A Lab Tested the Peels We Throw Out. In 10 of 12 Foods, They Held More Fiber.

Every kitchen has a bin moment: the watermelon rind after the slices are gone, the knobbly pineapple skin and its woody core, the squash seeds scraped into the trash. We read a study published this week that put those scraps through a fibre lab, set its numbers against the U.S. food tables, and checked what the extra fibre would actually do once it reached the gut. The question people type is simple: are peels high in fiber?

Below: what the King’s College London team measured and the one vegetable that broke the pattern, why fibre from skins and stalks behaves the way it does in the bowel, and which scraps are realistic to eat rather than a dare.

The study: The discarded parts beat the flesh on fibre in 10 of 12 fruits and vegetables

Researchers at King’s College London measured total dietary fibre in two parts of 12 types of raw fresh produce: the part people usually eat and the part most people throw away, which the team calls the “root-to-tip” part (peels, skins, seeds, stalks, rinds and cores). The paper, with research assistant Georgia Browne as first author and project lead Dr Rachel Gibson and Dr Balazs Bajka among the authors, appeared on October 29, 2026 in the International Journal of Food Sciences and Nutrition (PubMed 42956817; journal page).

According to the study abstract, the fibre in the discarded parts ranged from 2.10 g per 100 g in watermelon skin to 9.25 g per 100 g in butternut squash skin and seeds, and it exceeded the fibre in the commonly eaten part in 10 of the 12 items. In its October 30 release, King’s put the watermelon rind figure at roughly the fibre in a slice of wholemeal bread, and the squash skin and seeds at nearly as much as a bowl of bran flakes.

The largest gap King’s reported was pineapple: the skin and core “contained 453.7% (five times) more fibre than the flesh,” the release says. Our arithmetic: 453.7 percent more means about five and a half times as much. For squash, a rough cross-check lines up: USDA FoodData Central lists raw butternut squash at 2.0 g of fibre per 100 g, so the skin-and-seed figure is about 4.6 times that, by our arithmetic, with the caveat that the two numbers come from different labs and samples. An earlier write-up by Gibson and Browne for the British Dietetic Association in September 2025 described butternut squash seeds as almost 700 percent higher in fibre than the flesh.

The scraps are not a sliver of what we buy. The discarded parts made up between 25 and 48 percent of the total weight of each fruit or vegetable, King’s said, and Gibson summed it up in the release: “That means people are discarding a significant portion of the produce we buy.”

The exception: Broccoli stalks were the one vegetable where the florets won

Broccoli broke the pattern. Its stalks contained 28.4 percent less fibre than the florets, according to King’s, the same exception Gibson and Browne flagged in their 2025 BDA article. The abstract counts two items out of 12 where the discarded part did not come out ahead; the release names only broccoli, and because the full paper was not open to us, we are not naming the second.

Less is not none. A stalk still carries fibre, which is why the team’s own suggestions include using broccoli stalks in stir-fries rather than binning them, alongside leaving the skins on kiwi fruit and adding bell pepper seeds to dishes, per the release.

The gut part: Skin and stalk fibre is the bulking kind, and the bowel notices it

Peels, rinds and stalks are mostly plant cell wall, the tough structural material a fruit or vegetable uses to hold its shape. Our reading: much of that is the less fermentable kind of fibre, and that matters for how the bowel responds. A 2016 review of intervention trials in Nutrients by de Vries and colleagues found that less fermentable fibres increased stool weight more than easily fermented ones, and that vegetable fibre added as much stool weight as cereal fibre, both more than fruit fibre (PubMed 26950143).

The same review found that in people whose gut transit took 48 hours or longer, each extra gram of cereal, fruit or vegetable fibre shortened transit by roughly 30 minutes; in people already under 48 hours, fibre did not change transit time (PubMed 26950143). The part of fibre that does get fermented feeds gut bacteria, which turn it into short-chain fatty acids, a fuel for the cells lining the colon, as a 2018 review in Cell Host & Microbe by Makki and colleagues sets out (PubMed 29902436).

One caution from the authors themselves: all 12 items were tested raw, and cooking can change both how much fibre is measured and how easily it is digested, so the figures describe the raw scrap, not a roasted or simmered one.

The everyday version: Which scraps are realistic to eat, and how far they move the number

Scale is the honest test. The U.S. Nutrition Facts label uses a Daily Value of 28 g of fiber, according to the FDA. By our arithmetic, 100 g of raw watermelon rind at 2.10 g would cover about 7.5 percent of that, and 100 g of squash skin and seeds at 9.25 g about a third. In the UK, only 4 percent of adults reach the 30 g recommendation and the average is 16.4 g a day, King’s said, a gap of about 13.6 g by our arithmetic.

Nobody is being asked to chew through a pineapple skin. The easy wins are the ones the team named: kiwi eaten skin-on (USDA lists green kiwifruit flesh at 3.0 g of fibre per 100 g before the skin is counted), squash roasted with its skin and seeds, broccoli stalks sliced into a stir-fry, pepper seeds left in. King’s first tested the idea in its own campus cafés, where 29 root-to-tip recipes went on the menu in spring 2025 and the team’s early figures showed a meal’s fibre rising by up to around 15 percent, according to the project page.

The study’s focus group of 40 university staff and students also explains why this may stick: participants saw the fibre benefit as a stronger reason to change than cutting food waste, the abstract reports, while naming “mental discomfort” about eating these parts and worries about digestibility as barriers, per the release. Washing produce well before eating the skin is the plain first step. Anyone adding a lot of fibre at once tends to feel it as gas and bloating, which is why a gradual increase with enough fluid is the usual approach.

How we checked this: Sources and method

We read the King’s College London release of October 30, 2026; the study record on PubMed (PMID 42956817, title matched to the paper) and its abstract on Europe PMC; the King’s project page of May 2025; Gibson and Browne’s British Dietetic Association article of September 2025; the de Vries 2016 review (PMID 26950143) and the Makki 2018 review (PMID 29902436), each matched by title; USDA FoodData Central entries 169295 and 168153; and the FDA’s Daily Value table. The paper’s full text and per-item table were not open to us, so this page prints only the figures stated in the abstract and the King’s release. Comparisons with USDA figures and all percentages of the Daily Value are our arithmetic. This article is educational, describes what the cited sources say, and is not medical advice.