The Cancer Research
Growing research supports including foods such as winter squash, that are rich in beta-carotene and other carotenoids, as part of healthful eating habits that can reduce cancer risk. In population studies, higher blood levels of carotenoids are linked with a lower risk of overall cancer. Recent studies have started investigating genetic differences that influence people’s absorption of these important compounds.
Interpreting the data
After a systematic review of the global scientific literature, AICR/WCRF analyzed how vegetables and its nutrients affect the risk of developing cancer.
- Evidence categorized as “convincing” or “probable” means there is strong research showing a causal relationship to cancer—either decreasing or increasing the risk. The research must include quality human studies that meet specific criteria and biological explanations for the findings.
- A convincing or probable judgment is strong enough to justify recommendations.
- There is probable evidence that a diet high in non-starchy vegetables and fruits DECREASES the risk of:
• aerodigestive cancers overall (such as esophageal; mouth, pharynx and larynx; lung; stomach and colorectal cancers).
- Evidence categorized as “limited suggestive” means results are generally consistent in overall conclusions, but it’s rarely strong enough to justify recommendations to reduce the risk of cancer.
- Limited evidence suggests that non-starchy vegetables may DECREASE the risk of:
- Estrogen receptor-negative (ER-) breast cancer
- Bladder cancer
- Limited evidence suggests that foods containing carotenoids may DECREASE the risk of:
- Lung and estrogen receptor-negative (ER-) breast cancers
Limited evidence suggests that foods containing vitamin C may DECREASE the risk of:
- Lung cancer (in people who smoke) and colon cancer
- Limited evidence suggests that foods containing beta-carotene may DECREASE the risk of:
• Lung cancer
Ongoing Areas of Investigation
Laboratory research is extensive on squash compounds such as beta-carotene and vitamin C.
Carotenoids act as antioxidants themselves and stimulate the body’s own antioxidant defenses, decreasing free radical damage to DNA that can lead to cancer. Very high levels in cell studies, however, can have an opposite effect, promoting damage from oxidation.
Beta-carotene and alpha-carotene promote cell-to-cell communication that helps control cell growth. These carotenoids also increase carcinogen-metabolizing enzymes and stimulate the self-destruction of abnormal cells. The body uses beta-carotene and alpha-carotene to form vitamin A, which helps protect against cancer through the immune system and expression of genes that regulate cell growth.
Lutein and zeaxanthin are antioxidants that are especially concentrated in the eyes, brain and skin. Limited animal studies suggest that they may help protect against skin cancer
Beta-cryptoxanthin functions as an antioxidant in cell studies, protecting against free radical damage to DNA that can lead to cancer. Some cell studies suggest that it may help decrease inflammation. Beta-cryptoxanthin has decreased growth of several types of cancer cells in limited studies of isolated cells and animals. The body uses beta-cryptoxanthin to form vitamin A, which helps protect against cancer through the immune system and expression of genes that regulate cell growth.
Vitamin C is a powerful antioxidant. In lab studies, it protects cells’ DNA by trapping free radicals, and it helps renew the antioxidant ability of Vitamin E. In cell studies, vitamin C also inhibits formation of carcinogens and supports the immune system.
Human studies related to winter squash and cancer risk compare groups of people who consume relatively high and low amounts of total vegetables, squash, and/or levels of carotenoids and vitamin C.
People who eat more vegetables and fruits have a lower risk of a wide range of cancers. This probably reflects combined protection from many different nutrients and compounds they contain.
Carotenoids: In population studies, higher blood levels of total carotenoids and of beta-carotene are linked with a lower risk of overall cancer. Blood levels may more accurately reflect the consumption of carotenoid-rich foods than diet questionnaires, and they include differences in how much is absorbed from food. However, it may be that the lower cancer risk is seen because blood levels of these compounds is recognized as a signal of greater overall vegetable and fruit consumption.
- Lung cancer: Population studies link higher dietary and blood levels of beta-carotene or total carotenoids with a lower risk of lung cancer. Larger studies now show protection less clearly than earlier studies, and the AICR/WCRF Third Expert Report categorizes this link as Limited Suggestive. Additional research is needed.
- Breast cancer: Some population studies also link higher levels of carotenoids (including beta-carotene) in the diet to lower the risk of breast cancer, mainly to estrogen receptor-negative (ER-) forms of breast cancer. The AICR/WCRF Third Expert Report categorized this link as Limited Suggestive. It is possible that an effect of carotenoids on ER-positive (ER+) tumors is simply masked by the hormonal influences that dominate the risk of ER+ breast cancer.
Vitamin C: Population studies comparing people with higher and lower levels of vitamin C in their diets, and especially levels circulating in their blood, link higher amounts with lower overall risk of cancer. This effect is larger comparing people with very low levels to moderately increased levels than comparing people with moderate and much higher levels. Higher levels of vitamin C from foods are linked with a lower risk of lung cancer among people who smoke tobacco, although not in those who used to smoke or who have never smoked. People with more vitamin C in their diet are also less likely to develop colon cancer. That’s even after adjusting for other risk factors for colon cancer, such as alcohol, red meat and tobacco. Evidence for both lung and colon cancer is rated as Limited Suggestive in the AICR/WCRF Third Expert Report, and more research is needed.
Tips for Selection and Preparation
- When selecting squash such as acorn, compare equal sizes and purchase the heavier one to get more edible flesh.
- Inspect squash to find those with a hard rind and no soft spots.
- For canned pumpkin, make sure you buy pure pumpkin and not high-sugar pumpkin pie mix.
- For most winter squash, it’s easy to cut them in half, remove seeds and bake.
- Butternut, delicata and spaghetti squash can be peeled and cut in cubes for stir-fries, casseroles, stews, pasta, soup, and even enchiladas.
- Roast in the oven alone or with other vegetables, drizzled with a bit of olive oil.
- Baked squash halves are perfect for stuffing. Try apples, raisins and cinnamon, or spinach or kale with soy and ginger.
- Steam, microwave or bake winter squash and then purée. Enjoy it as a side dish or to thicken and flavor soups and stews. Or use canned or frozen squash or pumpkin that is already puréed.
- Top “strings” of spaghetti squash with pasta sauce.
- Seeds from winter squash and pumpkin make a great snack or salad topping. Rinse, air dry, add a touch of oil, and bake at 350°F for 15-20 minutes.
- Borel P, Desmarchelier C. Bioavailability of Fat-Soluble Vitamins and Phytochemicals in Humans: Effects of Genetic Variation. Annual Review of Nutrition. 2018;38(1):69-96.
- World Cancer Research Fund / American Institute for Cancer Research. Continuous Update Project Expert Report 2018. Wholegrains, vegetables and fruit and the risk of cancer. Available at: dietandcancerreport.org.
- Bohn T. Carotenoids, Chronic Disease Prevention and Dietary Recommendations. International Journal for Vitamin and Nutrition Research. 2017;87(3-4):121-130.
- Kaulmann A, Bohn T. Carotenoids, inflammation, and oxidative stress–implications of cellular signaling pathways and relation to chronic disease prevention. Nutr Res. 2014;34(11):907-929.
- Bouayed J, Bohn T. Exogenous antioxidants – Double-edged swords in cellular redox state: Health beneficial effects at physiologic doses versus deleterious effects at high doses. Oxid Med Cell Longev. 2010;3(4):228-237.
- Buscemi S, Corleo D, Di Pace F, Petroni ML, Satriano A, Marchesini G. The Effect of Lutein on Eye and Extra-Eye Health. Nutrients. 2018;10(9):1321.
- Thomas SE, Johnson EJ. Xanthophylls. Advances in Nutrition. 2018;9(2):160-162.
- Moran NE, Mohn ES, Hason N, Erdman JW, Jr, Johnson EJ. Intrinsic and Extrinsic Factors Impacting Absorption, Metabolism, and Health Effects of Dietary Carotenoids. Advances in Nutrition. 2018;9(4):465-492.
- Burri BJ, La Frano MR, Zhu C. Absorption, metabolism, and functions of β-cryptoxanthin. Nutr Rev. 2016;74(2):69-82.
- Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, D.C. : National Academies Press;2000.
- Lu JM, Lin PH, Yao Q, Chen C. Chemical and molecular mechanisms of antioxidants: experimental approaches and model systems. Journal of cellular and molecular medicine. 2010;14(4):840–860.
- Aune D. Plant Foods, Antioxidant Biomarkers, and the Risk of Cardiovascular Disease, Cancer, and Mortality: A Review of the Evidence. Advances in Nutrition. 2019;10(Supplement_4):S404-S421.
- Aune D, Keum N, Giovannucci E, et al. Dietary intake and blood concentrations of antioxidants and the risk of cardiovascular disease, total cancer, and all-cause mortality: a systematic review and dose-response meta-analysis of prospective studies. Am J Clin Nutr. 2018;108(5):1069-1091.
- World Cancer Research Fund/American Institute of Cancer Research. Continuous Update Project Expert Report 2018. Other dietary exposures and the risk of cancer. Available at dietandcancerreport.com.
- Farvid MS, Chen WY, Rosner BA, Tamimi RM, Willett WC, Eliassen AH. Fruit and vegetable consumption and breast cancer incidence: Repeated measures over 30 years of follow-up. International Journal of Cancer. 2019;144:1496-1510.
- Balic A, Mokos M. Do We Utilize Our Knowledge of the Skin Protective Effects of Carotenoids Enough? Antioxidants (Basel). 2019;8(8):259.