Swedish researchers found that using high- and medium-fat cow’s milk and coconut milk in spinach smoothies may make it easier to digest the antioxidant lutein from the leafy green. These liquids significantly improved lutein liberation – increasing the availability of the nutrient by releasing it from its food source. 

Other drinks, such as soy milk, harmed the available lutein content of spinach smoothies compared to water.

“Cow’s milk, in general, performs better than plant-based products,” lead author Rosanna Chung, assistant professor in the Department of Health, Medicine and Caring Sciences at linköping University, Sweden, tells NutritionInsight. 

She adds that it has become popular to use plant-based liquids or to add fibers to spinach smoothies, but that “most plant-based liquids do not help lutein to release and some may even have negative effects on lutein levels in smoothies.” 

“We previously found that consumption methods could make a significant difference in the levels of lutein from spinach. Using the optimal consumption method, i.e. smoothie-making, we now found that food companions also matter significantly.”

Lutein is an antioxidant with anti-inflammatory properties, commonly found in spinach and kale. Studies focused on the potential of lutein in eye health and found it may delay age-related macular degeneration progression. 

Lutein health benefits 
In the study, nclick="updateothersitehits(Articlepage,External,OtherSitelink,Mixing spinach with cow or coconut milk maximizes antioxidant content, study suggests,Mixing spinach with cow or coconut milk maximizes antioxidant content, study suggests,334387,https://www.mdpi.com/2072-6643/15/3/779, article,Mixing spinach with cow or coconut milk maximizes antioxidant content, study suggests);return no_reload();">published in Nutrients, the researchers explain that lutein needs gastric juice or food components to be liberated from the plant material and absorbed by the gut. 

“Lutein is not soluble in water while our food and gut environment is full of water. For lutein to ‘come out’ of spinach, it has to be emulsified either by other food components or our digestive juices,” explains Chung. 

“Many previous studies showed that lutein could suppress inflammation, so increased daily intake of lutein could potentially be beneficial to people suffering chronic inflammation.” 

She adds that lutein is better protected against degradation after being emulsified and should be consumed immediately. 

Earlier research suggested that if the US population with vision problems consumed a daily lutein and zeaxanthin supplement, it could have prevented 21,022 cases of transitioning to severe vision impairment states. 

Set-up of the study 
Humans cannot synthesize lutein, which needs to be consumed through foods. In previous research, the research team demonstrated that heating spinach breaks down the compound, wheras mixing the vegetable in a smoothie makes more lutein available for consumption. 

"Lutein is a bioactive compound. We have studied lutein in a similar way to studying a pharmaceutical drug. In this study, we looked at lutein liberation from fresh spinach,” notes Chung.  

The researchers blended spinach with 14 commercially available dairy and plant-based liquids. They added digestive enzymes to the smoothies to stimulate human digestion and compared the remaining lutein levels available for the body to absorb to smoothies made with water. 

“We could see that only four of the 14 examined products increased the liberation of lutein from spinach compared to water,” adds Chung.

The researchers note they previously demonstrated dairy fat could improve the lutein content in spinach smoothies. However, yogurt did not improve lutein liberation, likely due to the fermentation process. 

Plant-based milk trend 
The Plant based Foods Migration Analysis report indicated that 43% of plant-based consumers who are increasing or new to plant-based products choose plant-based milk over conventional dairy milk. 

“Plant-based liquids have become increasingly common in smoothies,” adds co-author Jan Neelissen. “We saw that soymilk was less effective than water when liberating lutein in spinach smoothies. In other words, soymilk had a negative effect on lutein liberation in our study.” 

The researchers note the negative effect of soymilk on lutein liberation may be due to the protein and fiber content in soymilk. The authors suggest soy protein interfered with the bile acid in the digestion model, as experimental studies indicated soy protein might adversely affect bile acid function and reduce dietary fat absorption.  

The other plant-based products examined in the study did not affect lutein liberation compared to water. 

“The current model could only indicate which liquids would result in higher levels of lutein available for absorption,” concludes Chung. “In our next study, we are planning to measure how much could actually enter human bodies using different liquids to make spinach smoothies.”