The best liquids to maximise antioxidant content in spinach smoothies

Different market products give very different results when it comes to liberating the antioxidant lutein from spinach in smoothies. Researchers at Linköping University, Sweden, have examined 14 common dairy and plant-based products and found that only four of these increased lutein liberation. Compared to water, some drinks had a negative effect on the lutein content in spinach smoothies.  

Most people know that spinach is good for your health. One substance found in, for instance, spinach and kale is lutein. Several experimental studies have shown that lutein can suppress processes linked to inflammation, and there is now ample research indicating that chronic low-grade inflammation is an important risk factor in cardiovascular disease. 

“Lutein is a bio-active 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,” says Rosanna Chung, assistant professor in the Department of Health, Medicine and Caring Sciences at Linköping University, who led the study published in Nutrients.

As our bodies cannot produce lutein, the researchers behind the study were interested to find out how to optimise lutein intake from foods. 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. 

The type of liquid used in the smoothie can also affect lutein content. As lutein dissolves in fat but not in water, it needs gastric juice and/or other food components to be liberated from the plant material and absorbed by our intestines. The researchers suspected that some components in our food, such as fat, carbohydrates, proteins and fibres, can affect the amount of lutein available for absorption. The effect of products such as yoghurt may also differ due to fermentation. However, products widely available on the market are rarely examined in scientific studies.   

The researchers have examined the effects of various products available from food shops on the amount of lutein liberated in smoothies. Both dairy and plant-based liquids were tested.

The researchers blended 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. 

“We could see that only 4 of the 14 examined products increased the liberation of lutein from spinach compared to water. Cow’s milk with a high fat content as well as coconut milk improved lutein liberation. Yoghurt, however, which is regarded as comparable to cow’s milk and is often used in cafés and similar, did not show particularly good results,” says Rosanna Chung.

The fact that yoghurt 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.

“Plant-based liquids have become increasingly common in smoothies. We saw that soymilk was actually less effective than water when it comes to liberating lutein in spinach smoothies. In other words, soymilk had a negative effect on lutein liberation in our study,” says Jan Neelissen, doctorate student and one of the researchers behind the study. 

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

It is important to note that whereas the results from this study indicate how much lutein is available for the body to absorb, no conclusions can be drawn as to how much lutein is actually absorbed. Therefore, the researchers will be conducting a human study in which they will measure the amount of lutein absorbed from smoothies made with different products.

The study was supported by project grants from the Dr P Håkanssons Stiftelse, the Swedish Heart-Lung Foundation and the Swedish Research Council, and was carried out in collaboration with researchers at the Occupational and Environmental Medicine Unit of the University Hospital in Linköping.

Article: The Effects of Dairy and Plant-Based Liquid Components on Lutein Liberation in Spinach Smoothies, Jan Neelissen, Per Leanderson, Lena Jonasson och Rosanna W. S. Chung, (2023), Nutrients, published online on 2 February 2023, Vol. 15, 779, doi: 10.3390/nu15030779