POP Study: dietary habits and gut microbiome of the Spanish population

The microbiota can be defined as the conjunction of living microorganisms that can be found in a concrete place, such as the gut, skin, vagina, etc. These microorganisms contain not only bacteria but also fungi, viruses, etc. (2). Among the different human body sites, the gut corresponds to one the largest locations, as it measures approximately 400 m², in which your microbes relate with you and the environment (22).

Gut microbiota is important because it ferments the fiber and starch that are present in your diet and it uses them to produce several components that are beneficial for your health such as:

• Short chain fatty acids or SCFAs: the more common ones are propionate, acetate and butyrate and they serve as food for the cells present in our colon, known as colonocytes. Correct feeding of this cells allows for a correct functioning of our gut barrier (12; 23)
• Vitamins: some examples are vitamin K, B5, B9 and B12. We are not able to produce these vitamins by ourselves, so we obtain them thanks to our microbes. Vitamin K has a role in producing the elements that we need for blood clothing (21). Vitamins B5 and B12 are relevant for normal brain function (12).
• Antimicrobial agents: they are substances that help to kill, or at least prevent from growing, possible harmful organisms that may enter our body (1).

Microbiota is also involved in the proper development of our immune system by allowing us to distinguish between “our bacteria” and “foreigners”, and it serves as physical barrier against pathogens (12).

There are several things that can affect your microbiota, such as age, gender, the place where you live, with who and even your lifestyle (6). However, among them, diet seems to be one of the most relevant and easier to modify.

As a country bordering the Mediterranean Sea, Spain has traditionally followed the Mediterranean diet (MD). This diet is rich in plant-based foods such as fruits, legumes, whole grains, vegetables, and nuts. It emphasizes healthy unsaturated fats, primarily from olive oil, and includes frequent fish consumption, moderate intake of dairy products and fermented beverages, and a low intake of meat and meat-derived products (5).

Despite its benefits, adherence to the Mediterranean diet (MD) in Spain appears to have declined over time, shifting towards a more Western dietary pattern. This shift is characterized by a decrease in the consumption of whole grains and an increase in processed foods, sugar, saturated fat, and animal protein (15; 25; 26). This type of diet has been pointed out by some organizations as the cause of morbidities and health problems worldwide (9) that may be consequence, at least in part, of the loss of microbiota equilibrium known as “dysbiosis”.

POP Study is a prospective longitudinal study carried out in Vall d'Hebron University Hospital in Barcelona, Spain that involves healthy volunteers aged between 18 and 75 years old coming from different regions of Spain. It was initiated in December 2020 and is still active today.

a. Which are the objectives of the study?

The main objectives of the study are:

• Obtain a general insight of Spanish dietary patterns.
• To try to look for relationship between diet and your microbiota, which means to try to understand how food choices can modify your microbiome and how these changes can affect your health.
• Study how other factors such as demography and anthropometrics relate with microbiota.
• Use this information to compare with patients suffering from other diseases to figure out what has happened to their microbiome and try to re-store it up to “healthy” levels (work in progress).

b. Study design

We recruit participants coming from the 17 Autonomous Communities of Spain as well as Ceuta and Melilla cities through announcements on social media (Facebook, Twitter, Instagram). We ask all participants to donate a stool sample and fill in an online food frequency questionnaire to assess habitual diet 3 times over the period of 1 year (baseline, 6 months and 12 months).

• The answers from the questionnaires are used to obtain information about diet, lifestyle and anthropometrics.
• Stool samples are processed to get microbiome data.

c. Study update

To date, we have recruited 1,017 volunteers from various regions across Spain, forming the "POP" cohort. Dietary information is available for nearly all participants. However, due to budget constraints, bacterial analysis was conducted on a randomized subset of 500 baseline fecal samples, with fungal analysis performed on 98 samples. We are currently working hard to secure additional funding to sequence the remaining samples.

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