Tomatoes are certainly nutritious — a good source of the antioxidants lycopene and beta-carotene. But consider this: if you eat a tomato without adding a little fat — say a drizzle of olive oil — your body is unlikely to absorb all these nutrients.
Scientists at Iowa State University figured this out a while ago. They recruited graduate students to eat bowls of salad greens with tomatoes and various types of salad dressings — from fat-free to regular Italian. "Basically once a month for several months we'd show up first thing in the morning," recalls participant Gregory Brown, now a professor of exercise science at the University of Nebraska. Researchers put IV lines into the participants' veins and drew blood samples before and after they'd eaten the salads in order to get precise measurements of the absorption of nutrients.
"The salads all tasted the same to me," says Brown. But when researchers went back and analyzed the blood samples they realized that people who had eaten fat-free or low-fat dressings didn't absorb the beneficial carotenoids from the salad. Only when they had eaten the oil-based dressing did they get the nutrients.
Carotenoids are the pigments responsible for red-, yellow- and orange-colored fruits and vegetables. And carotenoids are also found in dark green vegetables such as spinach. The compounds convert to Vitamin A in the body, and studies have found that carotenoids have anti-oxidant activity which may help protect cells from damage caused by free radicals. Human studies have linked high consumption of fruits and vegetables to reduced risk of cancer.
Beta-carotene researchers were not particularly surprised by the findings of the fat-free vs. regular Italian salad dressing study. "We already knew that carotenoids were fat soluble," explains Wendy White, a professor of Human Nutrition at Iowa State University. The results helped reinforce the idea that a little fat is healthy.
Chop And Chew
There are other ways to help maximize the absorption of carotenoid nutrients. Chopping or grating breaks down the plant material. "The finer the particle size ... the better the absorption of beta-carotene," explains White.
The findings of nutrition research often go against the grain of trendy food ideas. For instance, many people have heard that raw vegetables are best. But if you're eating carrots, it may be helpful to cook them gently. The heat can soften the food allowing more nutrients to be released.
A recent study in the Journal of Food Science suggests that some cooking methods may be better than others. Researchers at the University of Murcia in Spain cooked 20 different kinds of vegetables six different ways. Then they analyzed how well the foods retained antioxidants. They found that microwaving helped maintain the antioxidants, whereas boiling and pressure cooking led to the greatest losses.
Green beans, beets and garlic all did well with heat — maintaining beneficial phytonutrients after most kinds of cooking. The antioxidant value in carrots actually increased after cooking.
Experts explain that boiling may allow nutrients to leach into the pan water that people end up tossing out, especially with water-soluble nutrients such as Vitamin C and the B Vitamins.
Eat Plenty Of Colors
As testing methods have become more sensitive, scientists have the ability to peer into our foods and tally up all the phytonutrients that may be beneficial. But experts say the ways in which our bodies may use and absorb these compounds are complicated. Therefore, many experts say it's best not to fixate too much on how food is prepared. Instead, focus on eating more plant foods — of all colors.
Jeffrey Blumberg, an antioxidant expert at Tufts University, says "What's important is that you find a way to cook that's palatable to you so you're getting lots of plant foods."
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LINDA WERTHEIMER, host:
This is MORNING EDITION from NPR News. I'm Linda Wertheimer.
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And I'm Steve Inskeep.
It is Monday morning, when we look at your health. And today we'll examine how safe or unsafe our food supply really is. First, we'll find out about getting the most nutritional value from the fruits and vegetables you eat this summer. NPR's Allison Aubrey has some tips on how to maximize your body's absorption of beneficial vitamins and nutrients.
ALLISON AUBREY: If you've always assumed that tomatoes are very nutritious, you're absolutely right. They're a good source of the lycopene and beta-carotene. But consider this: if you eat a tomato without adding a little fat -say a drizzle of olive oil - your body will not absorb all these nutrients.
Greg Brown, who is now a professor of exercise science, learned this the hard way. He's not a big fan of tomatoes, but back when he was in graduate school at Iowa State he volunteered for a nutrition study, agreeing to eat big bowls of salad greens with tomatoes and dressing. All in exchange for a few hundred bucks.
Professor GREG BROWN (Exercise Science, University of Nebraska): Basically, once a month for several months we'd show up first thing in the morning. They would put an IV line our vein. And then we would eat the salad. And then once an hour, every hour for the next 12 or 13 hours they got a blood sample to evaluate the absorption of nutrients from the salad.
AUBREY: What Brown did not know at the time is that some of the salads had fat free or reduced fat dressings, while others had regular oil-based Italian. A distinction that turned out to be very significant when researchers went back and analyzed the blood samples they found that when people ate the fat free salads there was virtually no absorption of the carotenoids like beta-carotene or lycopene.
Study author Wendy White says the findings validated what researchers already really knew. Carotenoids are fat soluble, so we need a little fat to get the benefits. She says there are a few other ways to maximize the absorption of these healthy compounds, namely by chopping and chewing, which breaks down the plant material.
Professor WENDY WHITE (Nutrition, Iowa State University): The finer the particle size, the more finely chopped or homogenized or properly chewed the carrot the better the absorption of beta-carotene.
AUBREY: White says the interesting thing about nutrition research is that it often goes against the grain of trendy food ideology. For everyone who has warmed to the idea that raw veggies are best, well, when it comes to carotenoids - the beneficial pigments found in everything from carrots to sweet potatoes, spinach and tomatoes - White says actually cooking can be helpful. Heat can soften the plant material and help release the nutrients.
Prof. WHITE: Heat treatment in general isn't really destructive to beta-carotene or lycopene. So at boiling temperatures that are lower, carotenoids are very stable to heat treatment.
AUBREY: So is one way of cooking better than another? Well, a recent study in the Journal of Food Science suggests that microwaving is the method that helps maintain the highest level of antioxidants. Researchers in Spain tested 10 vegetables and found that among other veggies, green beans, beets and garlic all retained their nutrients. Experts say with the microwave the heat treatment is mild and quick. And microwaving may also help preserve water-soluble nutrients such as vitamin C and the B vitamins.
Jeffrey Blumberg is an antioxidant expert at Tufts University. He explains that if you boil a pot of veggies, some of the beneficial water soluble compounds may end up in the water that's tossed out.
Professor JEFFREY BLUMBERG (Tufts University): You can leach out those antioxidants and other nutrients and lose them that way.
AUBREY: But his advice is really not to focus so much on how your vegetables are prepared.
Prof. BLUMBERG: What's important is that you find a way to cook them that's palatable to you so you're getting lots of those plant foods and not worrying whether you should have them raw or microwaved or so on.
AUBREY: If you eat enough greens and fruits and eat for taste - meaning sticking with the things that appeal to you - you might find, as salad eater Greg Brown did, that you gravitate towards a healthy balance.
Allison Aubrey, NPR News. Transcript provided by NPR, Copyright NPR.