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Daniele Del Rio, PhD

How would you react if I told you that the world-famous proverb “An apple a day keeps the doctor away” was, in fact, missing one very relevant bit? The whole sentence should go like “An apple a day keeps the doctor away for John, but not for Jay!”. You think I am kidding… Well, I might be, but, in a few years, this new version of this saying might become closer to the real thing than you might imagine.

“A diet that might work for me could be unbearable for someone else, and this is something we have known for decades.”

The reason before this “renovation” is linked to the fast-emerging concept of “personalized” or “precision” nutrition. To be completely honest, well-trained dietitians and nutritionists already strongly personalize their dietary recommendations, their prescriptions, and their advice. They do so based on their patient’s age, sex, taste, habits, fitness and on several other factors that, altogether, make us unique individuals. A diet that might work for me could be unbearable for someone else, and this is something we have known for decades. We have known for decades also that some genetic conditions make some of us unable to introduce certain nutrients, like in the case of phenylketonuria or favism, where the body of the persons suffering these conditions lack specific enzymes and, therefore, cannot eat or drink foods and beverages containing the molecules that by these enzymes should be processed, or that can be damaged because they are somehow not protected by said enzymes. The new frontier, instead, that has emerged only very recently, is defined by what we were unable to see and measure until a few years ago, namely the gut microbiota. I’m sure you have read a lot about it in this blog; therefore, I will not bore you too much by adding information you already know. I will, however, explain the point of view of a nutritionist working with polyphenols and interested in the gut microbiota… So, let’s go back to our proverb!

“These polyphenols are not real “nutrients” for us, as we have not evolved wise strategies to efficiently absorb them at the small intestinal level.”

Apples and other fruits contain polyphenols, and these non-nutritive phytochemicals are produced by plants sometimes in response to stress, including the type of stress they undergo when they get in contact with invaders, like parasites, viruses, bacteria, and fungi. These molecules seem to protect the plant and its fruits from external attacks by some sort of toxicity towards the invader organisms. So, when we eat the fruit, we introduce these phytochemicals and because, as mentioned a few lines ago, they are not real “nutrients” for us, we have not evolved wise strategies to efficiently absorb them at the small intestinal level (i.e., we do not have specific transporters like we might have for vitamins). Unabsorbed in the upper intestinal tract, these compounds of plant origin flood the large intestine, where they encounter the locally residing, and now world famous, gut microbiota.

“…only some of us, or, better, some of our gut microbiota, are able to break down polyphenols to generate the active metabolites possibly impacting our health.”

Among the multitude of microorganisms of all sorts inhabiting that tract of our gastrointestinal system, some have learned how to break down polyphenols to escape their toxicity and, perhaps, also to get some energy out of the reaction. Well, it recently turned out (also thanks to some research carried out in my lab in Parma) that it might just be the products of these reactions that, when absorbed within our body, exert the putatively beneficial health-promoting effects of specific fruits and vegetables (besides, of course, the merits vitamins, minerals, fiber, and of the very low caloric density of plant foods). The problem (and the sense of all this blurb I’m writing) arises when only some of us, or, better, some of our gut microbiota, are able to break down polyphenols to generate the active metabolites possibly impacting our health. And this is what is really happening, not just with apples, but even more so with other fruits, containing different classes of polyphenolic compounds: we do not all produce the same metabolites, or the same proportion of metabolites derived by the specific plant phenolics that were present in the fruits we ate. We call these difference in the capacity of our gut microbiota to produce metabolites from molecules introduced via the diet “metabotypes”.

“Falling within a different polyphenol metabotype might mean that the apple you just ate will not have the same “anti-doctor” activity as it will have on me.”

Well, we are not quite at the point where your nutritionist has the information needed to advise you about what specific fruits and veggies are better for you… But the more we understand about our gut ecosystems and metabotypes, the closer we get to that moment, that might radically revolutionize our vision of personalized nutrition.