Who hasn't heard the saying "An apple a day keeps the doctor away"? But have you ever wondered why apples are good for you? The credit largely goes to polyphenols (molecules with anti-inflammatory, anti-diabetogenic and anti-carcinogenic properties) found in the flesh and especially the skin. Now, a study has looked in depth at what happens to these compounds when we eat this fruit. The research was conducted by the Edmund Mach Foundation in collaboration with the Council for Agricultural Research and Analysis of Agricultural Economics (Crea), funded by the Ager Melo project and just published in the journal Food Research International. It has uncovered the complex transformations of polyphenols into other chemical forms that are bioavailable to the human body, highlighting the decisive role of the gut microbiota in the beneficial action of these bioactive compounds. The findings provide essential information for mapping nutrikinetics — that is, the transit through the human body of the molecules that protect health.

How apple polyphenols are metabolised

Researchers from the FEM Food Quality and Nutrition Department and the Crea Food and Nutrition Research Centre followed a group of 12 healthy volunteers who, on two separate occasions, consumed either a simple high-quality apple juice or a polyphenol-enriched apple juice, with the aim of assessing how the polyphenols present in the apple were metabolised. The research showed that none of the phenolic compounds present in apple juice are found in the body in their original form (that is, as they appear in the apple). These compounds are instead variously metabolised in humans into 110 different chemical forms that appear first in the bloodstream and subsequently in the urine.

The fundamental role of the gut microbiota

The researchers were then able to establish that while 40 per cent of the metabolites originated from human metabolic processes, the remaining 60 per cent required the action of gut bacteria in order to enter circulation. "The quantity and persistence of each of these molecules in biological fluids (blood and urine) proved highly variable from one individual to another — the researchers explain — not only due to genetic differences, but also due to differences in the composition of the gut microbiota." The metabolites derived from microbial metabolism proved more persistent, meaning they were capable of remaining in circulation for much longer periods. In addition, an interesting correlation was observed between the composition of gut bacteria, measured through metagenomics experiments, and the quantity of circulating metabolites. The composition of the gut microbiota therefore appears to be an important factor in mediating the beneficial effects of apple consumption.