When we published an editorial on apple pomace earlier this year, the focus was on a deceptively simple idea: that the solid waste left behind after pressing apples for juice or cider is not waste at all, but a sophisticated matrix of dietary fibre and phenolic compounds with real potential to improve public health.

Mario Martinez described how this material could be incorporated into bread, how it survived baking and how its components were released or crucially, retained during digestion and reshaped the gut microbiota.

A new paper from the same research group, just published in Food Chemistry, takes the story a significant step further.

From "what survives digestion" to "what does it actually do"

The earlier research established that a large proportion of apple pomace polyphenols are not absorbed in the upper gut, they pass through largely intact, still entrapped within the cell wall matrix. At the time, this was framed primarily in terms of the gut microbiome: compounds reaching the colon can reshape microbial ecology in potentially beneficial ways.

The new study asks a different question. For that small but significant fraction of phenolics being released before reaching the colon, what are they doing to the cells lining the small intestine? And the answers are striking.

Using a carefully validated in vitro system, physiologically digested apple and pomace fractions applied directly to intestinal cell models, the researchers showed two things.

First, bioaccessible polyphenols from both whole apple and apple pomace inhibit the transport of glucose across the intestinal wall. The effect is primarily driven by dihydrochalcones and hydroxybenzoic acids, the same compound families that are particularly well-retained in pomace after cold pressing.

Second and more surprisingly, post-intestinal digestion apple pomace and not whole apple, stimulates the secretion of GLP-1, the appetite-regulating hormone that has become one of the most talked-about targets in modern medicine.

Why GLP-1 matters right now

If you have followed health news over the past two years, you will have heard about GLP-1 receptor agonists, drugs like semaglutide, marketed under names including Ozempic and Wegovy. These pharmaceuticals work by mimicking GLP-1, a hormone naturally produced by specialised cells in the distal gut lining that signals satiety, slows gastric emptying, and stimulates insulin release. The results in clinical trials for obesity and type 2 diabetes have been dramatic enough to reshape how medicine thinks about metabolic disease.

What the new research suggests is that apple pomace, when digested under physiologically realistic conditions, can stimulate the body's own GLP-1-producing cells - not at pharmaceutical levels, but measurably and significantly. Gastric digesta from both whole apple and pomace increased GLP-1 secretion by around 50% compared to controls. More importantly, pomace digesta retained this effect after intestinal digestion, increasing GLP-1 by 179% compared to the glucose control - an effect that whole apple digesta did not show.

Two fractions of the same fruit. Two distinct physiological mechanisms.

That distinction is worth dwelling on. Why would pomace outperform the whole apple in this regard?

The answer lies in the same structural feature that makes pomace interesting in the first place: the cell wall matrix and the distinct phenolic profile. During cold pressing, sugars, free acids and some phenolics leave with the juice. What remains is a concentrate of cell wall-bound polyphenols, many of them still physically entrapped within the fibre scaffold.

This structure appears to protect certain bioactive compounds from degradation and absorption in the upper gut, allowing them to arrive at the distal small intestine, where GLP-1-producing L-cells are most concentrated, in a form that can still stimulate hormone secretion (while preventing other phenolics from reaching these cells and inhibited the GLP-1 secretagogue effect).

Put differently, the very property that initially seems like a limitation (reduced bioaccessibility compared to juice) turns out to be a functional advantage when the goal is to reach and activate cells further down the intestinal tract.

This reinforces an important concept: Bioactivity is not just about composition - it is about structure, digestion and metabolite availability. Sometimes innovation isn't about finding "new" molecules. It’s about understanding food matrices more precisely.

A methodological contribution as well as a nutritional one

It is worth noting that this study also solves a practical problem that has frustrated researchers in this field for years. Applying digested food samples directly to cell models is technically difficult: the digestive enzymes, bile salts and other components present in a realistic food bolus are toxic to cultured cells at the concentrations needed to see meaningful polyphenol effects.

Previous studies typically worked around this by using concentrated extracts or purified standards, which is rather like testing the effects of orange juice by using isolated vitamin C tablets. And when working with real food matrices, digesta must be diluted to levels that compromise physiological relevance.

The new paper introduces a laboratory-implementable purification procedure, freeze-drying digesta and re-solubilising in methanol to precipitate proteins while concentrating polyphenols, that doubled detectable polyphenol concentrations without inducing cytotoxicity. This makes the cell-based results significantly more physiologically meaningful than much of the existing literature and the method itself is likely to be adopted more widely.

What this means in practice

Taken together with the earlier work on pomace in bread, these findings begin to sketch a coherent picture. Apple pomace is not simply a fibre supplement. It is a structured delivery system for cell walls and polyphenols that, when properly formulated into staple foods, may provide habitual, repeated exposure to compounds capable of modulating glucose absorption and appetite-regulating hormones, the two central targets of the most successful metabolic drugs currently in clinical use.

The effects are yet to be tested in vivo and no one is suggesting that a slice of pomace-enriched bread is equivalent to a weekly drug injection. But the logic of preventive nutrition has always been about cumulative exposure, small, repeated effects that shift population-level risk over years rather than months. On that basis, the case for taking apple pomace seriously as a functional food ingredient is now considerably stronger than it was.

This blog post accompanies the editorial "Apple pomace: a health-promoting reservoir" by Mario M. Martinez (University of Valladolid). The research described is from: Lopez-Rodulfo et al., "Selective detoxification of digesta revealed how cold-pressed apple fractions modulate transepithelial glucose transport and stimulate GLP-1 secretion," Food Chemistry (2026). https://doi.org/10.1016/j.foodchem.2026.148588