Yet, it's the way food is prepared that ultimately determines the net absorption and availability of valuable compounds from foods such as spinach, which we have commonly been taught is a nutritious and beneficial food in our diet.
Now, new research from Linköping University in Sweden reveals that eating spinach in the form of a smoothie or juice, combined with some fat, is actually the most efficient way to absorb lutein from spinach in our diet. Chopping the spinach, before preparing the smoothie, was found to release the greatest amount of lutein .
The findings were published in the journal, Food Chemistry.
Since elevated levels of fat-soluble lutein are found in dark green vegetables, researchers decided to compare various ways of preparing fresh spinach to maximize the lutein content in the finished food product.
We know that people with coronary artery disease (CAD) (narrowing of the arteries caused by fatty plaques) have chronic, low-grade inflammation that can be measured in the bloodstream. Measuring this degree of inflammation is important since it is linked to an increased risk of heart attack and stroke.
Previously, researchers at Linköping University had studied the role of lutein in reducing inflammation in blood vessels, particularly in the immune cells of patients with CAD. Their research demonstrated that that lutein can be stored in immune cells, allowing the body to build up a reserve of lutein. Increasing dietary intake of lutein would therefore be a reasonable approach to increasing lutein levels.
In the new study, the researchers decided to figure out the most effective food preparation method to maximize lutein, using spinach since it contains relatively high levels of lutein and is widely consumed. But similar to many other nutrients, lutein is also degraded by heat.
"What is unique about this study is that we have used preparation methods that are often used when cooking food at home, and we have compared several temperatures and heating times. We have also investigated methods of preparation in which the spinach is eaten cold, such as in salads and smoothies", said Lena Jonasson, professor in the Department of Medical and Health Sciences and consultant in cardiology.
The researchers embarked on various methods of food preparation to figure out the most efficient way to retain the highest levels of lutein from spinach. Using fresh baby spinach, they used steaming, frying, or boiling it up to 90 minutes. They then proceeded to measure the lutein content at different times of food preparation.
Researchers compared different temperatures and heating times in food preparation, since spinach cooked in a soup or stew is not heated to as high a temperature or for as long as spinach in a casserole.
The study revealed that heating time is crucial, especially when spinach is boiled. Spinach retains less lutein, the longer duration of boiling. But the method of preparation is also critical: when spinach is fried at high temperature a large amount of the lutein is degraded after only 2 minutes.
Since warming up meals in a microwave oven is obviously quite common, the researchers discovered that reheating the food in a microwave, to some extent, compensated for the loss of lutein in cooked food. It turns out that lutein is released from the spinach as the plant structure is degraded by the microwave. But the researchers found that it's better not to heat spinach, if you can avoid it .
They noted that one of the best ways to maximize the amount of lutein is to make a smoothie and add healthy types of fat from dairy products.
"Best is not to heat the spinach at all. And even better is to make a smoothie and add fat from dairy products, such as cream, milk or yogurt. When the spinach is chopped into small pieces, more lutein is released from the leaves, and the fat increases the solubility of the lutein in the fluid", said Rosanna Chung, lead author of the study.
