
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Delgado directed her career towards clinical research through nutrition and today collaborates with gastroenterologists and pediatricians to study the relationship between that invisible community, diet, and disease. Much of her work focuses on cow’s milk protein allergy (CMPA), one of the most common in childhood. She recently participated in the summer courses of the Menéndez Pelayo International University (UIMP), at the Magdalena Palace in Santander, where she addressed the role of the human microbiome, the set of genes of those microorganisms, and their interaction with the environment.
Question. What is the relationship between microbiota and cow’s milk protein allergy in infants?
Answer. We don’t know which came first: the chicken or the egg—whether an altered microbiota favors the allergy or vice versa. It is very difficult to study because as soon as symptoms begin, milk is replaced by hydrolyzed formulas [less allergenic], but we have obtained very interesting results.
There are allergies that cause skin reactions, atopy, or anaphylaxis and are detected with skin tests or immunoglobulin E (IgE) analysis, but our group mainly works with those not mediated by this immunoglobulin, which present gastrointestinal symptoms and are more difficult to diagnose. We look for biomarkers in stool to identify and monitor these children less invasively than with blood tests or biopsies. We have found, for example, that they have fewer bifidobacteria, which are beneficial, and more potentially pathogenic bacteria than healthy children or those with IgE-mediated allergies. We don’t know why, but it seems a kind of vicious circle is generated. Additionally, we detect markers of intestinal inflammation in their stool.
We need biomarkers because, although many of these allergies disappear over time, they would help identify when the child has outgrown them, avoiding unnecessary dietary restrictions —which can cause deficiencies in calcium or essential amino acids— and oral challenge tests. The disease also represents a significant burden for families —mine is one of them— because whey proteins are additives in many products.
Q. You emphasize that these allergies must be differentiated from lactose intolerance.
A. They have nothing to do with each other. Allergy is an immune reaction to milk proteins, while intolerance is a digestive problem due to lack of lactase, the enzyme that hydrolyzes lactose [a milk sugar], and usually occurs in adulthood. It is common for schools to confuse them and give children with CMPA lactose-free yogurts.
Q. You have also highlighted the importance of the first 1,000 days of life. What happens during this period and what role does the microbiota of breast milk play?
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A. It is a critical period, covering the first two or three years, during which the microbiota is established from the bacteria the baby receives from the mother, family, and environment. That is why now it is encouraged that hospitalized newborns spend more skin-to-skin time with their parents. Breast milk contains oligosaccharides that feed bifidobacteria, very abundant in the first months; infant formulas only include some, without reaching their complexity. Its composition also varies among women, ethnicities, or geographic regions, making it a key factor in that initial colonization.
As the child grows, those bifidobacteria produce compounds that facilitate the establishment of other groups, such as bacteroides, with the introduction of new foods: this is called “cross-feeding,” where some bacteria prepare the ground for others to thrive and establish a balanced microbiota that protects against infections. That is why there is also a warning about excessive hygiene: we have tended to sterilize everything and raise children almost in a bubble, when reasonable contact with the environment helps the immune system and microbiota mature, and a child with a balanced microbiota has fewer respiratory infections and otitis.
Q. Although the most well-known is that of the digestive system, what relationship does the microbiota have with the nervous and endocrine systems?
A. It is a bidirectional relationship. On one hand, the microbiota produces neurotransmitters, such as serotonin or GABA, which act on the nervous system. On the other hand, our hormones modify the microbiota, while some bacteria can transform hormones, both those of the organism itself and those we take from outside. That is why, for example, two women can respond differently to the same hormonal treatment for menopause. However, we still do not fully understand these interactions.
Q. Does that initial microbiota determine the one we will have as adults?
A. Once those microbial communities settle, it is difficult to modify them, except in situations like prolonged antibiotic treatments. That is why the first years are so important: that microbiota, which constantly interacts with our cells, will be the basis of the one we will have as adults, along with genetics and the environment. Also, it is during this period that the foundations of many immune-related diseases that may manifest years later are established.
If you have a varied diet, exercise, sleep well, and maintain a healthy lifestyle, your microbiota will also be healthy. Stress, sedentary lifestyle, diet, or sleep cycles influence it, just as it influences our health. To have a healthy microbiota, you must have healthy lifestyle habits.
Q. Does it make sense to take probiotics when prescribed an antibiotic?
A. From my point of view, not during the treatment. They may make more sense afterward, after prolonged treatments or recurrent infections, to help restore that balance and hinder the proliferation of resistant bacteria. It is like a forest fire: the antibiotic wipes out much of the ecosystem, and the probiotic tries to “reseed” it. But that recovery takes time; it is not achieved in a single day.
They are also not necessary for everyone; if you take an antibiotic for a few days for a throat infection, a probiotic will bring you little benefit. Additionally, beneficial bacteria can also be obtained through fermented foods, such as pickles, yogurts, or cheeses. In any case, probiotics are dietary supplements, not medicines, and their regulation is less strict. It would be necessary to check if the product you buy really contains what the label indicates.