November 2, 2026, 8:50 am | Read time: 5 minutes
Whether it’s kiwi skin, broccoli stalks, or pumpkin seeds, many parts of fruits and vegetables end up in the trash during preparation, even though they are generally edible. These leftovers could provide valuable nutrients. Researchers at King’s College London examined how much fiber is in such food scraps and whether consumers would be willing to eat them more often.
Food Scraps Often Contain More Fiber
The study shows: In ten out of twelve fruits and vegetables examined, the parts usually discarded contained more fiber than the parts typically consumed. The difference was particularly noticeable in pineapple, watermelon, and butternut squash. The findings suggest that using such plant parts could help increase fiber intake while reducing food waste.
How Was the Study Conducted?
Researchers at King’s College London examined the so-called “root-to-tip” principle, which involves using as many edible parts of a plant as possible instead of discarding peels, stalks, or leaves.1
In the United Kingdom, at least 30 grams of fiber are recommended–only four percent of adults meet this target. The scientists examined twelve fruits and vegetables, including broccoli, bell peppers, kiwi, pineapple, and watermelon. A chef prepared the foods as they would typically be prepared. The researchers then determined in the lab how much fiber the discarded parts contained. They compared these values with existing nutritional information for the parts usually consumed.
Additionally, between November 2024 and January 2025, they conducted six group discussions with a total of 40 students and staff from King’s College London. They aimed to find out what might motivate people to eat plant parts that are usually discarded.
Where the Most Fiber Is Found
The fiber content of the examined food scraps varied significantly. The highest value was found in the peel, seeds, and inner tissue of butternut squash, with 9.26 grams per 100 grams. This was followed by kiwi skin with 7.96 grams and celeriac peel with 6.79 grams. Bell pepper scraps, as well as pineapple peel and core, also contained relatively high amounts of fiber, each with around 6.70 grams.
However, not all examined scraps were richer in fiber than the parts usually consumed. The broccoli stalk contained about 28 percent less fiber than the florets. For fennel, it was about eleven percent less.
Moreover, a large percentage difference does not automatically mean that a food contains a lot of fiber. For example, watermelon rind provided about five times as much fiber as the flesh, but overall it was only 2.10 grams per 100 grams.
In all examined food scraps, insoluble fibers predominated. These can aid digestion by increasing stool volume and stimulating bowel activity.
Whether people would actually eat such plant parts more often, however, depends not only on their nutrient content. In an accompanying questionnaire, 85 percent of participants stated that taste often or always influences their food choices. For 75 percent, price played an important role, and for 70 percent, health was a factor. Environment and sustainability were less decisive, influencing purchasing behavior only sometimes, rarely, or not at all for 55 percent.
Many participants saw health benefits as a good reason to eat peels, stalks, and leaves more often. However, some doubted that such information would actually change their eating habits. The researchers therefore suspect that people might be more likely to accept such plant parts if they are used naturally in familiar dishes.
Surprise in Calorie Content
In addition to fiber, the researchers also analyzed the energy content. The result: In eight out of twelve varieties, the scraps contained more calories than the parts usually consumed–especially bell pepper scraps, celeriac peel, and butternut squash. The reason lies mainly in nutrient-rich seeds and seed inclusions. However, since the calories are bound in solid cell structures, the body probably does not fully absorb them.
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Study Assessment
The researchers used a recognized laboratory method for their investigation. However, only one specimen of each fruit and vegetable was analyzed–for cauliflower, the waste proportion had to be estimated based on broccoli data due to a lack of delivered florets. Differences due to origin, variety, or ripeness were thus not considered. Additionally, the comparison values came from a nutritional database and were not measured in the same laboratory experiment.
It also remains unclear how cooking or other preparation methods affect the fiber. The survey results cannot be easily generalized to the entire population, as only university members participated.
Another point is safety: Peels and other plant parts can be contaminated with pesticides, heavy metals, or natural toxins. Whether such substances were present in the examined samples was not tested.
Also interesting: Fibermaxxing–are fibers more important than protein?
What Previous Studies Show
A study from Denmark suggests that discarded plant parts can make a difference.2 There, 23 healthy participants were given whole apples, apple pomace, or apple juice in phases. Pomace is the press residue from peel and core left over from juice production. After consuming apples and pomace, total and LDL cholesterol tended to be lower, while clear juice increased it by about seven percent. The researchers attribute this to the fiber. However, the study was small.
Whether people would eat such scraps was examined in another study by King’s College London.3 In 2025, the university cafeterias offered root-to-tip dishes. Of 727 surveyed students and staff, about 38 percent chose such a dish. More than three-quarters of them would eat it again. Taste and health were especially important to them. Again, only university members were surveyed, and so far, only a summary is available.