Have you ever heard that you should eat more (or only) raw vegetables? Proponents say they retain their vitamins and antioxidants. Cooking “kills” the good stuff, they say, rendering them nutritionally useless.
Or, perhaps, you’ve been told you should focus on cooked vegetables. They argue that the fiber is partially broken down, easing straining on your digestive system, and that certain nutrients are more bioavailable.
To me, it doesn’t have to be complicated: just eat more vegetables however you like to eat them. But that’s probably not the answer you’re looking for…
With few exceptions, almost every nutrition professional agrees that your health can benefit from eating more veggies. Clearly, I feel the same way, otherwise I wouldn’t have made an ebook teaching you how to make veggies taste good, so you automatically eat more of ‘em (grab your FREE copy here).
Alas, the field of nutrition is full of rabbit holes and once you start down the path of switching up your diet, inevitably you’ll find yourself questioning if you’ve made the right decision(s).
Questions that commonly come up with clients are things like:
- Should I only focus on green vegetables? Eat the rainbow? Ditch starchy veggies entirely?
- Only raw? Only cooked? A combo of both?
- Cut out cruciferous vegetables because they are bad for my thyroid? Or eat more of them because they prevent cancer?
When you start digging into the weeds – or, more accurately, googling yourself into a stupor – you’ll pretty much always be left with more questions than answers.
In this post, I want to focus on just one of these head scratchers: Is it healthier to eat raw or cooked vegetables?
I first became interested in this topic in college (while studying nutrition) when I picked up a copy of the book Enzyme Nutrition at a tag sale. It was full of highlighter and notes scribbled in the sidebar. Whoever had kept this 1985 classic on their bookshelf clearly had read through it a few times. This book is often cited by raw foodists as evidence that we should eat all of our food – or at least the majority of it – entirely raw.
Admittedly, I did attempt a raw food diet after reading this book, though it only lasted a few days. My stomach was not happy, I literally could not eat enough to stay full, food prep took hours, and overall, I decided there was no way this was a rational or sustainable choice (unless perhaps you live on a tropical island with oodles of fatty coconuts, seafood that’s yummy eaten raw, and fruit… preferably with a live-in raw food chef).
Ultimately, I decided a raw food diet was not for me, but it did have me thinking a lot more about the nutritional benefits of raw vs. cooked foods, especially vegetables.
What’s Healthier: Raw or Cooked Vegetables?
Potential benefits of raw vegetables
- Enzymes remain intact
- Preserves vitamin C and other nutrients that are sensitive to heat
- More antioxidants/phytochemicals
- Maintain crisp texture and vibrant color
Potential benefits of cooked vegetables
- Kills bacteria and viruses (you might not know this, but raw vegetables, particularly leafy greens are the #1 cause of food poisoning outbreaks)
- Inactivates anti-nutrients
- Enhanced digestibility and nutrient bioavailability
- Better flavor
- Easier to chew
Why this question is hard to answer
The above listed benefits can also be a downside or may not be all that important in the context of a mixed diet.
For example, while enzymes remain intact in raw vegetables, they require A LOT more chewing (mechanical breakdown) and microbial fermentation in your gut to split open the cell walls and access the nutrients in them. Cooked vegetables may have less enzymes, but the process of cooking has already helped your digestive system gain access to the nutrients more easily.
I’m not saying cooked is always better. It’s simply different. I’ll give you a few more examples from the research on specific vegetables below.
Cruciferous veggies include broccoli, cauliflower, cabbage, kale, mustard greens, collard greens, Brussels sprouts, rutabaga, kohlrabi, turnips, radish, and several others. Cooking can be both a good thing or a bad thing, depending on which compounds you’re interested in and what health goals you’re working on.
On the plus side, large increases in carotenoid content have been observed in red cabbage and cauliflower after cooking. (J Sci Food Agric, 2014) This is similar to what’s been observed in tomatoes.
On the other hand, other compounds decrease with cooking (especially boiling, where compounds can leach into the cooking water). Glucosinolates, the sulfur-containing phytochemicals found in cruciferous veggies, tend to decline with cooking.
Glucosinolates are the giotrogenic (or “anti-thyroid”) compounds that interfere with iodine usage in your body, but these are the same compounds that have anti-cancer effects.
I take a “best of both worlds” approach with these and eat some raw and some cooked. I’m not worried about an occasional tender kale salad in the summer, some coleslaw with pulled pork, or thinly sliced kohlrabi chips to scoop up wild salmon ceviche. But, I’d say I eat the majority of cruciferous veg cooked out of personal preference.
If you have known or suspected thyroid problems, you might want to limit your intake of raw cruciferous vegetables. It’s really a case by case decision to be made with a knowledgeable practitioner.
Tomatoes are among the most cited veggies that retain their health-giving properties or may actually be healthier when cooked due to increased bioavailability of certain compounds.
In one study, researchers measured vitamin C, total antioxidants, and bioaccessible lycopene (a type of carotenoid known for its cancer-fighting potential) in raw tomatoes and those that were cooked for 2, 15, and 30 minutes. (J Agric Food Chem, 2002)
As expected, vitamin C content dropped as the tomatoes were cooked for longer periods of time. After 30 minutes of cooking, vitamin C content was 30% lower compared to fresh tomatoes. <–I actually expected that to be a greater decline. Did you?
However, longer cooking times increased the amount of lycopene and other antioxidants. For example, after 2 minutes of cooking, concentrations of lycopene were 1.5x higher and after 30 minutes were 2.6x higher.
Similarly, total antioxidants went up with longer cooking times. After 2 minutes, total antioxidants were 1.3x higher and after 30 minutes, were 1.6x higher.
In other words, cooking made tomatoes more nutritious. The authors noted that a reduction in vitamin C might not be so worrisome as it contributed “<0.4% of total antioxidant activity, indicating most of the activity comes from the natural combination of phytochemicals.”
They concluded: “These findings indicate thermal processing enhanced the nutritional value of tomatoes by increasing the bioaccessible lycopene content and total antioxidant activity and are against the notion that processed fruits and vegetables have lower nutritional value than fresh produce.” (J Agric Food Chem, 2002)
Much like cruciferous veggies, I eat a little of both. I personally don’t think twice about sinking my teeth into a juicy heirloom tomato in the summer, but during the winter, I use mostly cooked tomatoes (canned in BPA-free or glass containers). Pale pink tomatoes shipped in from who knows where have no flavor, in my opinion, so I’d rather have tomatoes that were picked and canned at peak ripeness. If I get some extra lycopene doing that, I consider it a bonus.
Potatoes and sweet potatoes
I’m not sure who would want to eat raw potatoes, but nonetheless, you can rest assured that potatoes retain most of their antioxidants (up to 97%) when cooked. (J Agric Food Chem, 2009) The skin contains most of the antioxidants, just ensure you buy organic, since pesticide residues concentrate in the skin and can’t necessarily be washed off.
In contrast, sweet potatoes tend to have a reduction in carotenoids with longer cooking times, though it’s also noted that the carotenoids isomerise (“change shape” in non-technical terms) and are actually better absorbed after cooking. (J Sci Food Agric, 2014)
There’s no question here. I only eat cooked potatoes, though I tend not to eat a ton of them in general. Since they’re in season in the winter, I tend to eat them more frequently during that time of year.
There’s no denying that leafy greens are full of health benefits, but there are also some downsides. Many of them, including spinach and kale, are high in oxalates and tannins, both of which can interfere with mineral absorption (like calcium and iron) and may also contribute to the formation of kidney stones when eaten in excess.
Cooking leafy greens significantly lowers oxalate levels. For example, one study noted that oxalate levels decreased by 38% to 87% from vegetables such as spinach, Swiss chard and Brussels sprouts after cooking (J Agric Food Chem, 2005). However, while this may be good for your kidneys, it does not appear to significantly increase calcium bioavailability. This may be explained by the fact that tannin levels seem to remain stable even after greens are cooked. (Food Chem, 2015)
On the other hand, iron bioavailability increases significantly after cooking leafy greens. This can be further enhanced when greens are cooked in cast iron pans. One study reported a 4-fold increase in bioavailable iron after cooking leafy greens in cast iron. (Food Chem, 2004) Yet another reason to cook in cast iron!
The trade off is that levels of vitamin C consistently decrease after cooking greens. We’d expect the same for antioxidants, but that’s not always the case. Some report a decrease while others report an increase in antioxidants.
In a study on kale, vitamin C and antioxidants (except beta carotene) decreased significantly after cooking by 89% and 39%, respectively. (Acta Scien Polo Techn Alimen, 2012)
My problem with the above study is that they overcooked the kale by boiling it for 12-15 minutes. Shorter cooking time – and frankly, preparation by any method other than boiling – is associated with fewer losses in nutrients. Indeed, I found a separate study that steamed kale leaves for 5 minutes and they found that total antioxidants increased by 86%. (Food Chem, 2016) In this same study, the effects of boiling, steaming and stir-frying (all for similar amounts of time) were compared. Both steaming and stir-frying preserved antioxidants (carotenoids, anthocyanins) better than boiling.
They concluded: “The effects of the cooking process can be positive since cooking softens the vegetable tissues, facilitating the extraction of bioactive compounds.” (Food Chem, 2016)
Similarly, a study on tropical leafy greens commonly consumed in Nigeria (which are traditionally always consumed in cooked form) found an increase in total antioxidants and antioxidant activity after steaming. The total increase in antioxidants after cooking ranged from 18.3% to 85.7%, depending on the variety of greens tested. Vitamin C, however, decreased significantly after cooking in all samples. (J Food Proc Preserv, 2011)
They conclude: “Cooked vegetables have much better hygienic quality, and due to chemical reactions during cooking, they become more digestible and have an increased nutritional value.” (J Food Proc Preserv, 2011)
Lastly, any leafy greens that are cruciferous veggies (kale, collard greens, etc) will have a decrease in goitrogens after cooking, as discussed earlier.
All in all, it appears there are both positive and negative effects of cooking greens. Raw ones definitely have more vitamin C and may have more antioxidants (especially if you only like greens when they’re cooked to a mush!). On the flipside, cooked greens may have less kidney-irritating oxalate, more bioavailable iron, will still be a good source of antioxidants (minus those that are boiled-to-death), and may prove easier to digest.
I’m back to my “some raw, some cooked” mantra.
Onions and garlic
Onions appear to have an increase in flavonols, a type of antioxidant, when cooked by certain methods. Sauteed onions have a 7% higher flavonol content and baked onions have a 25% flavonol content compared to raw. Boiling onions, however, decreases flavonol content by 18%. (Int J Gastro Food Sci, 2016)
Baking onions is also a better choice than boiling when it comes to preserving folate, a B-vitamin. Onions that have been baked in the oven retain 85% of their folate compared to 65% when boiled. (Food Chem, 2003)
Like onion, garlic also retains antioxidants when cooked. (Food Chem, 2017)
I’m a huge fan of cooked onion, but not so much for raw onions. Same goes for garlic. If I have too much of either (raw), I’ll be burping up onions and garlic for a day. No thanks.
One study looked at antioxidant levels and antioxidant activity (yes, these are two different things) for six varieties of oyster mushrooms. Interestingly, total antioxidants often declined with cooking, but in some varieties, pressure cooking significantly improved antioxidant activity. (J Food Sci Technol, 2015)
I can’t say I pressure cook mushrooms all that often, but maybe it’s time to try some mushroom soup in the Instant Pot?
In a large review of studies on the effects of cooking on phytochemicals, polyphenols consistently decreased after cooking among a wide range of vegetables tested. Boiling tended to be linked to the greatest loss in polyphenols, while steaming has a lesser effect. This was true for zucchini, carrots, potato, broccoli, cauliflower, and red cabbage. (J Sci Food Agric, 2014)
Yep, this is one of the downside of cooking. Some antioxidants decrease upon cooking.
Or if you want to get technical about it: “The final effect of cooking on phytochemical concentration depends on the processing parameters, the structure of food matrix, and the chemical nature of the specific compound.”
I happen to not be a fan of the flavor and texture of most steamed (or boiled) veggies. All I know is that I’ll eat way more of them if I prepare them in a way that tastes good.
This is why I think it’s reasonable to eat some of your veggies raw and some of your veggies cooked.
Which veggies should you eat raw? Which veggies should you eat cooked?
I like to keep things simple and eat as much food in season as possible. That means the time of year and climate play a role in what veggies I’m going to eat cooked or raw. I also take into consideration what I like to eat (no, I don’t “force it down” if I don’t like it), how well it digests raw vs. cooked (this is a personal thing you have to observe for yourself), and if the nutritional trade offs are relevant to me.
The majority of veggies grown – or eaten – in the winter are better eaten cooked. Think about it for a minute. Before we had a global food supply with bell peppers being shipped from Chile in November, you either ate bell peppers in the summer or canned them and ate them cooked in the winter. Most veggies that you could store for later cooking were hardy things like winter squash, onions, potatoes, and carrots. If you draw from Ayurvedic and Traditional Chinese Medicine principles, you’ll notice they suggest eating more warming foods during cold weather. In other words, raw veggies tend to be a “no no.”
Now, a lot of this depends on where you reside on the globe. When I spent a few winters in Alaska, I felt better and more satisfied eating cooked vegetables in the winter, such as roasted Brussels sprouts rather than a green salad (not like I could even find fresh greens worth eating up there that time of year!). When I lived in a more temperate climate, like Southern California, produce availability was better and I was less averse to eating raw veggies in December.
Nonetheless the root vegetables (potatoes, sweet potatoes, winter squash, turnips, beets, rutabaga, etc.) and most of the cruciferous vegetables, are often tastier and easier to digest when eaten cooked.
On occasion, I’ll have something like a raw Asian beet slaw or sesame-ginger kale salad or tangy coleslaw or balsamic carrot slaw in the winter if monotony is setting in (I’m a slaw fan, can you tell?), but for the most part, I eat most veggies cooked in the winter.
Come springtime, with fresh baby beets, tender baby kale, and the warm(er) weather setting in, I do more raw. This trend continues through the summer.
It goes without saying (but I’ll say it anyways) that some veggies are best enjoyed raw, like cucumbers and lettuce, which is a palatability thing more than a nutritional decision. Notice that these are in season in the summer when the weather is warm.
We all have our own tolerance for raw or cooked (or fermented) veggies. I personally do not digest raw broccoli, cauliflower, or onion very well – at least not in large quantities. That means these are usually cooked in my house.
Roasted Curried Cauliflower? YES! Raw cauliflower crudite? NO.
Balsamic Glazed Broccoli? YES! Raw broccoli salad? NO.
Onions cooked into just about everything I eat? YES! Raw onions on a burger? NO.
You might have your own preferences or digestive constraints to contend with. Just go with it and don’t worry about missing out on your vitamin C or a certain antioxidant. You’ll probably make up for it elsewhere.
A fair amount of folks with digestive issues fare much better with cooked vs raw vegetables. Cooking helps break down some of the fiber and irritating compounds in vegetables, letting your digestive system do less work to access nutrients with ease.
Anti-nutrients and secondary compounds
Unlike animals, plants don’t have claws or sharp teeth to defend themselves. They rely on chemical warfare to deter a hungry deer from eating them to the ground (though, the plants in my yard seem to have forgotten this).
Some of these “secondary compounds” have beneficial effects, often doubling as antioxidants, but can be harmful when eaten in excess. As discussed above, the cancer-fighting glucosinolates from cruciferous vegetables double as thyroid-harming goitrogens. If you eat lots of cabbage slaw, put kale in your smoothies every day, or nosh on raw broccoli like it’s your job, it may contribute to thyroid problems in the long run. Both cooking or fermenting helps reduce goitrogens. (A hat tip to sauerkraut and kimchi, perhaps?)
Fennel is delicious as a slaw. It’s also really really good roasted.
Raw carrots are an easy snack. They are also so tasty when roasted under a whole chicken or fermented with ginger and jalapeno.
Bell peppers are great dipped in hummus or spinach dip. I also love them as Indian-spiced stuffed bell peppers.
Do you think raw or cooked vegetables are healthier?
Or, do you take a “best of both worlds” approach, like me, and do a mix. Let me know your thoughts in the comments below.
Until next week,
PS – Feel like you should eat more veggies, but not sure how to make ’em taste good? Make sure you snag a copy of my free ebook, “Veggies: Eat Them Because You Want To, Not Because You Have To” via the box below.
8 CommentsLeave a comment
As always, Lily, your blogs are so informative and helpful! Good to read the studies that each veggie varies so much in the gains or losses of cooking vs not cooking. Eating raw in large quantities doesn’t work well for my digestion, so I tend to cook veggies more often. But I do appreciate raw veggies prepared in fun ways. I am with you — I think mixing it up is a good way to go! And to do what works for us as individuals. As long as our daily food intake involves a high portion of veggies, I think we are on a good path.
Thanks Kim. 😉
I really enjoyed reading this post. Thank you! It was eye-opening to read about the veg that can cause thyroid issues. I do eat those vegetables (mostly cooked) and those greens (usually raw), on a daily basis. What do I need to watch out for in regards to thyroid issues? Is this a life-threatening thing?
As always, thank you for the great info and recipes. We are putting together our list for transfer orders, so maybe we’ll see you soon. 🙂
Fascinating as ever! Thank you Lily. Same as you, I tend to mix it up. Intuitively I will eat cruciferous veggies more often cooked (perhaps my Thyroid is sensitive) although I really do like a bit of raw broccoli and will snack on it when preparing it for steaming. Generally, I only steam – never boil – and only very lightly so most things still have a crunch and green leaves are only wilted not turned to mush. This is not an intellectual decision but a taste preferential for me. And I do tend to be careful with the Solanaceae family – Potatoes, Tomatoes, Peppers, Aubergines, and the like (the bright red of tomatoes and peppers makes me think WARNING!! so I tend to heed it) and from somewhere way back I learned that Solanaceae are better combined with dairy/calcium… which kind of makes sense if you look at traditional recipes??? Thanks Kim
Great post thanks! I love my fried kale in my cast iron pan 😉 😉 i love love love reading your posts discovering the research is in line with common sense and traditional ways (doesn’t it always?!). I buy most of my veg as organic from store or farmers markets which I guess means in season. I can’t stomach raw broccoli or cauliflower either but raw onions yum! I loving veggies now since learning I can add healthy fats! My favourite way to cook veggies is definitely roasted like peppers (capsicum) and onions red and white with a free range chicken on top. I’m not great with mushrooms but have just got a new pressure cooker! So will look up a mushroom soup recipe to try. Thanks again!
I really enjoy all of the research based answers and knowledge I gained from this post.
I am definitely a best of both worlds veggie eater.
Great post, as always Lily! I do exactly as you, letting the seasons dictate my veggie choices, except I do like broccoli salad in the winter and raw onions on my burger ;). Thanks so much for all your dedication!
Great article! Thanks a lot for ding such an awesome research.