Scientists uncover the surprising power of apple peels to fight diabetes and oxidative stress — but how do they compare to existing drugs?
Study: Apple peels as an edible source of phenolic bioactive compounds with antidiabetic and antiglycation properties. Image Credit: SandaloFilms / Shutterstock
A recent study published in the journal Food & Function investigated apple peels as a source of phenolic compounds for industrial applications.
Type 2 diabetes (T2D) accounts for a majority (90%) of diabetes cases, and estimates suggest that there will be 783 million people with diabetes by 2045. As such, strategies involving modifiable factors, such as healthy diet and lifestyle behavior, are needed to stall the widespread surge in diabetes and related complications. Herbal supplements and plant-derived foods have been associated with lower odds of developing T2D.
Polyphenols are among the most common components in plant-based foods. They have antioxidant and survival functions in plants. Due to their anti-inflammatory and anti-radical properties, these compounds might help prevent metabolic and non-communicable diseases. Phenolic compounds are classified into the following major classes: phenolic acids, tannins, flavonoids, lignans, and stilbenes.
Plant residues, such as food byproducts and waste, are rich in phenolic compounds. As such, they can be extracted to produce food supplements, nutraceuticals, and medicinal products. While peels from some vegetables and fruits are edible, they are primarily residues in the food industry. Moreover, peels represent a substantial source of phytochemicals and nutrients, with greater bioactivity than pulp. However, since the Folin–Ciocalteu assay measures only certain components and may overestimate total phenolics due to interference by sugars and ascorbic acid, complementary techniques such as HPLC-MS, which identified 37 specific phenolics in this study, provide a more detailed picture.
About the study
In the present study, researchers investigated whether edible apple peels can be used as a source of bioactive phenolic compounds. First, they collected fresh apples of six varieties between August and October 2022. Commercial apple cultivars included Verde Doncella and Pinova. Local autochthonous cultivars included Borau 01, Amarilla de Octubre, Manzana Helada, and Esperiega de Ademuz.
Phenolic compounds were extracted by ultrasonication of apple peels, with methanol as the solvent. The Folin–Ciocalteu assay was used to determine the total phenolic content (TPC), though the authors note this method can overestimate phenolics due to interference from sugars and ascorbic acid. Further, 37 individual phenolic compounds were quantified using high-performance liquid chromatography-mass spectrometry (HPLC-MS). Since only a subset of all possible phenolic compounds was measured, actual total phenolic content could be higher.
Next, an MTT assay was performed to determine cellular viability using Caco-2 cells. In addition, the team analyzed the capacity (of the extracts) to inhibit pancreatic lipase, α-amylase, α-glucosidase, and advanced glycation end products (AGEs). The radical scavenging activity of the extracts against 2,2-diphenyl-1-picrylhydrazyl (DPPH) was also investigated. For context, the study compared the biological activities of the apple peel extracts to reference compounds, including acarbose (α-glucosidase inhibition, IC50 = 220 μg/mL), gallic acid (α-amylase inhibition and antioxidant, IC50 for DPPH = 0.47 μg/mL), and aminoguanidine (AGE inhibition, IC50 = 55 μg/mL).
Findings
The apple peel samples with the best TPC were Amarilla de Octubre, while the Borau 01 and Pinova had the lowest TPC. Phenolics detected in samples included flavonols, anthocyanins, flavan-3-ols, cinnamic acids, and dihydrochalcones. Notably, stilbenes, hydroxybenzoic acid, and flavanones were not detected in any sample.
Consistently, HPLC-MS analysis indicated Amarilla de Octubre as the best sample with three times higher phenolics than Borau 01. Amarilla de Octubre had the most phenolic compounds (16), while Borau 01 had 14 compounds, and others had 15. Chlorogenic acid, hyperoside, phlorizin, procyanidin B2, epicatechin, and delphinidin 3,5-diglucoside were the most abundant phenolic compounds.
Samples from commercial apple varieties were a better source of anthocyanins and flavonols but had a lower proportion of cinnamic acids, flavan-3-ols, and dihydrochalcones. By contrast, local samples had a lower anthocyanin and flavonol content than commercial samples. Amarilla de Octubre was the most abundant in anthocyanins and flavonols among local samples.
MTT assay revealed that Esperiega de Ademuz, Amarilla de Octubre, and Manzana Helada slightly decreased cell viability at high concentrations. Overall, apple peel extracts were considered non-cytotoxic.
Only four extracts (Pinova, Verde Doncella, Manzana Helada, and Amarilla de Octubre) inhibited α-glucosidase. Amarilla de Octubre extracts had the lowest half-maximal inhibitory concentration (IC50) (625.51 μg/mL), indicating the highest activity, though this was weaker than the reference drug acarbose (220 μg/mL).
Only Amarilla de Octubre extracts inhibited α-amylase, while none inhibited lipase. However, Amarilla de Octubre’s α-amylase inhibition (IC50 = 1796.51 μg/mL) was considerably less potent than gallic acid (IC50 = 909 μg/mL), the reference inhibitor.
Further, all samples could inhibit the formation of AGEs, with Amarilla de Octubre exhibiting the highest activity (IC50 = 158.69 μg/mL) and Pinova being the least effective. Nevertheless, Amarilla de Octubre’s antiglycation effect did not surpass aminoguanidine (IC50 = 55 μg/mL).
Notably, the researchers found no direct correlation between TPC and α-glucosidase or AGE inhibition, suggesting other compounds or synergistic effects may drive bioactivity. The study also hypothesizes that triterpenes, such as oleanolic and ursolic acids, naturally present in apples, may contribute to the observed enzyme inhibition.
In addition, all samples scavenged DPPH radicals, and consistently, Amarilla de Octubre had the highest activity (IC50 = 23.78 μg/mL); Pinova and Borau 01 had the lowest activity. However, gallic acid, used as a reference, had a much lower IC50 (0.47 μg/mL), indicating significantly stronger antioxidant capacity than any of the apple peel extracts.
Conclusions
In summary, apple peels from six cultivars were rich in phenolic compounds and showed antioxidant and antidiabetic properties without impacting cellular viability at physiological concentrations. The most abundant phenolic compounds were hyperoside, procyanidin B2, and delphinidin 3,5-diglucoside.
The local cultivar, Amarilla de Octubre, had the highest phenolic content and activity in assays, suggesting potential utility in developing pharmaceuticals and nutraceuticals. However, while Amarilla de Octubre showed promising bioactivity, its effects did not exceed those of established reference compounds.
This suggests that apple peel extracts may serve as complementary, rather than primary, interventions for managing type 2 diabetes and oxidative stress. The role of other bioactive compounds, such as triterpenes, also merits further investigation.