Added fats and oil were shown to enhance the absorption of fat-soluble vitamins, as well as beta-carotene and three other carotenoids, said the team writing in American Journal of Clinical Nutrition.
The study, led by Professor Wendy White at Iowa State University, found that eating a salad with soybean oil allowed the body to better absorb eight different micronutrients including alpha- and beta-carotene, lutein, lycopene and vitamins A, E and K.
Furthermore, the research showed that the more oil added, the more nutrients were absorbed. Yet when participants were asked to eat the same salad without added oil, the team found that the absorption of nutrients was much lower.
"The best way to explain it would be to say that adding twice the amount of salad dressing leads to twice the nutrient absorption,” said White, an associate professor of food science and human nutrition.
However, she warned that salads should not be drenched with dressing. Instead, people should be consuming the U.S. dietary recommendation of about two tablespoons of oil per day.
It was at this amount of oil that White and her team saw the best results.
“For most people, the oil is going to benefit nutrient absorption. The average trend which was statistically significant, was for increased absorption,” said White, commenting that there were varied responses from participants.
Maximal absorption of nutrients was found at 32 g of soybean oil, which was the highest amount studied.
The study, which was funded by Unilever, monitored 12 healthy, non-smoking women between the ages of 19 and 39, from June to September 2009. The report follows previous findings that showed vegetable oil is necessary for carotenoid absorption from salad and vegetables.
Participants were screened for health and lifestyle factors, as well as eating disorders, and were asked not to consume foods that are good sources of carotenoids, phylloquinone, retinoids and tocopherols for three days.
Following this, on day 4, the women consumed a standardised weighed diet of conventional foods that were low in carotenoids and fat-soluble vitamins.
On day 5, the women consumed the test salad in the morning, after fasting overnight, as well as bottled water.
All salads had equivalent vegetable composition (48 g spinach, 48 g romaine lettuce, 66 g shredded carrots and 85 g cherry tomatoes) but had different dressings containing different amounts of soybean oil. The amounts of oil studied were 0, 2, 4, 8, or 32 g.
Blood samples were taken and measured to assess nutrient absorption.
They found that across the entire 0–32-g range, soybean oil was linearly related to values for a-carotene, lycopene, phylloquinone, and retinyl palmitate.
"Across the entire 0–32-g range of soybean oil, the average absorption of a-carotene, lycopene, and retinyl palmitate and vitamin A could be largely predicted by the amount of soybean oil added to fresh vegetables of dietary importance," wrote the team.
They concluded that more research is needed to better understand the huge differences in carotenoids bioavailability among healthy individuals.
Source: American Journal of Clinical Nutrition
Published online, DOI: 10.3945/ajcn.117.153635
’Modeling the dose effects of soybean oil in salad dressing on carotenoid and fat-soluble vitamin bioavailability in salad vegetables’
Authors: Wendy White, Yang Zhou, Agatha Crane, Philip Dixon, Frits Quadt and Leonard Flendrig