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‘EAT FOOD, NOT too much, mostly plants’.
That was journalist Michael Pollan’s short answer in 2008 to what we humans should eat in order to stay maximally healthy.
In his book ‘In Defence of Food,’ Pollan urged us to eat real food, arguing that escaping from the Western diet can free us from most diet-related chronic disease, at the same time as enriching our lives.
Almost three decades later, a new paper from world-renowned researchers at APC Microbiome based in University College Cork, confirms what Pollan was advocating all those years ago. The paper in Nature Reviews collates the latest evidence on how much of what Pollan was advocating is based on what is called our gut microbiome – the community of microorganisms living in our gut.
The research team, led by Yolanda Sanz and John Cryan, describe how the gut microbiome acts as the mediator between what we eat and our ultimate health.
What is revolutionary about this area of research is the idea that the gut microbiome needs to be fuelled by our diet, and that it is the microbiome that decides how much nutrition we get from our food.
Several reviews on the role of diet in shaping our microbiome have been published. Some have focused on the role of specific nutrients in our diet, like fibre, proteins, fats, vitamins and minerals. Some have focused on the role of the microbiome in influencing certain diseases such as inflammatory bowel disease, autoimmune or metabolic diseases or diseases affecting brain health.
This review gives an overview of the role of nutrition in modulating the gut microbiome at different scales – in terms of overall diet, food types and nutrients…
The research outlines how the diversity and quantity of healthy plant-based foods in the diet are key to microbiome variety and diversity.
Fruit, vegetables, grains, and, to a certain extent, dairy products were found to have the strongest association with a healthy gut microbiome. The consumption of healthier and plant-based food items, especially fruit, was associated with a greater diversity within the gut, and it is the level of diversity that is considered key to a healthy microbiome.
Conversely, a lower diversity was usually observed with the consumption of less healthy foods such as high sugar drinks and fried products, red and processed meat, or refined grains.
Association with dairy products is less clear, with differences between yoghurt (fermented), which promotes a healthy microbiome, and other dairy products such as cheese and butter, believed to be less effective.
The results at the food level partly echo those at the diet level. Healthier plant-based foods are those that contribute to healthier dietary patterns, for which there is solid evidence of better human health and greater microbiome diversity. Conversely, foods such as red meat, foods high in fat, salt and sugar that contribute to Western diets or unhealthy dietary patterns showed lower microbiome diversity.
Dietary fibre is the prime source of energy for the gut microbiome, which through fermentation provides short chain fatty acids (SCFAs) that benefit the host.
Dietary protein is also an important contributor to a healthy gut microbiome, though not as important as fibre. The ratio of protein to fibre intake is considered to have more influence on a healthy gut microbiome than dietary fibre alone. This is why it is important to look at the diet as a whole rather than just individual foods or nutrients.
The lipid content of the diet is another important driver of gut microbiome composition. High fat diets, usually rich in saturated fat (as found in meats and dairy products such as cheese and butter) have mostly been associated with lower microbiome diversity and increased abundance of unhealthy bacteria species that potentially have adverse effects on human health. On the other hand, monounsaturated (found in olive oil or avocado), and polyunsaturated fats (found in salmon, vegetable oils, nuts and seeds) have more favourable effects.
In addition to nutrients, plant-based foods also contain bioactive compounds such as polyphenols. These are found especially in fruit, vegetables and wholegrains as well as coffee, tea, red wine and dark chocolate. In terms of red wine, the polyphenol content is potentially beneficial to a healthy gut microbiome. However, the alcohol content has been documented as having an adverse effect.
The researchers explain how the interplay between the diet and the gut microbiome contributes to the regulation of the intestinal immune system and the integrity of the gut. Any breach in this system can lead to low-grade inflammation and a breach in the gut barrier function, or ‘leaky gut’, that allows unhealthy pathogens and other toxins to penetrate the gut wall. This can trigger inflammation and changes in the normal flora of the gut wall that can be the precursor of both problems within the gut itself and more systemic chronic disease in other organs throughout the body.
The researchers specifically reference the impact of a healthy microbiome and the so-called gut-brain-axis that explains the diet-sensitive mechanisms underpinning brain health that potentially underpin many disorders, including depression, anxiety, attention deficit disorder, epilepsy, Alzheimer’s and Parkinson’s disease.
The gut microbiome also plays a role in the cardio-metabolic system that is linked to appetite, satiety and diseases such as diabetes, cardiovascular disease and obesity.
In the interim, the best advice for a healthy gut microbiome appears to focus on lots of fibre and lots of variety. That means lots of fruit and vegetables, nuts and seeds, wholegrains including bread, pasta, rice; legumes including beans, peas and lentils, some dairy products such as fermented yoghurt and minimal quantities of ultra-processed foods and drinks, high in saturated fat, sugar, salt and additives.
‘Eat food, not too much, mostly plants’ was Michael Pollan’s advice in 2008 to keep it simple when choosing food. These seven words seem to be the best advice years later, for maintaining a healthy microbiome, which plays that key mediator role between what we eat and our ability to bolster our defences against chronic disease.
Dr Catherine Conlon is a public health doctor in Cork and is the former director of human health and nutrition, safefood.
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