Pumpkins, Squashes, Vitamin A and Cancer

pumpkins and squashes

We think of fruits as being sweet and juicy, yet the two fruits Americans love to eat most on Thanksgiving are neither.  Intensely colorful and tart cranberries, despite being so dry and sour that they taste absolutely terrible in their natural state, are always invited to Thanksgiving dinner.  But cranberries are not the only unusual fruits donning the Thanksgiving dinner table–what fall feast would be complete without a bright orange member of the squash family?




We think of squashes as vegetables because they are dry and starchy rather than sweet and juicy, but because they contain seeds, they are actually fruits in disguise. The “Cucurbita” include not only squashes, pumpkins, and gourds, but also zucchini, yellow squash, cucumbers, and melons.  Let’s take a closer look at these vibrant autumn unfruity fruits.


Those of you familiar with my philosophy about vegetables know that I view them as untrustworthy and deserving of suspicion (see my vegetables page)—but what about fruits?  Plants which rely on animals to disperse their seeds tend to wrap their seeds in fruits to entice hungry mobiles.  If all goes as planned, an unsuspecting creature will eat the seedy fruit, walk away, digest the fruit, and later deposit the seeds, along with a nice meadow-muffin of natural fertilizer, in just the right spot so they can germinate and grow into mature plants.  Not the most romantic method of reproduction, but so effective that it’s been around for hundreds of millions of years. Plants want and need animals to eat their fruits, so it would not be in their best interest to invest fruits with toxic compounds—it would be unwise to sicken or kill your reproductive helpers.  Therefore, hypothetically speaking, the chemicals in fruits should be gentler on our systems than those in vegetables.

Pumpkins and other squashes are not particularly appealing fruits in their natural state–they generally need to be cooked in order to be palatable. Of course plants in the squash family also have vegetable parts–stems, roots, and leaves, but we don’t tend to eat those parts, probably because they don’t taste very good.  In fact, even many fruits in this family don’t taste very good–when was the last time you enjoyed a nice steaming helping of gourd?  There are many plants and plant parts that we do not and should not eat, whereas nearly all animals are completely edible.


The Cucurbita moschata include several varieties of pumpkins and squashes, including butternut squash.  The poor pumpkin family–they have had every inch of their beings, from stem to seeds, violated by scientists who hope to discover magical, life-saving ingredients.  We look today specifically at the fruit flesh of pumpkins and squashes, since that’s what we like to eat on Thanksgiving.


Carotenoids (such as alpha and beta carotene) are responsible for the beautiful orange color of pumpkins and squashes.  We think of orange foods like carrots and sweet potatoes as excellent sources of vitamin A, but you may be surprised to learn that plant foods contain no vitamin A at all, at least as far as humans are concerned.  Herbivores (vegan animals) and many other animals possess an enzyme that can convert carotenoids to vitamin A, but we don’t.  In our bodies, plant carotenoids have to jump through a series of biological hoops in order to become the active form of vitamin A (retinol) that our bodies can use.

Carotenoids in fibrous foods like pumpkin are trapped within plant cell walls made of indigestible cellulose (insoluble fiber), so that even after cooking and digesting pumpkin, a maximum of only 25% of the carotenoids are freed from the rigid matrix of plants.  Fat is required for the absorption of these compounds (pass the butter please), but even if we eat enough fat with our pumpkin or squash, we can only absorb 8% of their carotenoids.  Furthermore, once inside our bloodstream, only about 50% of the absorbed beta carotene is converted into active vitamin A.  So when you see vitamin A content listed on the side of a can of pumpkin, multiply that by 25% and then by another 8% and then by another 50% and what you are left with is only about 1.5% bioavailable vitamin A, whereas vitamin A from animal sources is in the form of “retinyl esters”, not carotenoids, and we easily convert these to active vitamin A, therefore they are completely bioavailable.  Good food sources of vitamin A include egg yolks and dairy products, but the best source of vitamin A is actually liver.  Unfortunately not a pretty orange vegetable, but hey, looks aren’t everything.


Well, maybe, if you drop one directly onto a tumor cell…

Pumpkin’s latest claim to fame is cucurmosin, a chemical within pumpkin flesh that is toxic to cancer cells under laboratory conditions.  Cucurmosin is a “Ribosome Inactivating Protein”, or RIP for short.  RIP’s are aptly named because they are deadly.  RIP’s kill cells by stopping their ribosomes dead in their tracks.  Ribosomes are the protein manufacturing assembly lines within all cells (not just cancerous cells), so if they are taken out of commission, all cellular activity grinds to a screeching halt and cells die.  Two especially potent RIP’s are notorious poisons—Shiga toxin (from bacteria) and ricin (from castor beans).  Luckily, cucurmosin is a kinder, gentler variety of RIP, because it has a harder time breaking into cells to get at their ribosomes.  However, when it does, it is just as merciless as other RIP’s. Pumpkins and squashes use cucurmosin to ward off invaders such as bacteria and viruses.  But wait…healthy cells also contain ribosomes—does cucurmosin kill them too?

It may be reassuring to know that, in laboratory studies, it takes a much higher dose of cucurmosin to kill healthy cells than to kill cancer cells.  Also reassuring is that cooking probably destroys cucurmosin.  How much cucurmosin is there in raw pumpkin?  Is it destroyed by digestion?  If not, how much of it do we absorb?  We don’t know.


Happily I have found no evidence that compounds within the fruity flesh of pumpkins and related orange squashes are harmful to our health, provided we eat them cooked.  This is in keeping with the theory that the fruity parts of plants are less likely to irritate our bodies than the vegetable bodies of plants.

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Coming soon to the Food and Health Blog:

Ketogenic Diets for Weight Loss and Health—a review of The Art and Science of Low Carbohydrate Living by Dr.s Phinney and Volek.

The role of diet in eating disorders.

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Barbieri L et al.  Ribosome-inactivating proteins in edible plants and purification and characterization of a new ribosome-inactivating protein from Cucurbita moschata.  Biochimica et Biophysica Accta 2006; 1760: 783-792.

Fleshman MK et al. Carotene and novel apocarotenoid concentrations in orange-fleshed Cucumis melo melons: determinations of β-carotene bioaccessibility and bioavailability. J Agric Food Chem 2011; 59(9):4448-54.

Fleshman MK et al.  An LC/MS method for d8-beta-carotene and d4-retinyl esters:  beta -carotene absorption and its conversion to vitamin A in humans. Journal of Lipid Research  2012; 53: 820-827.

Hou X et al. Atomic resolution structure of cucurmosin, a novel type 1 ribosome-inactivating protein from the sarcocarp of Cucurbita moschata. J Struct Biol  2008; 164(1): 81-7.

Nielsen K and Boston RS. Ribosome-inactivating proteins:  a plant perspective. Annu. Rev. Plant Physiol. Plant Mol. Biol. 2001. 52:785–816

Stirpe F and Battelli MG.  Ribosome-inactivating proteins: progress and problems.  Cell Mol Life Sci 2006;  63: 1850–1866.

Supriya V et al.  Determination of bioaccessibility of b-carotene in vegetables by in vitro methods.  Mol Nutr Food Res 2006; 50: 1047-1052.

Tang G.  Bioconversion of dietary provitamin A carotenoids to vitamin A in humans. Am J Clin Nutr 2010; 91(suppl):1468S–73S.

Yadav M et al.  Medicinal and biological potential of pumpkin: an updated review.  Nutrition Research Reviews 2010; 23:184­190.




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  • Someone, Somewhere

    Very interesting post, Dr. Ede.

    Question: If plants “want” animals to eat their fruits, to disperse their seeds, how can we explain the unpalatability of most Cucurbita, compared to most fruits? I believe I’ve heard that some fruits are “designed” to be eaten by certain animals (e.g., certain berries are toxic to humans but not birds, who will disperse the seeds more widely). Could that be the case with many Cucurbita, that they are “designed” to be eaten by animals other than ourselves?

    Another hypothesis I can think of is that Cucurbita are designed to be as palatable as possible, but their nutrient content is limited by the relative lack of sunlight and heat available in the winter (after all, most Cucurbita are eaten by humans in the fall and winter, when sweet fruits are less plentiful). Perhaps non-human animals—who can’t cook—are more likely to eat non-sweet Cucurbita when are extremely ripe (perhaps already a bit rotten?) or partially digested by bacteria?

    • http://diagnosisdiet.com/ Dr. Ede

      Hi SS

      Interesting ideas–just spent some time googling about animals and pumpkins and find lots of evidence that a wide variety of animals enjoy eating pumpkins (even before soft and beginning to rot), so they must enjoy the taste of raw pumpkin more than we do? It is absolutely true that certain plants go out of their way to put substances in their fruits to attract one kind of predator and repel another (chili peppers are the best example–there have been excellent experiments supporting the notion that birds are the best seed carriers for these plants and are not affected by their spicy taste the way we are).

      • Someone, Somewhere

        Interesting. Thanks for the info.

        • Hey

          ”Cucurbita” is a genus, not family. ”Cucurbita moschata” is a species, not a genus.

          • Hey

            and it’s cucurmosin, not curcurmosin

          • http://diagnosisdiet.com/ Dr. Ede

            Dear Hey,
            Thanks for the edits:)

  • Carol L

    I was flicking through the tv today and Dr. Oz had a dietician telling people to eat a 1/4 carrot to get their daily vitamin A. I thought you would have something on the bioavailability of Vitamin A fom plant vs animal sources, and of course you do. Thanks for making such a great searchable site. Off to find some liver.

    • http://diagnosisdiet.com/ Dr. Ede

      Hi Dr. L!
      Oh, I’m so glad to know that you found exactly what you were looking for:) Enjoy your liver and have a nice weekend! Oh, are you going to AHS this summer?

      • Carol L

        Go th AHS? …that will be a negotiation with the family.

  • Julie

    hi Dr. Ede,
    I’m curious about your statement “Happily I have found no evidence that compounds within the fruity flesh of pumpkins and related orange squashes are harmful to our health, provided we eat them cooked.” …

    Is there evidence that raw squash is unhealthy?

    • http://diagnosisdiet.com/ Dr. Ede

      Hi Julie
      Thanks for your pumpkin question! I am referring to the cucurmosin in fresh, uncooked pumpkin, which has the potential to be damaging to our ribosomes, but which is destroyed in the cooking process.