Diet and gut microbiota in cardiometabolic health: Studies from the Danish Diet, Cancer and Health – Next Generations Cohort and the sub-cohort MAX

Doctoral thesis, 2023

Cardiometabolic diseases (CMD) such as heart disease and diabetes, are the leading cause of death worldwide. Diet affects the risk of CMD but also the composition of bacteria in the gut. It is therefore possible that the relationship between what we eat and the risk of getting diseases is mediated through the bacteria in our gut. When studying the relationship between diet and risk of disease, researchers often use self-reported dietary data derived from questionnaires. These are subject to measurement errors since they rely on people’s memory of which foods and beverages they have consumed. It is therefore important to assess how accurately dietary intake is measured based on a food questionnaire. Another problem, relating to the importance of gut bacteria, is that we do not know how stable the bacterial populations within an individual are over time, and we therefore do not know how to best take samples for analysis of the levels of different bacteria. More knowledge is therefore needed to optimize the design of dietary studies in a way that makes it possible to demonstrate with greater certainty connections between diet, bacteria in the gut and development of disease. In my thesis, dietary data from a Danish population were evaluated, and the stability of gut bacteria was assessed. In addition, I investigated whether there is a relationship between individuals’ plant-based dietary patterns and their levels of gut bacteria, as well as whether there is a connection between diet, various risk factors for CMD, and gut bacteria. My research shows that self-reported dietary data collected by a new food frequency questionnaire was of sufficient quality for use in research studies and that a simple ‘dietary quality score’ was suitable for categorizing individuals into groups with healthy, average or unhealthy eating habits. In addition, I found that 39% of our gut bacteria appears to be reasonable stable over time suggesting that these can be used in studies investigating bacteria in relation to diet and diseases. Furthermore, I found a relationship between plant-based dietary patterns and the level of some specific bacteria. In particular, high intakes of fruit, vegetables, wholegrains, cereals and nuts were associated with higher levels of bacteria that are known to ferment fibre in the gut. However, it was not possible to find an effect of the diet on risk factors for CMD via specific gut bacteria previously shown to modulate effects of plant-based foods on CMD.