You may wonder if cooking vegetables with heating destroys all the valuable nutrients in them. Yes, it is true. Some nutrients, including vitamin C, are lost when heated. However, the difference between cooked and raw vegetables is very small unless those are fully boiled and cooked. And not all the veggies need to be eaten raw. There are even some vegetables that have more nutritional value when cooked in the proper ways.

Fat-soluble nutrients
Why are we cooking? Cooking enhances foods’ flavor, making them more palatable for people. It can soften vegetables and benefit us easier to chew and digest. Cooking vegetables breaks down cell walls, making them easier to digest and absorb certain nutrients. Some veggies, like carrots and tomatoes, increase their bioavailability when cooked. Reducing the risk of foodborne illnesses by killing harmful bacteria and parasites is an aspect, too.
Vegetables that contain a lot of fat-soluble nutrients must be heated and eaten. Carrots, pumpkins, garlic, beans, tomatoes, eggplants, and spinach are good to be cooked. Heating or cooking allows fat-soluble nutrients to be consumed and absorbed into the body more virtually.
Fat-soluble nutrients such as beta-carotene and lycopene are not easily destroyed even when heated. Eating carrots raw can absorb about 10 percent of beta-carotene. Nonetheless, boiling it can increase absorption by more than 60 percent. Spinach is suitable for heat because it contains a lot of beta-carotene, and it is also rich in vitamin C, so it is better to bake it slightly. Suppose you cook them in boiling water for about 30 seconds. In that case, the cell wall is destroyed by heat, and beta-carotene components come out and are ready to increase bioavailability.
A study in Italy found that carrots contain more carotenoids in the form of beta-carotene when they are boiled than raw or steamed [1]. Pumpkins are also rich in beta-carotene, so it is better to boil them. When garlic is boiled, it contains more ‘S-allylcysteine,’ a carcinogen-inhibitory substance. Boiling garlic in water for 60 minutes produces S-allylcysteine four times more than that of raw garlic. Boiling beans increases protein by 6-7 percent.
For tomatoes, cooking is recommended. Lycopene in tomatoes is soluble in oil, so using a little oil when stir-frying or boiling it is helpful. When baking tomatoes, you can intake anthocyanin, an antioxidant. This anthocyanin is heat-resistant and does not be destroyed. According to a study by Cornell University, heating tomatoes at 88 degrees celsius for 30 minutes increases the antioxidant nutrients lycopene by 35%. These findings indicate thermal processing enhanced the nutritional value of tomatoes by increasing the bioaccessible lycopene content and total antioxidant activity [2]. However, in the pure lycopene model, the percentage of retained lycopene in tomato slurry decreased with time and temperature [3]. When exposed to cooking temperatures above 100 celsius, the lycopene becomes unstable.

[1] Giuseppe Maiani. Carotenoids: Actual knowledge on food sources, intakes, stability and bioavailability and their protective role in humans. Supplement: Selected Bioactive Plant Compounds in Human Nutrition. 2009, Volume53, IssueS2, Pages S194-S218.

[2] Veronica Dewanto 1, Xianzhong Wu, Kafui K Adom, Rui Hai Liu. Thermal processing enhances the nutritional value of tomatoes by increasing total antioxidant activity. J Agric Food Chem. 2002 May 8;50(10):3010-4.
[3] Chae Jin Lim1, Hyun Young Kim1, Cheol Ho Lee1, Yongung Kim2, Kyongwhan Back3, Jung-Myung Bae4, Shin Woo Lee1, and Mi-Jeong Ahn1†. Variation in Carotenoid Composition in Carrots during Storage and Cooking. J Food Sci Nut. Vol 14, p 240～245 (2009)

Water-soluble nutrients
On the other hand, raw vegetables generally retain more of their vitamins, enzymes, and minerals compared to cooked ones. Temperature-sensitive nutrients are likely stable in raw cuisines, and typically raw veggies have higher fiber content for aiding digestion and promoting fullness.
Many water-soluble nutrients should be considered before cooking, particularly water-soluble vitamins like B and C. Water-soluble vitamins and polyphenols are fragile against heat. Cauliflower and broccoli contain a lot of glucosinolates. These glucosinolates, known to have anti-cancer effects, are also water-soluble and weak against thermal stress.
Radish, commonly used in soup cooking, is almost completely devoid of nutrients when boiled. Diastase, the main ingredient in radish, is an enzyme that helps digestion, but it is so weak in heat that it is less effective at 50 degrees. Radish skin contains a lot of antioxidants. Peeling and boiling the skin especially increases nutrient loss. It is better to clean the outside with diluted vinegar and eat it right away, or if you feel uncomfortable, heat it up a little.
Dark leafy greens, such as lettuce and kale, contain a lot of folate, a type of vitamin B. Folic acid is quickly destroyed when heated, so it is better to eat it as a wrap or salad. Chives are rich in sulfated compounds, which help blood vessel health. Chives are better eaten raw because sulfated allyl substances is destroyed when heated to 70 degrees or higher.

Whether vegetables are best consumed cooked or raw depends on various factors, including the type of vegetable, personal preferences, and specific health goals. Both cooking methods have their advantages and disadvantages. It’s generally recommended to include a variety of both raw and cooked vegetables in your diet to maximize nutrient intake and enjoy the diverse flavors and textures that different cooking methods offer. Experimenting with different cooking techniques and combinations can help you find what works best for your taste preferences and nutritional needs.