Coconut milk is the plant-based milk that works best when it comes to liberating the antioxidant lutein from spinach in green smoothies, according to a new study that was recently published in the scientific journal Nutrients.

Lutein is the potent antioxidant that is abundant in dark leafy greens such as spinach and kale and is known to be beneficial for eye health. Several experimental studies have also shown that lutein can suppress processes linked to inflammation, which is an important risk factor in cardiovascular disease. And because smoothies are commonly used by consumers as a method to increase fruit and/or vegetable consumption, researchers at Linköping University in Sweden wanted to determine which milk is best for absorption. 

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The research group previously showed that preparation methods where the spinach is heated break down some of the lutein, whereas mixing it into a smoothie makes more lutein available for absorption. 

“Lutein is a bio-active compound,” Rosanna Chung, study lead author and assistant professor in the Department of Health, Medicine and Caring Sciences at Linköping University, said in a statement. “We have studied lutein in a similar way to studying a pharmaceutical drug. In this study, we looked at lutein liberation from fresh spinach.”

For the study, the researchers examined 14 common dairy and plant-based milk products and found that only four of these increased lutein liberation. Compared to water, some mixers had a negative effect on the lutein content in spinach smoothies.  

The researchers suspected that some components in our food, such as fat, carbohydrates, proteins, and fiber can affect the amount of lutein available for absorption. The effect of products such as yogurt may also differ due to fermentation. However, products widely available on the market are rarely examined in scientific studies.   

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For the study, the researchers tested both dairy and plant-based liquids by blending spinach with the various products. They then used a method simulating human digestion in the gastrointestinal tract, by adding digestive enzymes. Finally, the amount of lutein available for the body to absorb in smoothies made with various products was compared to that in smoothies made with just water. 

Which milk is best for smoothies?

Compared to blending spinach with water alone, the study found that coconut milk without additives significantly improved lutein liberation by 42 percent and coconut milk with additives improved lutein liberation by 25 percent. Comparatively, high-fat and medium-fat cow’s milk were found to improve lutein liberation by 36 percent and 30 percent, respectively. Adjustment models suggested that the effects of coconut milk and cow’s milk were derived from fat and protein, respectively. 

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“We could see that only four of the 14 examined products increased the liberation of lutein from spinach compared to water,” Chung said. “Cow’s milk with a high fat content as well as coconut milk improved lutein liberation. Yogurt, however, which is regarded as comparable to cow’s milk and is often used in cafés and similar, did not show particularly good results.”

The fact that dairy yogurt is not that good at liberating lutein may have to do with the fermentation process. It also turned out that plant-based drinks, often made from nuts, legumes or oats, showed significantly different results.

Among the plant-based milk, the researchers also discovered that soymilk was less effective than water when it comes to liberating lutein. “Plant-based liquids have become increasingly common in smoothies,” Jan Neelissen, doctorate student and one of the researchers behind the study, said in a statement. 

“We saw that soymilk was actually less effective than water when it comes to liberating lutein in spinach smoothies,” Neelissen said. “In other words, soymilk had a negative effect on lutein liberation in our study.” 

The other plant-based products in the study did not affect lutein liberation compared to water. As a general piece of information, the researchers also noted that smoothies should be consumed as soon as possible, because lutein breaks down quickly.

Now that the researchers have discovered how much lutein is available for the body to absorb, they will be conducting further research to measure the amount of lutein that is actually absorbed by the bodies from the smoothies made with the different milk products.

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