“Take our quiz to find out.
If you subscribe to the tenets of the raw food diet, or even if you don’t, you may have heard the phrase, “When you cook it, you kill it.” Many people believe that applying heat to vegetables — whether by sautéing, boiling, steaming, frying, roasting or grilling — zaps their nutrition.
But Emily Ho, a professor of nutrition and director of the Linus Pauling Institute at Oregon State University, said that while it’s true that certain cooking methods can change the amount of nutrients you get from many vegetables, it’s not always for the worse. And in fact, heat from cooking can enhance some nutrients and other beneficial compounds your body will absorb.
Think you know which vegetables are better eaten raw versus cooked? Take our quiz to find out.
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There’s a reason this leafy green is called “the Popeye vegetable,” Dr. Ho said. It’s packed with vitamin C, magnesium, vitamin B6, iron and calcium. And when eaten cooked, you’ll absorb more iron and calcium. Spinach is loaded with oxalate, a compound that binds to and blocks the absorption of calcium and iron in the gut. But high temperatures from cooking help remove some of that oxalate, reducing this effect.
Evidence suggests that most forms of cooking will make spinach healthier, but of course there are some trade-offs. While cooking can increase antioxidant capacity in spinach, it can also result in the loss of some B and C vitamins, Dr. Ho said. Though on the whole, cooked spinach has a higher nutritional value, she said, especially because cooking shrinks the leaves down, allowing you to eat more of them.
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When raw garlic is sliced, chopped or smashed, a variety of healthful compounds — including allicin, which is responsible for the allium’s distinct odor and pungency — are activated through an enzyme reaction, and deactivated when garlic is cooked. In one 2015 study, researchers found that allicin was dramatically reduced when garlic was stir-fried, boiled or simmered — though chopping or slicing the garlic before cooking helped to preserve some allicin.
Allicin (along with garlic’s other active ingredients) has been associated with a variety of health benefits, including a healthier heart and decreased cancer risk, though more research is needed to confirm those links. And while raw garlic may have a slight health advantage over cooked, most people don’t eat enough garlic to make a difference in their health anyway, Dr. Liu said, so prepare it in whatever way is the best for you. Cooked garlic is much more pleasant and easier to eat, Dr. Ho said.
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Cooked carrots supply more carotenoids, antioxidants that give vegetables their vibrant colors, than they do when they are raw. But how you cook them matters, according to a 2008 study in The Journal of Agricultural and Food Chemistry — which found that boiling better preserved carotenoids in carrots, while deep frying had the opposite effect. The study also found that boiling, steaming and deep frying increased the carrots’ overall antioxidant levels. A 2021 study found that steaming and microwaving carrots for 10 minutes concentrated the potassium and sodium content in carrots.
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As with garlic, onions supply more beneficial compounds called thiosulfinates when raw, said Alexander Michels, a research associate at the Linus Pauling Institute at Oregon State University who studies micronutrients. One 2012 studyshowed that the heat from cooking broke down those compounds, especially when the onions were crushed or chopped beforehand. And a 2016 review found that boiling onions led to a decrease in antioxidants compared with other methods.
On the other hand, the 2016 review noted that most cooking methods, including sautéing and baking, increased the flavonoid — or antioxidant — levels in onions, especially if they were cooked with lower heat, and for just five minutes or less. But because raw onions also contain antioxidants, Dr. Michels said, eating them raw seems to give you a slight nutritional edge.
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Beets are rich in dietary nitrates, Dr. Michels said, nitrogen-based compounds that have been linked to a range of health benefits, including lower blood pressure. Beets also contain betalains — pigment compounds that give beets their signature deep hue and that have antioxidant, anticancer, anti-inflammatory and liver protective properties — as well as flavonoids.
But you won’t get as many of these benefits if the beets are overcooked, Dr. Michels said — especially if they are boiled. Studies suggest that boiling can reduce the levels of vitamin C, folate, flavonoids and betalains.
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Raw green beans contain a protein called lectin, which disrupts the breakdown and absorption of nutrients, Dr. Ho said. Also known as “anti-nutrients,” lectins disturb digestion and can cause symptoms like nausea, vomiting, upset stomach, diarrhea, bloating and gas — and can make foods less nutritious by interfering with the absorption of minerals, especially calcium, iron, phosphorus and zinc. Cooking — especially under high heat or with water-based methods like stewing or boiling — inactivates lectins, giving green beans more nutritional value. One 2009 study found that baking, microwaving, griddling and pan-frying green beans increased the availability of certain types of antioxidants.
Green beans also taste better and are easier to digest when cooked, Dr. Ho said, which can encourage people to eat more of them.
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These crunchy, leafy stalks are full of vital nutrients: antioxidants, fiber, folate, potassium and vitamins A, C and K. In one 2009 study, researchers found that cooking celery — whether via boiling, pressure-cooking, baking, griddling or frying — increased its antioxidant levels. And in another, published in 2018, cooking increased levels of vitamin K in all kinds of vegetables, including broccoli, potatoes, onions and carrots. While there’s not much research on how folate and potassium react to cooking, one 1989 study found that folate levels were more available when celery was boiled.
But many people turn to celery for the fiber content, Dr. Ho said, which is lost through cooking. And as with most vegetables, cooking also reduces levels of vitamin C.
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As with other cruciferous vegetables — like cauliflower, broccoli, brussels sprouts and cabbage — kale is loaded with compounds called glucosinolates. When you chop or chew kale, an enzyme is released that converts glucosinolates to new compounds called isothiocyanates, which can trigger anti-inflammatory, antioxidant and anticancer pathways in the body, Dr. Ho said. Heat from cooking, however, destroys those enzymes, preventing that reaction and making isothiocyanates less available.
Kale also supplies plenty of vitamin C and antioxidants, which are similarly degraded by cooking. One 2018 study also found that various cooking methods like boiling, steaming, microwaving and pressure cooking reduced levels of potassium, magnesium, calcium, iron, zinc and copper.
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Tomatoes are one of the richest sources of lycopene — a powerful antioxidant that has been linked to reduced risks of certain cancers as well as cardiovascular disease, including stroke. According to Dr. Liu, heat from cooking can change the shape of lycopene, making it easier for the body to absorb. A 2015 studyfound that steaming was especially good at increasing the antioxidant capacity of tomatoes, and a 2010 study showed that microwaving was also effective.
As with other vegetables, cooking can reduce the levels of vitamin C in tomatoes, but the trade-off is worth it, Dr. Liu said. You can easily get vitamin C from other produce like citrus fruits, strawberries and bell peppers.“