Professor Charlotte Erlanson-Albertsson at Lund University in Sweden said thylakoid, a compound in spinach and other green leaves, slows down food digestion and therefore makes people feel fuller.
Erlanson-Albertsson and colleagues said thylakoids are believed to slow down fat digestion, meaning the whole intestine has time to get involved. Once food enters the distal intestine, satiety hormones are released and sent up to the brain, telling people they are full. However, processed food tends to only utilize the upper intestine, so the key hormones are not released.
"I like to say our intestines are unemployed," Erlanson-Albertsson said in a statement.
However, the spinach has to be crushed, filtrated and centrifuged, freeing the thylakoids from the plant's cells, since the body can't break it down from fresh spinach directly. The final product is a natural water extract.
The researchers found if human test subjects were given a shot of the spinach extract in the morning, they felt less hungry and had fewer cravings during the day. In addition, the study subjects subsequently found it easier to stick to three meals a day, compared to the control group that was given a shot without the active substance.
The thylakoid group also had had higher levels of satiety hormones in their blood, and had more stable blood glucose levels, the study said.
Erlanson-Albertsson said the powerful effect of thylakoids couldn't be traced to just one active ingredient.
"It contains hundreds of substances -- galactolipids, proteins, vitamin A, E, K, antioxidants, beta-carotene, lutein, and so on," she said.
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