The Healthiest Way to Cook

Doesn’t it seem like when it comes to nutrition there are more opinions than facts to go around? Every day we hear new theories about diets, and supplements, and the best foods to eat. My role is to take the mystery out of good nutrition, and look at the science. Welcome to the Nutrition Facts Podcast. I’m your host Dr. Michael Greger.  And I’m here to bring you an evidence-based approach to the best way to live a healthier longer life. 

We know there’s lots of great fresh ingredients out there to use in our favorite recipes. But what’s the best way to cook them so that they retain their nutrients? Today we start with the best way to cook greens.

The main purpose of cooking vegetables is to make them more edible, palatable, and digestible.” The downside, though, is that “cooking may adversely affect the levels of nutrients, especially the heat-sensitive and water soluble ones.” But even if you boil greens for 10 minutes, the drop in antioxidant capacity, for example, which is a rough proxy for phytonutrient retention, isn’t that much. Yes, there’s a significant drop in each case, a 15 to 20 percent drop, but most of the antioxidant power is retained, even if you boiled lettuce for 10 minutes. The single nutrient that drops the most is probably vitamin C.  Collards start out so vitamin C-packed that even collard greens boiled for 10 minutes have twice as much vitamin C compared to even raw broccoli.

The vitamin C in spinach really takes a hit. Even just blanching for five minutes can cut vitamin C levels more than half, with more than 90 percent dissolving away into the water after 15 minutes, though most of the beta carotene, which is fat soluble, tends to stay in the leaves. But just keeping it in a regular plastic bag, like you get in the produce aisle, can protect it. The refrigeration is important, though. Even in a bag, a hot day can wipe out nearly 50 percent. Not as bad as drying, though, which can wipe out up to 90 percent of the vitamin C, suggesting that something like kale chips may pale in comparison to fresh, though vitamin C is particularly sensitive. Other nutrients, like beta carotene, are less affected across the board.

Cooking by microwaving and steaming preserves the nutrition more than boiling.  A little steaming or microwaving hardly has any effect compared to raw, though boiling even two minutes may cut antioxidant levels nearly in half. Watercress is a cruciferous vegetable, though—a cabbage and broccoli-family vegetable—so it’s prized for its glucosinolate content, which turns into that magical cabbage compound sulforaphane. What does cooking do to it? Fresh is best, but steaming’s not bad, with microwaving coming in second, and then stir-frying and boiling at the bottom of the barrel. The glucosinolates in other cruciferous vegetables are also significantly affected by boiling. The researchers conclude that red cabbage is best consumed fresh, and look, not just in salads. As I talked about in How Not to Die, I always keep a red or purple cabbage in my crisper to cruciferize my meals, slicing off shreds and putting it on basically anything. But if you are going to cook it, steaming may be the best bet, “so as to retain the optimum benefits of the health-promoting compounds.”

Other nutrients we look to greens for are the eyesight-preserving nutrients like lutein, which I’ve talked about before, and folate, particularly important for women of child-bearing age; and vegetables are the main natural source. It’s been estimated that approximately half the folate is lost during cooking, which may be true for boiling broccoli, or stir-frying spinach or mustard greens. But the folate in stir-fried kale holds up better, only losing about a quarter, similar to steamed broccoli florets. But note that broccoli starts out so high that even boiled broccoli has more folate than raw spinach. But check out broccoli leaves. Not only do they start out among the highest levels, the levels actually go up a bit when you cook them. No one’s ever looked at the folate concentration in broccoli leaves, which ironically are commonly just cut off and thrown away, yet contribute great concentrations of this vitamin. Therefore, we should make sure to eat them.

Note they also compared thinly-sliced kale to kale just torn into larger pieces, to determine if a larger surface area of exposure would promote greater losses of folate in kale. However, no effects were found, so slice away. Here, they just looked at stir-frying. What about the effect of other cooking methods on kale? There’s lots of studies on cooking cabbage and broccoli. However, very little information has been available on the queen of greens…until now.

First of all, fresh versus frozen. “The freezing process is generally regarded as destructive to antioxidant compounds.” One just assumes that frozen would have a lower antioxidant capacity compared to fresh, but kale breaks all the rules. The frozen kale showed a higher antioxidant capacity than fresh. And not just by a little; we’re talking 60 percent more. Wow! Okay, so what happens when you cook it? If you start out normalizing the starting levels at 100 percent, blanching and steaming actually boost the antioxidant content, whereas microwaving or even boiling doesn’t seem to do much so you can boil kale without losing out on its antioxidant punch. I told you kale’s a rule breaker.

But check out that blanching and steaming. Heat can disrupt the plant cell walls and all the little subcellular compartments, and spill out extra antioxidant compounds that may have been hiding. Now that’s usually counterbalanced by losses caused by high temperatures, but the kale compounds are looking pretty cruciferocious, and stood their ground.

In our next story –boiling, steaming, microwaving, air frying, and sous vide cooking are put to the test for nutrient retention.

I’ve made videos on how not to die from heart disease, how not to die from cancer, how not to die from other deadly diseases like diabetes, but some of the most popular videos on the site are like  “the best way to cook sweet potatoes.”

All right, then. What’s the best way to cook bell peppers? Here’s the antioxidant power of raw green peppers and red peppers, and microwaving or stir-frying doesn’t seem to do much, though with boiling, there’s a drop. But then, if you measure the antioxidant activity of the leftover boiling water, the antioxidants weren’t destroyed, but just leached out into the cooking water. So, the researcher’s conclusion is that it’s “vital to consume the water used for boiling, in addition to the peppers, as bioactive compounds will be left over in the water.” But that’s not the take-away I get from this study. Drink the water or not, red peppers have nearly twice the antioxidant power of green, no matter what you do. So, while both peppers are, by definition, green-light foods, the red peppers, ironically, are even greener.

What about mushrooms? Probably best not to eat them raw, but what’s the best way to cook them? “Since cooking techniques clearly influence the nutritional attributes of mushrooms, the proper selection of cooking method may be a key factor to prevent or reduce nutritional losses. And, ”microwaving and grilling were established as the best processes to maintain the nutritional profile of mushrooms.” For example, a significant decrease was detected in the antioxidant activity of mushrooms, especially after boiling and frying, while grilled and microwaved mushrooms reached in some cases higher antioxidant activity.

Boiling had a similar negative impact on the antioxidant power of cauliflower, which serves as just kind of a rough proxy for how many phytonutrients of potential benefit we might be losing. Blanching was better, where the cauliflower here was dunked into boiling water for three minutes and then run under cold water to stop it from cooking. I had never heard of steam blanching, but same idea. Steam for three minutes, then cool off, which appears to be better, since you’re not immersing it in water. Though, note there’s not much difference between steaming for six minutes and steaming for three, and then running under cold water. Too bad they didn’t look at roasting—that’s how you make cauliflower taste good. In fact, I’ve got two recipes on roasted cauliflower in my How Not to Die Cookbook (for which all my proceeds go to charity, of course).

There are certain antioxidants we’re especially interested in, though. Like the eyesight and brain-protecting green vegetable compound lutein. Here’s the back of the eyeball. What lutein does is protect those sensitive light-sensing nerves by blocking the high-energy blue light rays, which helps us see better, and may help us think better too. So, researchers looked at the effects of four different cooking methods on lutein concentrations. The first thing you’ll notice is that broccoli has like 50 times more than cauliflower—not a surprise, since lutein is a plant pigment, and cauliflower is too white. Here is it graphically, so you can appreciate the difference.

Then they compared boiling, steaming, microwaving, and sous vide cooking, which is like a fancy name for boiling in a plastic bag. And, boiling actually made lutein levels go up! How is that possible? Heat can actually disrupt the cell walls, and all the little subcellular compartments that can enhance the release of antioxidant compounds. Sous vide was similar; microwaving detrimental, at least for the broccoli, and… steaming the superstar, nearly doubling lutein levels.

Heat isn’t the only way to liberate lutein from greens. If you finely chop spinach, you can double the amount of lutein released during digestion in this experimental model. And make a green smoothie, or pesto, or some kind of puréed spinach dish, and you may triple the bioavailability. But you have to watch the heat. Steaming or boiling is one thing, but super high heat, like stir-frying, can reduce lutein levels to nearly nothing.

Frying is also bad for the purple pigments in blue potatoes, even air-frying; they just seem sensitive to extremely high heat. These special antioxidant plant pigments appear to be sensitive to really high temperatures; so, we should try to avoid frying, especially deep frying. That was one of the conclusions of an expert panel on cooking methods: avoid deep frying foods. Not only the nutrient losses, but all the added oil—not to mention the production of some toxic compounds at those temperatures. So, that continues to be a challenge to the food industry. What’s their solution? Forget deep-fat frying, let’s try frying in pure molten sugar. It’s like the SnackWell cookie phenomenon taken to its logical conclusion. Oh, you want low-fat? We’ll fry in sugar.

Finally, today we look at which are the gentlest cooking methods for preserving nutrients, and which cooked vegetables have more antioxidants than raw?

You may remember back in Volume 2, I compared the effects of different cooking methods on the phytonutrients in broccoli. Well, last year, food scientists outdid themselves. They looked at 20 different vegetables, six different cooking methods, and then looked at three separate measures of antioxidant activity. That’s over 300 separate experiments to figure out what’s the best way to cook our vegetables.

First, though, let’s figure out the worst, in terms of loss of antioxidant content; baking, boiling, frying, George Foreman, nuking, or pressure cooking? The worst is boiling.

What’s the second worst? The pressure cooking. When we use these wet cooking methods, some of the nutrition is lost into the cooking water. It may be less than you think, though. Averaged over those 20 vegetables, boiling removes only about 14% of the antioxidants. So, if you really like boiled broccoli, fine just eat one more floret. Seven florets of boiled broccoli has all the antioxidant power of six florets of raw broccoli.

So, the best way to eat your veggies is really whichever way will get you to eat the most of them, with the exception of frying; that just adds way too many empty calories.

What’s the gentlest cooking method, though? Out of these remaining four, which preserves antioxidants the best? It was the microwave; preserving 97.3% of the antioxidants.

But that’s on average, across 20 vegetables. There was one vegetable whose antioxidants get clobbered, no matter how you cook it; up to 75% of the antioxidant power gone. Which is the one vegetable really best to eat raw; artichoke hearts. asparagus, beets, broad beans, broccoli? I hope we don’t have to eat raw Brussels sprouts.  Cauliflower, carrots, celery, eggplant, garlic, green beans, leeks, corn on the cob, onions, peas, bell peppers, spinach, swiss chard, or zucchini? The most vulnerable vegetable is bell peppers. Do try to eat those raw.

On the other hand, there were three vegetables that weren’t affected by cooking at all. You could even boil them, and lose no antioxidants. Can you guess at least one of the three? The three were artichokes, beets, and onions. Boil away. Asparagus actually gets honorable mention here. Unaffected by all but frying, so you can boil asparagus, too.

Final question, and perhaps the most interesting. There are two vegetables that, no matter what you do to them, they increase in antioxidant value. They become healthier. Which two are they?

First, the honorable mention: green beans. With the exception of boiling and pressure cooking, they actually increase in antioxidant power when you cook them, so microwaved green beans are actually healthier than raw green beans.

But which two vegetables always increase in value, no matter how you cook them? Carrots and celery. So, when we make a nice vegetable soup, we’re actually boosting the nutrition.

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For recipes, check out my “How Not to Die Cookbook.” It’s beautifully designed, with more than 100 recipes for delicious and nutritious meals. And all the proceeds I receive from the sales of all my books goes to charity. NutritionFacts.org is a non-profit, science-based public service, where you can sign up for free daily updates on the latest in nutrition research via bite-sized videos and articles.

Everything on the website is free. There’s no ads, no corporate sponsorship. It’s strictly non-commercial. I’m not selling anything. I just put it up as a public service, as a labor of love – as a tribute to my grandmother, whose own life was saved with evidence-based nutrition. Thanks for listening to Nutrition Facts. I’m your host, Dr. Michael Greger.