Lettuce Antioxidants are Concentrated in the Outer Leaves
Not only are the outer leaves of a head of lettuce are edible, they are also good for you. In fact, research suggests that the outer leaves contain more antioxidants than the inner leaves because they get to absorb more sunlight. For the very same reason, "loose" or "open leaf" lettuce varieties, such as such as romaine and red oak leaf, have been shown to contain more antioxidant nutrients than varieties with more tightly-packed heads, such as iceberg. To get the full scoop, keep reading.
Higher Exposure to Sunlight Equals Higher Levels of Antioxidants in Lettuce
In an interesting study published in the Spanish Journal of Agricultural Research in 2011, researchers from the University of Gaziosmanpasa in Turkey investigated how the arrangement of indivividual leaves on a lettuce plant influences the phytochemical content and antioxidant capacity of the plant. They looked at both red and green lettuce cultivars, and only studied plants that had been grown under the same environmental conditions. The researchers found that the outer leaves, which get the most sun, contained more phenolic compounds and had stronger antioxidant properties, as measured by total antioxidant capacity (TAC), than the inner leaves. This phenomenon was observed both in the green and red varieties.
Another study, published in Current Nutrition & Food Science, suggests that also the type of lettuce has an impact on the amount of antioxidant vitamins your get from your salad, with "loose" or "open-leaf" lettuces (such as romaine, red leaf, and butterhead) providing more antioxidant vitamins than varieties with more tightly-packed heads (such as crisphead or iceberg lettuce).
After comparing the nutritional value of different lettuce varieties, Beiquan Mou, the USDA researcher who conducted this study, went on to investigate whether the lower nutritional value of crisphead lettuce was linked to the shape of its head. He artificially kept leaves of some crisphead lettuce plants open so that they could not form heads, and conversely, the leaves of open-leaf romaine lettuce varieties were tied so that they would form a head structure and fewer leaves would be exposed to direct sunlight. After about five weeks, these plants, along with naturally-grown control plants, were analyzed for antioxidant vitamins such as beta-carotene and vitamin C. Mou discovered that when the heads of crisphead lettuce were kept open, their vitamin C and beta-carotene levels reached levels comparable to those of leaf or romaine lettuce. Conversely, the beta-carotene and vitamin C levels were significantly lower in the closed romaine leaves than in the open-leaf romaine plants.
The Love-Hate Relationship Between Lettuce Leaves and the Sun
But why do open-leaf varieties and the outer leaves of lettuce plants contain more antioxidants than lettuce leaves that are sheltered from the sun? As with so many other questions related to maximizing the nutritional value of our meals, we find an answer in Jo Robinson's award-winning book, Eating on the Wild Side. In this New York Times bestseller, Robinson explains that salad greens have a "love-hate relationship" with the sun: they need sunlight to grow, but the sun's UV rays can also destroy them. To protect themselves from the UV rays, they have to manufacture their own sunscreen—antioxidants that block the harmful effects effect of the sun's UV rays. And, the higher the exposure to UV light, the more antioxidants are produced by the leaves.
Studies cited:
1. S. Ozgen and S. Sekerci (2011). Effect of leaf position on the distribution of phytochemicals and antioxidant capacity among green and red lettuce cultivars. Spanish Journal of Agricultural Research, 9(3), 801-809.
2. B. Mou (2009). Nutrient Content of Lettuce and its Improvement. Current Nutrition & Food Science, 5(4), 242-248(7).
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