We all know it's important to eat plenty of vegetables.
That's very clear … but does it matter whether they're raw or cooked?
Comic Dick Gregory advocated raw-food diets in the 1970s … but his skeletal appearance was pretty off-putting.
In the 1980s, raw-food diets got a huge boost from advocacy by TV's vibrant, vivacious "Juiceman” (Jay Kordich), and they've been adopted by Hollywood stars like Woody Harrelson.
Advocates make strong claims about the benefits of eating veggies raw, but generally offer dubious explanations for those alleged effects.
However, cooking makes some nutrients more "bioavailable", and eating vegetables along with fats enhances absorption of carotenoids.
Carotenoids are the healthful, antioxidants in red/yellow/orange vegetables like peppers and squash, and in dark green vegetables such as spinach, kale, and chard.
So cooking makes sense from a nutritional standpoint, but what's the best way to cook vegetables?
What's the healthiest way to cook vegetables?
Many health authorities advise steaming vegetables.
Steaming minimize nutrient losses versus boiling in water, and avoids the calories added by cooking foods in oil.
- Boosts vegetables' antioxidant power
- Instantly improves people's blood sugar and fat/cholesterol profiles
Although other vegetable oils contain small amounts of antioxidants, such as carotenes, only extra virgin olive oil is truly rich in antioxidants.
Crucially, the uncommon antioxidants in EVOO – such as oleuropein and tyrosols – are virtually unique to olives, and deliver particularly powerful, clinically proven cardiovascular benefits.
Quick cardiovascular and metabolic benefits from EVOO
Rome's Sapienza University just reported more encouraging findings from a small clinical trial in 25 people (Violi F et al. 2015).
The results showed that eating a Mediterranean-style meal with extra virgin olive oil (EVOO)
brought immediate blood sugar and cholesterol benefits.
The Roman researchers divided the subjects into three groups:
- No Oil Group - Mediterranean lunch with no added oil
- EVOO Group - Mediterranean lunch plus 10 grams of added EVOO
- Corn Oil Group - Mediterranean lunch plus 10 grams of added corn oil
The team conducted blood tests before each meal and two hours after the meals.
In all groups, blood sugar levels rose after eating, which is normal.
But blood sugar rose much less after a meal with olive oil, compared to either of the other meals.
The blood tests also found lower levels of LDL ("bad”) cholesterol in the EVOO-meal group, compared to the two other meals.
EVOO differs sharply from corn oil, which may explain the trial results:
- Corn oil is low in antioxidants and extremely high in polyunsaturated omega-6 fats
- EVOO is extremely rich in antioxidants and monounsaturated fat, and quite low in omega-6 fats
The heart-healthy reputation of cooking oils high in omega-6 fats (e.g., corn, soy, cottonseed, and safflower) has suffered a series of scientific blows: see Heart Risks Raised by Omega-6 Excess
The effects of cooking methods on antioxidant content
Ten years ago, researchers at Italy's University of Parma tested various cooking methods to see how each affected the antioxidant power of vegetables.
The Italian team noted that standard antioxidant-content data comes from tests on raw, uncooked vegetables (Pellegrini N et al. 2009).
But cooked vegetables might provide substantially more or less antioxidant power, depending on cooking method and duration.
So they tested the effects of boiling, pan-frying and deep-frying on the "total antioxidant capacity” of potatoes, artichokes, eggplant, mushrooms and onions.
Total antioxidant capacity or TAC is a measure of how much antioxidant power a given chemical (such as vitamin C) or substance (such as a bit of puréed vegetable) displays in a test tube experiment.
In line with other studies, they found that boiling reduced the TAC of every vegetable (except artichoke).
In contrast, vegetables pan-fried (sautéed) in EVOO had substantially higher TACs, while foods deep-fried in EVOO had dramatically higher TAC values.
If you're going to cook with water, the results suggest you should steam vegetables instead of boiling them.
If you must boil veggies, be aware that antioxidant losses in them rose along with the volume of water used, so minimize the amount.
And the water used to boil veggies contains some of their lost antioxidants, so keep it to cook beans and grains or to make broths and sauces.
Spanish studies give extra-virgin olive oil a boost
Spanish researchers have performed some of the best studies in this realm.
For example, twin studies from Spain's University of Mucia found that cooking vegetables in water reduced antioxidant levels much more than cooking them in oil, and that cooking them in EVOO
resulted in the highest antioxidant content (Moreno DA et al. 2007; Jiménez-Monreal AM et al. 2009).
As the authors of one of those studies wrote,"... griddling, microwave cooking, and baking produced the lowest losses, while pressure-cooking and boiling lead to the greatest losses; frying occupies an intermediate position. In short, water is not the cook's best friend when it comes to preparing vegetables." (Jiménez-Monreal AM et al. 2009)
Recent studies from two other Spanish universities echo the results of their countrymen's earlier investigations, and of the 2009 Italian study.
#1 - Effect of cooking time and added EVOO
Researchers from the University of Barcelona prepared tomato sauce from scratch.
They cooked it using four different durations (5, 30, 45, or 60 minutes) and with or without extra virgin olive oil (EVOO), with these results:
- The TAC of tomato sauce dropped the longer it was cooked.
- Adding EVOO to tomato sauce boosted its TAC (antioxidant power).
As the Barcelona team wrote, "Higher levels of virgin olive oil in tomato sauce seemed to enhance the extraction of phenolic [antioxidant] compounds from the tomato, leading to higher phenolic contents in the sauces.” (Vallverdú-Queralt A et al. 2014)
Of course, addition of EVOO to tomato sauce would boost its antioxidant content, aside from any extraction effect on the polyphenol-type antioxidants in the tomatoes.
#2 - Effects of different cooking methods
Scientists from Spain's University of Granada joined with Mexican scientists to test the effects of four common cooking methods on the antioxidant capacity of vegetables.
They tested the raw vegetables before cooking to determine the variety and baseline levels of their antioxidants .
The researchers cooked about 4 ounces of potato, tomato, eggplant, and pumpkin in 4 different ways: deep-fried, sautéed, boiled in water, or boiled in a mixture of water and extra virgin olive oil (EVOO).
So that their test would mimic actual household cooking methods, they used the proportions of vegetable to either water or oil found in traditional recipes.
Across all three methods, the vegetables were cooked for 10 minutes, which produced results that resembled the findings from the University of Barcelona study and the prior Italian and Spanish studies:
- Cooking the vegetables in water invariably cut their antioxidant capacity.
- Deep-frying raised antioxidant capacity dramatically, while sautéing raised it substantially.
- Vegetables cooked in EVOO retained their original antioxidants, and gained antioxidant capacity, thanks to several particularly powerful types they absorbed from the EVOO.
As the Spanish-Mexican team wrote, "Deep frying and sautéing led to increased fat contents and total phenolic content, whereas both types of boiling reduced the same.” (Ramírez-Anaya Jdel P et al. 2015)
As the research results show, a deep-fried vegetable is likely to absorb more oil than a vegetable sautéed in oil.
And when you sauté absorbent veggies like pumpkin or potato, they will retain more oil than smooth, hard vegetables like carrot or peppers.
The evidence suggests these are the best ways to cook vegetables, in descending order of nutritional preference:
- Sauté veggies in EVOO, and use minimal oil to minimize calories.
- Sauté veggies in minimal amounts of any low-omega-6 oil, such as macadamia nut, canola, coconut, avocado, or hi-oleic sunflower.
- Steam your vegetables for the minimum time needed to soften them sufficiently.
- Boil veggies in minimal water, for the minimum time required to soften them sufficiently.
By the way, many Internet sources provide misinformation about the "smoke point" of olive oil.
The smoke point (temperature) of an oil matters, because smoke signifies the degradation of an oil's fats ... a bad thing for taste and health.
Of course, using EVOO for deep-frying is far from the ideal culinary use of a top-quality extra virgin olive oil!
What's the bottom line?
First, the idea that it's always healthier to steam vegetables than to cook them in oil isn't necessarily correct.
And when you cook with oil, you probably can't do better than extra virgin olive oil.
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