4 antioxidant

One of the most popular sources of "green" in a green smoothie is spinach.

In addition to a broad assortment of nutrients, spinach delivers important plant compounds such as the antioxidant lutein. Lutein supports eye health and heart health due to its anti-inflammatory properties.

A new study compared 14 liquids commonly used in smoothies to determine which ones would be best at liberating lutein from spinach for absorption by the human intestine. It revealed that not all spinach-based green smoothies are equal.

The findings show that spinach smoothies containing coconut milk with or without additives, and high-fat and medium-fat cow's milk are best for liberating lutein.

Coconut milk without additives increased lutein liberation by 42%, and with additives by 25%. High-fat cow milk increased lutein availability by 36%, while medium-fat cow milk did so by 30%.

Soy milk surprisingly had the opposite effect, reducing lutein liberation by 61%. For smoothies with soy milk plus additives, the result was better at 40%.

Almond milk and oat milk had no significant effect on lutein liberation.

The study was recently published in MDPI.

"Spinach is rich in dietary nitrates which helps to dilate the arteries, allowing for better blood flow and lower blood pressure," said Michelle Routhenstein, RD, heart health dietitian at EntirelyNourished.com.

Spinach contains vitamins C and K, as well as carotenoids from which our bodies produce vitamin A.

This dark leafy green also contains iron, folic acid, and calcium, as well as numerous plant-based compounds in addition to lutein. These include kaempferol, nitrates, quercetin, and zeaxanthin.

Cooking spinach can reduce its nutritional value, as can freezing, so raw, uncooked spinach in a smoothie offers the greatest health benefit.

Research has demonstrated that sufficient amounts of lutein can prevent or improve age-related macular degeneration. This is the leading cause of age-related blindness and otherwise poor vision.

In addition, Rosanna W. S. Chung, Ph.D., lead investigator of the new study, told Medical News Today:

"Many previous studies, including my own, showed indication that lutein could suppress the key process of inflammation, so increased daily intake of lutein could potentially be beneficial to people suffering chronic inflammation such as coronary artery disease."

To be absorbed efficiently by our digestive system, explained Dr. Chung, lutein requires a minimal amount of fat.

"However, we generally do not eat enough lutein-rich food to obtain enough lutein in our system for any noticeable health benefit," Dr. Chung noted.

She added that determining the optimal amounts of lutein for eye and heart health is a topic for future research.

The researchers did not test fruit juices, often used in smoothies, in the study.

"Lutein is not soluble in water, while our food and gut environment is full of water," noted Dr. Chung. Since lutein liberation requires emulsification with fats, "The components in fruit juices and fruits are unlikely to affect lutein liberation," she said.

However, very high-fat liquids tested in the study proved not to increase lutein liberation. These included whipped cream and Greek yogurt.

"Whipping cream and Greek yogurt are manufactured very differently, with significantly different compositions compared to cow's milk," Dr. Chung added.

"We think that the reason why whipping cream and Greek yogurt are less effective in lutein liberation is because the milk fat in these two products tends to aggregate, which affects their abilities to dissolve lutein. Milk fat tends to aggregate in high concentrations (whipping cream) and after fermentation (yogurt)."

– Rosanna W. S. Chung, Ph.D., lead author of the study

"Some types of nuts such as walnuts and almonds, contain certain types of soluble fibers which could reduce fat absorption. So, in theory, lutein could also be affected during co-consumption with these nuts," suggested Dr. Chung. This remains to be confirmed in further research, she noted.

Although having a high-fat liquid within the smoothie may seem counterintuitive, Routhenstein noted to MNT that fat is needed to aid the absorption later on.

"While plant-based milks were found in this study to be lutein reducers, it is important to note that lutein is a fat-soluble antioxidant. This means it needs fat to be absorbed, so you may be able to enjoy these plant-based milks as long as you are adding in a heart-healthy fat to help with lutein absorption," she said.

In addition, and beyond lutein liberation, fruits also deliver helpful nutrients in a smoothie, pointed out Routhenstein.

"Whole fruits that are rich in vitamin C like kiwi, mangoes, and strawberries can increase the absorption of plant-based iron found in spinach. It is best to use whole foods versus a fruit juice to avoid excess sugar intake that can increase inflammation in the body."

– Michelle Routhenstein, RD, heart health dietitian

The key to getting more antioxidants out of the foods people eat also lies in the method they consume them.

"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 found that food companions also matter significantly," Dr. Chung said.

"I hope people would start to pay more attention to these two important factors (i.e., consumption methods and food companions) when they eat," Dr. Chung added.

"We as scientists should also study these two factors a lot more," she said.