Dear Mark: Dehydrated Vegetables, Cooked Versus Raw, and Premature Graying and Copper

Vegetables can be hard enough to work into our diet without other factors making it even more difficult. Either we’re stuck with nutrient-sparse, weeks-old produce that has lost all semblance of flavor (but it’s certainly affordable!), or we’re inundated with a countertop full of beautiful vegetation straight from the farmers’ market that we can’t hope to consume in time. You think convincing a ten year old to eat a plate of fresh sauteed kale is hard? Try getting a ten year old to eat a plate of sauteed withered kale that’s been sitting in the fridge for a week. And what about cooked vegetables versus raw? Some say that raw produce is the only way to eat it, that if you cook a carrot you’re rendering its nutritional content null and void. Is it really that dire? Does it matter that much? Find out the answers to these questions, plus one on whether or not copper deficiency can trigger premature gray hairs, in this week’s edition of Dear Mark.

Let’s go.

Hi Mark,
I have checked some of your older posts for info and maybe I’ve missed it, but I was wondering about the nutritional value of dehydrated veggies. I know that when you eat dehydrated fruits (apple rings, banana chips, craisins, etc.), you are primarily eating sugar. What happens to veggies when you dehydrate them? I was thinking that some fun snacks would be carrot or zucchini “chips” for my kids. I found the recipe you posted for Diana’s zucchini chips, but how does the nutrient profile hold up? Do some veggies hold up to dehydration better than others (nutritionally speaking)?


According to one source on dehydrated vegetables, vitamin A is mostly retained, vitamin C is mostly lost, B-vitamins are somewhat reduced, and minerals are lost only if you hydrate the vegetables and discard the soaking liquid. Since you’re going to be eating the vegetables directly rather than cooking with them, you don’t have to worry about that. As I mentioned in a past Dear Mark that dealt with powdered, dehydrated green drink smoothies, dehydrated vegetables may lose some of the volatile nutrients (like vitamin C and the carotenes), but many more are retained, and even when you lose some nutrients you’re not losing all of them. Eating a dehydrated vegetable chip is going to be better than eating nothing at all.

Methods matter, of course. Air-drying, which most home dehydrators employ, results in more nutrient loss than vacuum drying and low-pressure super-steam drying. Using a lower temperature, even if you’re going with an air-drying dehydration process, will be gentler and therefore preserve more nutrients than using hotter temperatures.

Traditional sun-drying might have different effects. In one study, sun-drying greatly reduced the vitamin C content while increasing the antioxidant capacity, phenolic content, and free-radical scavenging ability of leafy green vegetables common to Nigeria. If you recall from my post a ways back on hacking the vitamin content of your produce, exposing green produce to artificial lighting (like you’d get in a supermarket) increases the phytochemical content of that produce. Perhaps sun-drying – which uses a far more powerful light source – confers similar benefits?

Overall, dehydrated veggies are a good option. Most of the important stuff (minerals, antioxidants) will be retained, as long as you’re gentle with your drying.

Hello Mark,

For breakfast I throw a good amount of peppers, onions and mushrooms in the skillet with olive oil to eat with my omelet. Is it ok to “fry” my veggies this way or should I just eat them raw? I do this because they taste better.


Don, I’d just say go for it and keep doing what you’re doing, but I know it’s fun to geek out a bit on this stuff from time to time. If you want to get nerdy about it, we can go through each vegetable and see how cooking does, or doesn’t affect the nutritional value. Shall we?


Raw onion is more effective at inhibiting platelet aggregation than cooked onion. Another study had similar results. In other words, both raw and cooked onion make your blood less “sticky,” but raw onion is better at it. Point to raw.

Onions sauteed in oil show a 7-25% increase in quercetin concentration (a powerful antioxidant) levels over raw onions. Point to cooked.

Sauteeing onions preserves more antioxidants (they “remained high” when compared to raw) than frying onions. Point to cooked (sauteed, specifically).


Compared to boiling or steaming, stir-frying without water is the optimal way to cook “colored peppers” and retain their antioxidant content. Point to cooked (sauteed, specifically).

In another study, grilling colorful Mexican peppers over open flame caused the largest increase in phenolic content, though spiciness (read: flavor) remained similar in grilled, boiled, and raw peppers. I’d say that grilling is pretty similar to sauteeing; wouldn’t you? Point to cooked.

Peppers are a rich source of vitamin C, but cooking can reduce that. Since you’re sauteeing for a short period of time, rather than boiling, the losses should be minimized. Still, it’s something to consider. Point to raw.


Though boiling mushrooms results in a significant loss of selenium (to the water), other forms of cooking (like sauteeing in a pan of olive oil) probably do not. Technically, point to raw (but you’re not boiling).

Another study (PDF), this time on Thai mushrooms, confirms that boiling negatively affects the antioxidant activity and polyphenol content, but that by consuming both the mushroom tissue and the boiling liquid (or broth) the lost nutrients can be salvaged. Again, you’re not boiling, so this may not even apply. Point to raw.

I would eat at least some of your produce in the raw state, just for variety and because it could be a good way to eat some dirt (as long as you’re getting produce from a farm you trust), but for the most part, you’re going to be just fine cooking your vegetables.

Dear Mark,

At the ripe old age of almost 28 I’ve been noticing a few gray hairs – mostly on my chest. I’ve heard that graying hair can be a sign of a copper deficiency, so I’m left to wonder what foods I might be lacking?  Or is this really just a matter of the genetic lottery?



You might be on to something. There’s a rare genetic condition called Menkes disease where a mutation disrupts the normal distribution of copper to surrounding tissues, thereby turning the hair gray (among other effects). It’s quite deadly, and autopsies of people who had Menkes show a total lack of copper in their hair. Menkes is also called “steely hair disease,” owing to the premature graying it causes. I’m guessing you don’t have Menkes, because you probably wouldn’t have outlived infancy. A study from this year shows that gray hairs from people suffering normal, premature graying also contain less copper than normal colored hair from the control group. Similar relationships for iron and zinc were not found.

Though the only way to really know if a copper deficiency is causing your graying would be to get your gray hairs tested at a lab for metal content, you could try adding in some copper-rich foods to your diet to see if they have an effect. Beef and lamb liver are the best sources of copper, but a little bit goes a long way. Other good sources include oysters, crimini mushrooms, and dark chocolate. Maybe plug your weekly food intake into a tracking program to see if your copper is low. You’ll want about 2 mg per day, but you don’t have to do it piecemeal. A half pound per week of beef or lamb liver all at once should get you there.

It might also be worth looking at whether your zinc intake is outpacing your copper intake. Zinc and copper compete for the same pathways in the body, so an excess of one could lead to a deficiency of the other. Chris Kresser just did a podcast on this subject, although he was looking at it from a standpoint of excessive copper and inadequate zinc.

That’s it for this week, folks. Keep sending in your questions and I’ll keep answering them. Take care and thanks for reading!

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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