Healthiest way to make a smoothie revealed by scientists (but it’s bad news if you’re a fan of trendy vegan milks…)

They’re loved by fitness fanatics everywhere.

But scientists say there’s a secret trick that can make green smoothies even more nutritious.

Although already packed full of spinach, kale and other leafy greens, mixing them with certain liquids can boost levels of lutein — an antioxidant that keeps our eyes healthy.

Tests showed levels of lutein were highest in smoothies made using coconut milk, followed closely by cow’s milk.

Oat, almond and soy alternatives, loved by the eco-conscious, were no better than water, Swedish researchers found.

They claimed that plant-based mixers, which have become ‘increasingly common’ for smoothies, can have a ‘negative effect’ on how much lutein the body absorbs.

From 2004, companies saw a boom in the popularity of green smoothies as the nation began a new trend of veggie-based drinks

The researchers made 14 other versions using dairy and plant-based mixers instead of water, including yoghurt, cow’s milk and coconut milk (pictured)

The body cannot produce lutein itself. It acts as an anti-inflammatory and studies suggest it boosts eye health.

To find out how to maximise the lutein content of spinach smoothies, Linköping University researchers made a green drink by blending spinach and water.

They then made 14 other versions using dairy and plant-based mixers instead of water, including yoghurt, cow’s milk and coconut milk.

They added digestive enzymes to the smoothies to simulate human digestion and measured the amount of lutein available for the body to absorb — called lutein liberation — for each smoothie.

While green vegetables can be packed with lutein, it needs gastric juice — made in the stomach to break down food — before it is released from the vegetable and absorbed by the body.

The results, published in the journal Nutrients, show that only four of the 14 smoothie mixers significantly boosted lutein liberation compared to water.

These include high-fat and medium-fat cow’s milk, which increased lutein liberation by 36 per cent and 30 per cent, respectively.

Meanwhile, pure coconut milk, without any additives, increased lutein liberation by 42 per cent, while the same milk with additives trigged a 25 per cent rise.

This is because components in cow’s and coconut milk encourage lutein to be released from the spinach and into the smoothie.

While other mixers, such as low-fat cow’s milk, Greek yoghurt and almond milk caused small increases in lutein liberation compared to water, the scientists said these were not statistically significant. 

And some mixers, such as pure soy milk and soy milk with additives, which are popular among vegans, caused lutein liberation to drop.

Other plant-based mixers, often made from nuts, legumes or oats, did not increase lutein liberation compared to water.

This means those making smoothies would be better off using water than plant-based milks, which had a ‘negative effect on lutein liberation’, the researchers said. 

The team believe their findings can be explained by the different fat, carbohydrate, protein and fibre content of the mixers, which affected the amount of lutein available for the body to absorb.

And the fermentation process yoghurt goes through could be to blame for yoghurt not being good at liberating lutein, they suggested.

Study author Rosanna Chung, assistant professor in experimental cardiology and nutritional immunology at the university, said: ‘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.’

The researchers noted that the study tested the amount of lutein made available for the body to absorb rather than the amount actually absorbed.

So they now plan to conduct a separate study in people which will measure the amount of lutein absorbed from smoothies made with different mixers.