Pumpkin seeds are a popular snack food and healthful source of nutrients like magnesium, zinc, and omega-3 fatty acids. However, some people wonder if the hard outer shell of pumpkin seeds is digestible, or if it comes out whole in stool.
This article provides a detailed look at pumpkin seed shells, digestion, and the evidence on whether the shells are digestible.
What Are Pumpkin Seed Shells Made Of?
Pumpkin seeds are enclosed in a hard, fibrous shell that helps protect the seed. The shell is made up mostly of cellulose and lignin.
Cellulose is an insoluble fiber and the most abundant molecule found in plant cell walls. It cannot be broken down by human digestive enzymes.
Lignin is another fiber found in plant cell walls that resists digestion. Together, lignin and cellulose make the pumpkin seed shell very sturdy and tough.
The shell also contains some protein, fat, and minerals like zinc, magnesium, and copper. However, levels are low compared to the seed contents inside.
How Resistant Starch Impacts Digestion
Digestion starts in the mouth, where chewing helps break food into smaller particles. Saliva contains enzymes like amylase that start breaking down carbs.
After being swallowed, food travels down the esophagus to the stomach. The stomach releases acids and enzymes like pepsin to further break down proteins.
Material moves from the stomach into the small intestine, where most digestion and absorption happen. The pancreas releases digestive juices like lipase to process fats, amylase to break down carbs, and proteases to handle proteins.
The walls of the small intestine also secrete enzymes that finish digesting peptides, carbs, lipids, and nucleic acids into absorbable molecules. These molecules cross the intestinal lining into the bloodstream.
Fiber and resistant starch move through the small intestine mostly intact since human digestive enzymes cannot break them down.
They enter the large intestine, where trillions of gut bacteria ferment them. Fermentation softens stool while producing beneficial compounds like short-chain fatty acids.
Finally, stool passes from the large intestine into the rectum and leaves your system during elimination.
Are Pumpkin Seed Shells Digestible?
Due to their resistant starch and fiber content, pumpkin seed shells are generally not digestible.
Your body lacks the enzymes needed to break down lignin and cellulose, the main components of the shell. Therefore, it passes through your system mostly intact.
Chewing helps break the shell into smaller particles but does not aid digestion. The fiber can absorb water in the intestines to create softer, bulkier stool.
Some small pieces of shell may become lodged in intestinal folds and cracks instead of moving through into stool. Larger pieces are more likely to come out looking similar to how they entered your system.
Evidence on Digestibility
Limited studies have examined pumpkin seed digestibility directly to know if humans can process the shell.
However, many studies confirm the indigestibility of other high fiber, resistant starch foods like seeds, skin, bran cereal, and raw vegetables.
Since pumpkin seed shells are mostly fiber, they likely behave similarly.
Here is some evidence that various fibrous foods pass through the human GI tract undigested:
- In a small study, radiolabeled cellulose given to human subjects was mostly excreted intact rather than digested or fermented.
- Scientists found that up to 61% of bran from certain high fiber cereals appeared visually intact in stool samples.
- Apple and potato skin fed to ileostomy patients came out looking much the same as when they entered the stomach.
- An experiment in 17 people showed that up to 91% of bean seed coats passed through the GI tract undigested.
Overall, research clearly demonstrates the digestion resistance of various plant fibers. This suggests pumpkin seed shells likely move through your system in a similar way.
Do Whole Pumpkin Seeds Pass Undigested?
While their shells are indigestible, the inner contents of pumpkin seeds can be broken down by digestive enzymes and absorbed into your body.
However, some sources claim you should chew the seeds well, or even remove the shells, to get the most benefit.
This is because whole seeds may pass quickly through your system before they fully digest. Evidence on this is mixed.
One study in pigs found 79–96% digestion of pumpkin kernel oil and proteins. The digestibility was lower than hulled pumpkin kernels but still relatively high.
A rat study also reported decent digestive utilization of lipids and proteins from whole pumpkin seeds.
In contrast, an experiment in cows showed lower digestibility for whole versus hulled pumpkin seeds. Just 54–73% of proteins were digested.
Overall, digestibility may be somewhat reduced in whole seeds, but your body can still absorb a majority of the nutrients. Thorough chewing is important to rupture cell walls so that enzymes can access the contents.
Potential Benefits of Pumpkin Seed Shell Fiber
While you don’t digest pumpkin seed shells, their fiber content offers some advantages as it moves through your digestive tract:
Fiber adds bulk and weight to stool. This helps reduce constipation by making elimination easier.
One study in 168 constipation-prone children found significantly increased bowel movement frequency when taking 10 grams of fiber from oat or corn bran per day.
Supports Regular Bowel Function
Adequate fiber intake is linked to lower risks of hemorrhoids, diverticulitis, and other conditions caused by straining during bowel movements.
Eating more fiber may also reduce your risk of colon cancer. In one 9-year study in over 47,000 men, each 10 gram increase in daily fiber intake was tied to a 16% lower risk of colorectal cancer.
High fiber foods like pumpkin seeds help promote feelings of fullness. Fiber can absorb water in your stomach and intestines, leading to increased satiety after meals.
In a study in 240 overweight or obese adults, a high fiber diet decreased hunger and increased fullness hormone levels.
Supports Gut Bacteria
The trillions of bacteria living in your intestines can use pumpkin shell fiber and other resistant starches as a food source.
Fermentation of these fibers creates beneficial short-chain fatty acids and intestinal gases. Both help nourish gut cells and optimize the growth of healthy bacteria.
Potential Digestive Downsides
While pumpkin seed shells likely move through your system undigested, are there any potential downsides to ingesting them?
Studies on the impacts of eating fully indigestible fiber like seed coats are limited. However, some possibilities include:
There are a few reports of seeds causing a bowel obstruction or blockage when eaten whole.
It’s possible for pieces of rigid, fibrous shells to lodge in an intestinal fold or small opening rather than passing through stool.
However, this seems to be very rare, as most people regularly eat seeds with shells without issue.
Nutrient Absorption Issues
Some doctors warn that seeds high in phytic acid like pumpkin seeds could potentially inhibit mineral absorption.
Phytic acid can bind to minerals like zinc, iron, and calcium and prevent their uptake into your bloodstream.
Still, this effect is unlikely to be significant if you eat a varied diet. Just sprinkle pumpkin seeds on dishes rather than eating a large quantity at once.
For some people, a high intake of indigestible fiber may lead to issues like gas, bloating, cramping, or diarrhea.
This is more likely if suddenly increasing your fiber intake rather than ramping up slowly. Drinking plenty of water helps move fiber smoothly through your colon.
How To Eat Pumpkin Seeds
Pumpkin seeds are highly nutritious and safe for most people to eat in moderation.
Chewing the seeds well helps break down the shells as much as possible before swallowing. However, expect portions of shell to pass through stool looking similar to when you ate them.
If you want to remove the shells, you can easily do so by hand. Simply crack off the wider outer shell, separate the inner seed coat, then snap off the bitter white tip.
Here are some healthy ways to enjoy pumpkin seeds:
- Sprinkle shells-on pumpkin seeds onto oatmeal, yogurt, salads, or baked goods
- Process hulled pumpkin seeds into milk, sauces, or spreads
- Blend shells-on pumpkin seeds into trail mixes, granola, or energy bars
- Add roasted pumpkin seeds to cooked dishes like stir-fries, soups, and chilis
Stick to a handful or two per serving if eating the shells. Drink plenty of fluids and up your fiber intake gradually to keep your digestive system regular.
The Bottom Line
Pumpkin seed shells are made up mostly of lignin and cellulose, two types of fiber that cannot be broken down by human digestive enzymes.
Therefore, the shells likely pass through your intestines undigested before being eliminated in stool.
While the shells themselves don’t provide any nutritional value, their fiber content can promote regularity, satiety, and healthy gut bacteria.
Pumpkin seeds can be eaten shells-on in moderation as part of a healthy, high fiber diet. Pair with adequate hydration and slowly increase fiber intake to avoid digestive issues.