It has long been said by many raw foodists that, by cooking vegetables many of the nutrients and enzymes are destroyed thereby rendering cooked vegetables much less nutritious. According to the Journal of Agricultural and Food Chemistry, after having conducted a study, it was found that cooking carrots, courgettes (zucchini), and broccoli is extremely beneficial increasing the level of antioxidants, ferulic acid, and particularly carotenoids. Antioxidants reduce the risk of heart disease, some cancers, and other diseases. Ferulic acid is an antioxidant that boosts the activity of other antioxidants and helps protect the skin along with providing other anti-aging benefits. Carotenoids are also antioxidants that benefit immunity, vision, bone growth, reproductive health, etc (Miglio, 2008).
"According to Consumer Reports, spinach contains oxalates that block absorption of iron and calcium when raw but when boiled up to 40% of the oxalates are destroyed. "A cup of cooked white mushrooms has about twice as much muscle-building potassium, heart-healthy niacin, immune-boosting zinc, and bone-strengthening magnesium as a cup of raw ones. That’s according to the Department of Agriculture’s nutrient database. Even mushrooms considered edible can sometimes contain small amounts of toxins, but they can be destroyed through cooking.
When it comes to asparagus, a study in the International Journal of Food Science & Technology found that cooking these stalks raised the level of six nutrients, including cancer-fighting antioxidants, by more than 16 percent. Another study in the International Journal of Molecular Sciences found that cooking asparagus more than doubled the level of two types of phenolic acid, which some studies have linked to lower cancer rates.
In reference to broccoli, cabbage, and other members of the Brassicaceae family, a 2012 study published in the Journal of Agricultural and Food Chemistry found that "steaming the veggies preserves myrosinase and therefore the cancer-fighting compounds you can get from them (plus other research has found that steaming also preserves these crucifers’ vitamin C). What’s more, a 2018 study from the same journal found that chopping broccoli and letting it sit for 90 minutes before you cook it also allows myrosinase to activate. When the vegetables were subsequently stir-fried, the researchers found, they did contain the cancer-fighting compounds" (Branch, 2019). Additionally, when cooking members of the Brassicaceae family, Indole-3-Carbonal increases. "I3C inhibits the growth of breast, prostate, colon, and cervical cancer cells" (2004).
Branch, J. (2019). Consumer Reports. https://www.consumerreports.org/fruits-vegetables/vegetables-that-are-healthier-cooked/
Miglio, C., et al. (2008). Journal of Agricultural and Food Chemistry. https://pubs.acs.org/doi/abs/10.1021/jf072304b?prevSearch=boiling+frying+steaming&searchHistoryKey=
Sarkar, F. (2004). Journal of Nutrition. https://academic.oup.com/jn/article/134/12/3493S/4688712