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?
SQUASHES ARE FRUITS
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” family includes 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.
FRUITS VS VEGETABLES: BRING IT ON.
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.
PUMPKINS AND SQUASHES, DECONSTRUCTED
The Cucurbita moschata sub-family (genus) includes 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.
DOES ORANGE = VITAMIN A?
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.
CAN PUMPKINS CURE CANCER?
Well, maybe, if you drop one directly onto a tumor cell…
Pumpkin’s latest claim to fame is curcurmosin, 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.
THE BOTTOM LINE ABOUT PUMPKINS AND SQUASHES
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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REFERENCES
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
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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.