Many people eat salads in place of regular meals … often due to a desire to control weight and/or deter chronic diseases.
As well as being low-calorie and fibrous – hence lean and satiating – the most popular salad greens and vegetables rank among the most “nutrient-dense foods Americans eat.
They come packed with vitamins and beneficial “phyto-nutrients” such as polyphenols (fruits, onions, beets, radishes) and carotenoids (carrots, tomatoes, spinach, lettuces, and other greens).
Firm proof is lacking, but epidemiological and cell consistently studies link phyto-nutrient-rich diets to reduced rates of cancer (least consistently), cardiovascular disease, and macular degeneration.
That's the good news … but there's bad news, too.
Our bodies can't easily absorb these phyto-nutrients, absent a bit of fat to boost their absorption.
And many popular salad dressings do a poor job of helping us absorb phyto-nutrients … judging by the outcomes of a novel clinical trial from Purdue University.
Fortunately, the Purdue trial shows that using a dressing made with the right type of oil boosts the nutritional benefit of salad veggies and fruits.
Better yet, the right kind of dressing oil does the job with smaller quantities of fat and calories, versus the wrong dressing oils.
Earlier findings set the stage
Eight years ago, an Iowa State University study found carotenoids more “bioavailable” – better absorbed during food digestion – when paired with full-fat dressing versus a low-fat or fat-free choice (Brown MJ et al. 2004).
One year later, a team at Ohio State University found that adding avocado fruit or avocado oil to salad and salsa greatly raised people's absorption of carotenoids ... from four- 15-fold, depending on the source of the avocado fat (fruit or avocado oil) and the carotenoid in question (Unlu NZ et al. 2005).
And it didn't take much avocado fat – which, like the fats in olive, canola, and macadamia oils, are mostly the monounsaturated type – to reach optimal absorption.
As they wrote, “Neither the avocado dose nor the lipid [fat] source [fruit or oil] affected carotenoid absorption … which is attributed primarily to the [monounsaturated] lipids present in avocado.” (Unlu NZ et al. 2005)
Now, clinical trial results reported by Purdue researchers affirm the Ohio State team's finding on monounsaturated avocado fat.
Critically, the Purdue team adds a true revelation.
When it comes to maximizing absorption of a salad's phyto-nutrients – while consuming the minimum amount of fat calories – the kind of oil in a dressing matters more than the quantity.
Purdue trial picks olive, canola, and macadamia nut oils
The Purdue University team divided 29 volunteers into three groups, and several sub-groups with those groups (Goltz SR et al. 2012).
Each group ate identical salads, but different groups got dressings made with different kinds and quantities of oil.
The dressings fell into three categories, depending on their oil's dominant fat type:
Within each of the three oil-type groups, the participants were further divided into three sub-groups, whose dressings provided different amounts of fat:
3 grams (1/10 oz; 27 calories)
8 grams (1/4 oz; 72 calories)
20 grams (3/4 oz; 180 calories).
After eating the salad, the volunteers' blood was tested for absorption of fat-soluble carotenoids – including lutein, lycopene, beta-carotene, and zeaxanthin.
Canola oil-based dressing came out on top, producing the same absoprtion as the other fats at much lower amounts.
In fact, the canola oil-based dressing yielded the same carotenoid-absorption boost when it included 3 grams of fat as it did with 20 grams of fat.
The results showed that small amounts of monounsaturated-fat-rich oil (canola) provide the highest carotenoid absorption.
“Even at the lower fat level, you can absorb a significant amount of carotenoids with monounsaturated fat-rich canola oil,” said study leader Mario Ferruzzi, Ph.D. “Overall, pairing with fat matters.” (PU 2012)
In contrast, saturated animal fat (butter) and polyunsaturated fat (soy oil) dressings required much larger amounts of fat to produce the same absorption benefit.
The soybean oil (rich in omega-6 polyunsaturated fat) was the least effective, with the largest amounts needed to match the canola or olive oils.
Although the butter-based dressing was also less efficient than canola oil, it performed better than the soy-based dressing.
Pick your dressing with care
According to Ferruzzi, “If you want to utilize more from your fruits and vegetables, you have to pair them correctly with fat-based dressings.” (PU 2012)
And he made a point pertinent to dieters: “If you have a salad with a fat-free dressing, there is a reduction in calories, but you lose some of the benefits of the vegetables.” (PU 2012)
Clearly, people craving lower-calorie dressing options – and optimal absorption of phyto-nutrients – should choose dressings that feature “high-mono” oils.
Olive oil has more mono fat than canola oil … and macadamia nut oil has an even higher proportion.
Ferruzzi and his colleagues now seek to see whether people absorb more nutrients if they eat vegetables at one time, or if consumption is spread throughout the day.
The Purdue team's research was funded by the U.S. Department of Agriculture.
Brown MJ, Ferruzzi MG, Nguyen ML, Cooper DA, Eldridge AL, Schwartz SJ, White WS. Carotenoid bioavailability is higher from salads ingested with full-fat than with fat-reduced salad dressings as measured with electrochemical detection. Am J Clin Nutr. 2004 Aug;80(2):396-403.
Goltz SR, Campbell WW, Chitchumroonchokchai C, Failla ML, Ferruzzi MG. Meal triacylglycerol profile modulates postprandial absorption of carotenoids in humans. Mol Nutr Food Res. 2012 Jun;56(6):866-77. doi: 10.1002/mnfr.201100687.
Purdue University (PU). No-fat, low-fat dressings don't get most nutrients out of salads. June 19, 2012. Accessed at http://www.purdue.edu/newsroom/research/2012/120619FerruzziSalad.html
Unlu NZ, Bohn T, Clinton SK, Schwartz SJ. Carotenoid absorption from salad and salsa by humans is enhanced by the addition of avocado or avocado oil. J Nutr. 2005 Mar;135(3):431-6.