Apple Polyphenols and the Peel
Everything nutritionally distinctive about an apple — the part that does not show up on a nutrition label — is a family of plant compounds called polyphenols, and they are concentrated overwhelmingly in and just beneath the skin. An apple's peel can carry several times the polyphenol concentration of its flesh, which is why peeling an apple throws away most of what makes it worth eating. Apples are also one of the leading dietary sources of quercetin in Western diets, and the only common food containing meaningful amounts of phloridzin — a compound isolated from apple bark in 1835 that turned out, nearly two centuries later, to be the structural ancestor of an entire class of modern diabetes drugs. This page covers what those compounds are, why variety and colour matter far more than most people expect, how much of them your body actually absorbs, and what browning, cooking, storage and juicing do to the lot.
Table of Contents
- The Five Polyphenol Families in an Apple
- Why the Peel Holds Most of It
- Variety Matters More Than You Expect
- Quercetin: Apples Are a Leading Source
- Phloridzin: The Compound That Became a Drug Class
- What Happens in People
- Browning, Cooking, Storage and Juice
- Wash It, Do Not Peel It
- Getting the Most From Apple Polyphenols
- Key Research Papers
- Connections
- Featured Videos
The Five Polyphenol Families in an Apple
"Polyphenol" is a structural label covering thousands of plant compounds built around phenol rings. Plants make them for their own purposes — defence against insects and fungi, UV protection, pigmentation, and the astringency that discourages animals from eating unripe fruit. An apple's polyphenol profile is unusually well characterised, and it sorts into five groups:
- Flavan-3-ols and procyanidins — usually the largest fraction by mass. These are catechin and epicatechin and the chains they form (procyanidins, also called condensed tannins). They are what makes an unripe or a cider apple astringent — that drying, puckering mouthfeel. The proanthocyanidin-rich Renetta Canada variety was chosen for the Reading cholesterol trial precisely because of this fraction.
- Hydroxycinnamic acids, principally chlorogenic acid — the same compound that makes coffee a major polyphenol source. Unlike most apple polyphenols, chlorogenic acid is found substantially in the flesh as well as the skin.
- Flavonols, principally quercetin and its sugar-attached forms (quercetin glycosides: rutin, hyperoside, isoquercitrin, quercitrin, avicularin). These are almost entirely in the skin. Peel an apple and you remove essentially all of its quercetin.
- Dihydrochalcones, principally phloridzin (phlorizin) and its aglycone phloretin. This family is effectively unique to apples among common foods — a genuine chemical signature. It has its own section below.
- Anthocyanins — the red pigments, found only in red-skinned varieties and only in the skin. Cyanidin-3-galactoside is the main one.
Total polyphenol content in apples is highly variable — it depends on the cultivar, on ripeness, on growing conditions, on sun exposure, and on how long the fruit has been stored. That variability is not noise to be averaged away; it is one of the most practically useful facts on this page, and the section on varieties returns to it.
Why the Peel Holds Most of It
Wolfe, Wu and Liu at Cornell published a direct comparison in the Journal of Agricultural and Food Chemistry in 2003, separating apple peel from flesh and measuring both. The peel carried substantially higher concentrations of total phenolics and total flavonoids, and correspondingly higher antioxidant activity in laboratory assays. Peels also had a much greater ability to inhibit the growth of cultured cancer cell lines than the flesh — a cell-culture result, worth stating plainly as such, since a compound acting on cells in a dish is a long way from a compound acting in a person.
Why is it concentrated there? Because that is where the plant needs it. The skin is the interface with insects, fungi, and ultraviolet light. Flavonols and anthocyanins are made in the epidermis in direct response to sun exposure, which is also why the reddest, most sun-exposed side of an apple carries the highest polyphenol load.
The practical consequences are concrete:
- Peeling an apple removes most of its quercetin and all of its anthocyanins, plus a large share of its total polyphenols and a meaningful part of its fibre.
- Applesauce made without skins is a fundamentally different food from applesauce made with them.
- Clear apple juice loses both the peel compounds and the pectin, which is why it performed so badly in the trial described on the cholesterol page.
- The most sun-exposed fruit from the outside of the tree carries more than the shaded fruit from inside the canopy — one small reason a farmers-market apple can genuinely differ from a commodity one.
Variety Matters More Than You Would Expect
Vrhovsek and colleagues quantified polyphenols across different apple varieties in the Journal of Agricultural and Food Chemistry in 2004 and found large differences between cultivars — the range across varieties was severalfold, not a few percent. Kschonsek and colleagues extended this in Antioxidants in 2018 by comparing old and new apple cultivars directly, and found that older varieties tended to carry higher polyphenol contents and higher in-vitro antioxidant capacity than modern commercial ones.
That result is not a coincidence, and it connects directly to the history of the apple. Twentieth-century commercial breeding selected hard for appearance, uniformity, storage life, shipping tolerance, and low astringency — and astringency is procyanidin content. Breeding the pucker out of an apple means breeding out a polyphenol fraction. Selecting against browning means selecting against the substrates that brown. The blandly sweet, uniformly red, indefinitely storable commodity apple is, on average, the polyphenol-poor one.
Some useful rules of thumb follow, offered as tendencies rather than laws, because growing conditions and storage shift the numbers:
- Red skin beats green or yellow skin for anthocyanins, by definition — a green apple has none.
- Tart, astringent, "old-fashioned" varieties generally carry more procyanidins than sweet modern ones. Russets, heritage varieties, and true cider apples sit at the high end.
- Cider apples are the extreme case — bred for tannin, and largely inedible raw for exactly that reason.
- A crab apple carries a very high polyphenol concentration for its size.
- Long controlled-atmosphere storage gradually reduces polyphenol content. A freshly picked local apple in September genuinely differs from the same variety pulled out of storage the following July.
The honest bottom line: eating a mix of varieties, favouring tart and heritage ones when you can get them, and buying in season, will get you more of these compounds than optimising within the supermarket's four options. But any apple with its skin on is far better than no apple, and the differences between varieties matter much less than the difference between whole fruit and juice.
Quercetin: Apples Are a Leading Source
Quercetin is one of the most-studied dietary flavonoids, and apples, onions and tea are its principal sources in most Western diets. The quercetin page covers the compound in depth; this section covers what is specific to apples.
The first thing to know is that apples do not contain much free quercetin. They contain quercetin glycosides — quercetin with a sugar attached — and which sugar it is changes how much you absorb. Hollman and colleagues demonstrated this in an elegant 1995 study in the American Journal of Clinical Nutrition using volunteers with ileostomies, which allowed the researchers to measure exactly how much of each form disappeared before reaching the end of the small intestine. Quercetin glucosides from onions were absorbed best; the free aglycone was absorbed less well; and the rutinoside form was absorbed least. Apple quercetin is largely bound to galactose, arabinose, xylose and rhamnose rather than glucose, and is therefore absorbed less efficiently than onion quercetin.
What is not absorbed in the small intestine is not wasted. It travels to the colon, where bacteria cleave the sugars and metabolise the flavonoid into smaller phenolic acids, which are then absorbed. Some of the biological activity attributed to dietary polyphenols is probably the activity of these microbial metabolites, not of the parent compounds — a theme picked up on the gut microbiome page.
A caution about dose. Quercetin supplements sell in 500–1000 mg capsules. Total dietary flavonol intake in Western populations is typically measured in tens of milligrams a day. Whatever an apple is doing, it is not doing it at supplement doses — and the epidemiology linking flavonoid intake to lower mortality is based on food intake, not on capsules. Do not read a supplement trial as evidence about apples, or the reverse.
Phloridzin: The Apple Compound That Became a Drug Class
This is the most remarkable story in apple chemistry, and it is entirely documented.
In 1835, Belgian chemists isolated a bitter white crystalline compound from the root bark of the apple tree and named it phlorizin (phloridzin). It was studied for decades as a curiosity. Then, in 1886, the physiologist Josef von Mering observed something strange: giving phlorizin to animals made them excrete large amounts of glucose in their urine, mimicking the appearance of diabetes, without the underlying disease. For a long time that made phlorizin a research tool for producing experimental glycosuria.
The mechanism took another century to nail down. Phlorizin is a competitive inhibitor of the sodium-glucose cotransporters, SGLT1 in the intestine and SGLT2 in the kidney. SGLT2 is responsible for reabsorbing most of the glucose the kidney filters out of the blood. Block it and glucose is excreted rather than reclaimed, and blood glucose falls.
Phlorizin itself was never usable as a drug — it is poorly absorbed orally, is rapidly broken down, and inhibits intestinal SGLT1 as well, causing gastrointestinal trouble. But it was the structural lead. Medicinal chemists used it as the starting scaffold to design selective, orally stable SGLT2 inhibitors, and the result is the modern drug class that includes dapagliflozin, canagliflozin and empagliflozin — medicines that turned out to do far more than lower blood glucose, with major trial benefits in heart failure and chronic kidney disease. Ehrenkranz and colleagues' 2005 review in Diabetes/Metabolism Research and Reviews traces the whole arc.
Now the essential caveat, and it must not be skipped. This history does not mean eating apples has a drug-like effect on blood sugar. Phloridzin is concentrated in the tree's root bark, in the leaves, and in unripe fruit and seeds — not in the flesh you eat. The amount in a dessert apple is small, its oral bioavailability is poor, and it is nowhere near a pharmacological dose. Anyone selling apple-peel extract as a natural SGLT2 inhibitor is trading on a genuine piece of pharmacological history to imply an effect that has not been shown at food doses. Apples appear in the diabetes epidemiology for reasons that are much more likely to involve fibre, satiety and displacement of worse foods — see the blood sugar section.
What the story genuinely illustrates is how often plant chemistry seeds pharmacology: aspirin from willow, metformin from goat's rue, digoxin from foxglove, and SGLT2 inhibitors from apple bark. That is a real and interesting lineage. It is not a dosing recommendation.
What Happens in People
Laboratory antioxidant assays are easy to run and mean very little on their own. The measurements that matter are the ones taken in people.
Endothelial function. Bondonno and colleagues at the University of Western Australia ran a randomised controlled trial published in Free Radical Biology and Medicine in 2012, testing flavonoid-rich apples and nitrate-rich spinach, alone and together, in healthy men and women. Apples augmented nitric oxide status and improved endothelial function — the ability of blood vessels to dilate appropriately, an early and meaningful marker of vascular health. This is a genuine acute human effect from a whole food.
Vascular markers over eight weeks. The Reading trial described on the cholesterol page used a proanthocyanidin-rich variety and found improved endothelium-dependent microvascular vasodilation and lower ICAM-1 against a sugar-matched control drink. Because the control was matched for sugar and energy, that result cannot be attributed to calories — the fibre, the polyphenols, or both are doing it.
Long-term flavonoid epidemiology. Bondonno and colleagues' 2019 analysis of the Danish Diet, Cancer and Health Cohort in Nature Communications found that higher total flavonoid intake was associated with lower all-cause mortality, with the association strongest in smokers and heavy drinkers. Knekt's earlier Finnish cohort work in the American Journal of Clinical Nutrition in 2002 found flavonoid intake — and apple intake specifically — inversely associated with several chronic disease outcomes. Hodgson and colleagues reported an inverse association between apple intake and all-cause mortality in elderly Australian women in the British Journal of Nutrition in 2016.
These are observational associations, and they must be read as such. People who eat more apples differ systematically from people who do not — in education, smoking, exercise, and overall diet quality. Statistical adjustment helps and never fully solves it. The history page covers the study that makes this point most memorably: a nationally representative US survey found daily apple eaters visited doctors less often in the raw comparison, and the association vanished once education and smoking were accounted for.
Ursolic acid deserves a brief, carefully bounded mention. It is a triterpenoid concentrated in apple peel wax, and a 2011 study in Cell Metabolism identified it by screening for compounds that reverse the gene-expression signature of muscle atrophy, then showed it increased muscle mass in mice. That is genuinely interesting mouse work. It is not evidence that eating apple skins builds muscle, and it is regularly misrepresented as such in supplement marketing.
Browning, Cooking, Storage and Juice
The brown colour a cut apple turns is a direct visual readout of polyphenol chemistry. When you cut the fruit you rupture cells and bring the enzyme polyphenol oxidase into contact with polyphenols that were previously kept separate from it. With oxygen present, the enzyme oxidises those polyphenols into quinones, which polymerise into brown pigments. In short: browning consumes the polyphenols.
Practical consequences:
- Cut apples just before eating. A sliced apple sitting out for hours has oxidised part of what you wanted.
- Lemon juice works — vitamin C is a competing substrate for oxidation and the acid slows the enzyme. It genuinely preserves polyphenol content, not just appearance.
- Heat destroys the enzyme. Blanching or cooking inactivates polyphenol oxidase, which is why cooked apple does not keep browning. Cooking loses some polyphenols to heat and leaching, but stops the enzymatic destruction.
- Non-browning apples exist. Arctic apples are genetically engineered to suppress polyphenol oxidase expression. They resist browning because there is less enzyme, not because there are fewer polyphenols — a distinction worth stating accurately either way you feel about the technology.
On processing:
- Whole raw apple with skin — the reference standard.
- Baked or stewed with the skin on — some heat-sensitive loss, but the peel compounds are still in the dish rather than in the bin. A good option, and the one to choose if raw apples make your mouth itch (see the allergy section).
- Applesauce with skins — retains much of the polyphenol content; loses the physical structure that produces satiety.
- Cloudy juice — retains more polyphenols than clear juice, because the particulate matter it keeps is partly peel-derived.
- Clear juice — commercially clarified specifically to remove the cloud, and much of the polyphenol content goes with it. Combined with the loss of pectin, this is why clear juice behaves so differently from fruit.
- Apple pomace — the pressing residue, rich in both fibre and peel polyphenols. It is largely a by-product fed to livestock; it performed best of all in the Danish LDL trial, which is a small indictment of how apples are processed.
- Dried apple — retains a good deal, though heat and long air-drying oxidise some. Fine in small quantities; concentrated sugar makes it easy to overeat.
Wash It, Do Not Peel It
The commonest reason people peel apples is worry about pesticide residue — and it is the wrong solution to a real question. Peeling does remove surface residue, but it also removes the peel's polyphenols, part of the fibre, and every anthocyanin in the fruit. You are trading a well-documented benefit for a poorly quantified risk.
The safety page works through what the residue monitoring data actually show, but the summary for this page is short: wash apples under running water, rubbing the surface with your hands or a brush for twenty to thirty seconds. Plain running water removes a large fraction of surface residue and, importantly, most surface microbes. Commercial produce washes have not been shown to outperform this meaningfully. Baking soda solution has been shown in laboratory work to remove certain surface pesticides more effectively than plain water, if you want to go further. If pesticide exposure is a serious concern for you, buying organic apples is a more logical response than peeling conventional ones — you keep the peel and its compounds.
There are two legitimate reasons to peel: an oral allergy syndrome reaction driven by peel-concentrated allergens, and cooking recipes that genuinely need it. Neither is a nutritional reason.
Getting the Most From Apple Polyphenols
- Eat the skin. This is the whole page in three words.
- Eat the whole fruit rather than drinking it. Clear juice loses both the pectin and much of the polyphenol content.
- Vary your varieties, and favour tart, astringent, heritage and red-skinned apples when you have the choice.
- Buy in season and locally when you can. Freshly harvested fruit has had less storage time to lose polyphenols.
- Cut just before eating, or acidulate with lemon juice if you are prepping ahead.
- Do not chase supplements. Apple polyphenol extracts and quercetin capsules deliver a dose the epidemiology says nothing about, without the fibre, water or bulk.
- Combine, do not isolate. An animal study of apple pectin and apple polyphenol concentrate found the two together outperformed either alone on lipids and cecal fermentation — a mouse-level result that nevertheless fits the general pattern that whole foods beat their fractions.
- Eat other polyphenol foods too. Onions for quercetin, green tea for catechins, blueberries and cranberries for anthocyanins and proanthocyanidins. Apples are a good contributor to a varied intake, not a category unto themselves.
Key Research Papers
Author names, titles and journals are plain text; only the DOI or PMID is a link. Every DOI below was verified against Crossref before publication. Animal and cell-culture studies are labelled as such.
- Wolfe K, Wu X, Liu RH. Antioxidant activity of apple peels. Journal of Agricultural and Food Chemistry. 2003;51(3):609-614. — doi:10.1021/jf020782a
- Boyer J, Liu RH. Apple phytochemicals and their health benefits. Nutrition Journal. 2004;3:5. — doi:10.1186/1475-2891-3-5
- Vrhovsek U, Rigo A, Tonon D, Mattivi F. Quantitation of polyphenols in different apple varieties. Journal of Agricultural and Food Chemistry. 2004;52(21):6532-6538. — doi:10.1021/jf049317z
- Kschonsek J, Wolfram T, Stöckl A, Böhm V. Polyphenolic compounds analysis of old and new apple cultivars and contribution of polyphenolic profile to the in vitro antioxidant capacity. Antioxidants. 2018;7(1):20. — doi:10.3390/antiox7010020
- Wojdyło A, Oszmiański J, Laskowski P. Polyphenolic compounds and antioxidant activity of new and old apple varieties. Journal of Agricultural and Food Chemistry. 2008;56(15):6520-6530. — doi:10.1021/jf800510j
- Hollman PCH, de Vries JHM, van Leeuwen SD, Mengelers MJB, Katan MB. Absorption of dietary quercetin glycosides and quercetin in healthy ileostomy volunteers. The American Journal of Clinical Nutrition. 1995;62(6):1276-1282. — doi:10.1093/ajcn/62.6.1276
- Boots AW, Haenen GRMM, Bast A. Health effects of quercetin: from antioxidant to nutraceutical. European Journal of Pharmacology. 2008;585(2-3):325-337. — doi:10.1016/j.ejphar.2008.03.008
- Guo Y, Bruno RS. Endogenous and exogenous mediators of quercetin bioavailability. The Journal of Nutritional Biochemistry. 2015;26(3):201-210. — doi:10.1016/j.jnutbio.2014.10.008
- Ehrenkranz JRL, Lewis NG, Kahn CR, Roth J. Phlorizin: a review. Diabetes/Metabolism Research and Reviews. 2005;21(1):31-38. — doi:10.1002/dmrr.532
- Bondonno CP, Yang X, Croft KD, et al. Flavonoid-rich apples and nitrate-rich spinach augment nitric oxide status and improve endothelial function in healthy men and women: a randomized controlled trial. Free Radical Biology and Medicine. 2012;52(1):95-102. — doi:10.1016/j.freeradbiomed.2011.09.028
- Bondonno NP, Dalgaard F, Kyrø C, et al. Flavonoid intake is associated with lower mortality in the Danish Diet Cancer and Health Cohort. Nature Communications. 2019;10:3651. — doi:10.1038/s41467-019-11622-x
- Knekt P, Kumpulainen J, Järvinen R, et al. Flavonoid intake and risk of chronic diseases. The American Journal of Clinical Nutrition. 2002;76(3):560-568. — doi:10.1093/ajcn/76.3.560
- Hodgson JM, Prince RL, Woodman RJ, et al. Apple intake is inversely associated with all-cause and disease-specific mortality in elderly women. British Journal of Nutrition. 2016;115(5):860-867. — doi:10.1017/S0007114515005231
- Kunkel SD, Suneja M, Ebert SM, et al. mRNA expression signatures of human skeletal muscle atrophy identify a natural compound that increases muscle mass. Cell Metabolism. 2011;13(6):627-638. — doi:10.1016/j.cmet.2011.03.020 (mouse study)
- Aprikian O, Duclos V, Guyot S, et al. Apple pectin and a polyphenol-rich apple concentrate are more effective together than separately on cecal fermentations and plasma lipids in rats. The Journal of Nutrition. 2003;133(6):1860-1865. — doi:10.1093/jn/133.6.1860 (rat study)
- Apple polyphenols and vascular function in humans — PubMed: apple polyphenols and endothelial function
- Quercetin glycoside bioavailability from food sources — PubMed: quercetin glycoside bioavailability
Connections
- All Food
- Apples
- Apples: Benefits Deep Dive
- Apples: History and Origins
- Quercetin
- Resveratrol
- Onions
- Green Tea
- Blueberries
- Cranberries
- Spinach
- Type 2 Diabetes
- Atherosclerosis