The way you cook your vegetables — or don’t — has a major impact on what nutrients you’re retaining when the food hits your fork. Some nutrients are water-soluble, some are fat-soluble, some degrade at high temperatures, and some are unavailable in raw form.
Take kale, held up as a superfood, as a case study: Boiled or braised kale loses much of its vitamin C, iron, and calcium. Frying it or baking it with oil diminishes its vitamin A. Because of the nutrient blocker oxalic acid, raw kale doesn’t afford you any of the iron and calcium it contains. Ultimately, the answer to the problem is that you should be eating kale in a variety of ways, because no one preparation is going to give you all the available nutrients.
Find out which vitamins and minerals you’re missing out on in these five vegetables with different ways of cooking them.
A study out of Zhejiang University has shown that out of the five most common ways of cooking broccoli — steaming, boiling, microwaving, stir-frying, and braising — the only one that didn’t account for major nutrient loss was steaming, followed next by pan-frying. Water-soluble nutrients like vitamin C are significantly decreased by wet cooking methods like braising and boiling. The results of the study showed that boiling and braising broccoli diminished the vitamin C content by 38% and 33%, respectively.
Glucosinolates are the cancer-fighting nutrients found in broccoli. This study also found that boiling reduces the compound by 41%; microwaving reduces it by a whopping 60%, while steaming had negligible effects on glucosinolates. If you’re going to be consuming the broth you’ve made by boiling the broccoli, you’ll regain some of the nutrients. Otherwise, you’re better off steaming your green, cruciferous vegetables. If you steam with a flavored liquid, it’ll even still taste good.
Other vegetables high in vitamin C affected by wet-cooking methods: bell peppers, kale, cauliflower, Brussels sprouts
2. Sweet potatoes
Are you choosing sweet potato fries over normal fries because they have more vitamin A? Unfortunately, you’re not really reaping any other benefits. Vitamin A is a fat-soluble nutrient, so a long oil bath is leaching much of that extra nutrition right out of the fry.
Baking your sweet potatoes is much better, though this can still account for some lost nutrition. Believe it or not, according to the European Food Information Council, you may be best off with boiled sweet potatoes for mashing to retain vitamin A.
Other vegetables high in fat-soluble vitamin A: squash, kale, carrots, red bell peppers
3. Leafy greens like spinach
All that being said, raw is not always better. There are some vegetables in which the absorption of nutrients is actually blocked by compounds within the vegetable, and those nutrients are only accessible through cooking. Many leafy greens fall into this category. Jeffrey Steingarten wrote at length about this effect in his book The Man Who Ate Everything in a chapter titled, hyperbolically, “Salad, the Silent Killer.”
Spinach, for example, is known for its high levels of iron and calcium. Ask anyone: Spinach is good for you. Turns out, though, that the oxalic acid present in spinach binds with those nutrients, and they can’t be absorbed by the body. The only way to unlock the nutrients is to cook the spinach, negating the oxalic acid. Sorry to tell you, but all those salads and smoothies you made with raw spinach just weren’t worth it. In fact, if you’re susceptible to kidney stones, WebMD says, the oxalic acid could cause their formation. Even PopEye ate cooked (albeit canned) spinach.
Other vegetables high in oxalates: soybeans, beets, sweet potatoes, chard, beet greens
4. Green peas
If you’re a gluten-free vegetarian with a nut allergy looking for more thiamin (B1) in your diet to help your body change carbs into energy, peas are one of the best vegetables for the job — unless you plan to boil them and drain the liquid, that is. According to the USDA, up to 70% of a food’s thiamin is destroyed by cooking and draining the cooking liquid. Eating them raw, though, means you can’t absorb the vitamin E they have to offer because of nutrient blockers. What’s a nutrient-scarce cook to do? Pea soup. Raw peas dumped into broth and then cooked and blended and eaten with that broth is your best bet.
Other vegetables with thiamin: squash, soybeans, and asparagus
We’ve already discussed many of the vitamins inherent in carrots and how they’re affected by various cooking methods, but one interesting finding remains: According to a study out of Italy, antioxidant compounds called carotenoids have been shown to increase by 14% in carrots when boiled.
Carotenoids are a type of nutrient the body can convert into vitamin A, and are particularly known for fighting off certain cancers and eye disease. Lutein, one particular singular carotenoid, was shown to increase by 11% when boiled; lutein absorbs damaging blue light as it enters the eye. Other forms of cooking, such as steaming and frying, showed deteriorated antioxidants.
Other vegetables containing carotenoids: sweet potatoes, orange squashes, broccoli, spinach, tomatoes
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