Leaf Colour Determines Lettuce's Antioxidant Effect

Antioxidants in red-leaf lettuce are fast-moving while green-leaf antioxidants are slow - but this does not affect the benefits of the nutrients, say researchers.

Antioxidants in red-leaf lettuce are fast-moving while green-leaf antioxidants are slow - but this does not affect the benefits of the nutrients, say researchers. The researchers, who published their results in the Journal of Agricultural and Food Chemistry, used electron paramagnetic resonance (EPR) techniques to observe the kinetic behaviour of antioxidants in three lettuce varieties - the green-leaf 'Batavia', the red and green-leaf 'Marvel of four seasons', and the red-leaf 'Oak leaf'.

The study found that the green-leaf lettuce contained water-soluble, antioxidant compounds that acted at a slow and intermediate speed, the red-leaf variety had compounds with intermediate and rapid kinetics, while the green-red-leaf of ‘Marvel of four seasons’ had three kinds of compounds - rapid, intermediate and slow.

Dr Usue Pérez-López, researcher of the Department of Plant Biology and Ecology of the University of the Basque Country said that the different speeds of uptake did not mean that some were better or worse. Rather, it is beneficial to eat a mix of different types of lettuce which can act over a long period of time, thus maximising their different but complementary characteristics. “If we eat foods that can generate free-radical activity, there will be some compounds that act to eliminate them more quickly. But at the same time, it is also important that our bodies should acquire foods with antioxidants that have slower kinetics so that the latter will continue to act over a longer period of time,” he said.

Functional food for the future

The researchers said their next step was to isolate the specific compounds and boost the nutraceutical value of each variety. Lettuce is already a rich source of antioxidants, containing different polyphenolic compounds such as flavonoids, anthocyanins as well as vitamins A and C. The scientists will subject the plants to short bursts of environmental stress – such as watering them with salinated water, raising CO2 concentrations or intensifying light, thus triggering an increase in the plants’ defence mechanisms - antioxidant activity.

“The aim is to maintain production and achieve greater quality in this production," said Pérez-López.

The study

The scientists grew three varieties of lettuce in the laboratory and measured the concentration of different antioxidant compounds. Using a 70% methanol solution, they obtained a hydrophilic extract from the fully expanded leaves of each cultivar. This was done in the dark to avoiding further photosynthesis or phytochemical effects on the leaf extracts. Using EPR they then determined the presence of antioxidants and their characteristics.

The results

The scientists found that the green lettuce displayed an intermediate and slow rate, red a fast and intermediate rate while the red and green lettuce had a fast, intermediate and slow rate. They said that the higher antioxidant capacity in the red and red/green lettuces may be explained by the higher concentration of phenols in these varieties. “At present, we cannot attribute a kinetic behaviour to a specific antioxidant, but we can suggest that the flavonoid quercetin accounted for the majority of the intermediate-rate antioxidants, whereas anthocyanins accounted for the majority of the fast-rate antioxidants.”

Source: Journal of Agriculture and Food Chemistry