In an article soon to be published in the Journal of Agricultural and Food Chemistry, it was shown that polyphenol extracts from strawberries and blueberries can induce weight loss in rats when they are put on a high-fat diet. Surprisingly, if the animals were consuming the fruit itself, no weight loss effects were observed. One reason might be that the fruit contains extra carbohydrates that could have inhibited the weight loss by increasing insulin. Even through strawberries and blueberries are low glycemic-load carbohydrates, they will still have an impact on insulin secretion.
A much more intriguing explanation is that the polyphenols are activates of AMP kinase, the key energy-regulating enzyme in every cell. If AMP kinase is turned on, then incoming nutrients are diverted to increased ATP synthesis. If the AMP kinase is turned off, the incoming nutrients get stored as fat. Thus it is possible that the increased concentration of polyphenols was sufficient to turn on the AMP kinase, whereas the lower concentration of the same polyphenols in the fruits was insufficient to induce an increase in AMP kinase activity. This helps explain the fact that people who eat larger amounts of fruits and vegetables (rich in polyphenols) have 30 percent less likelihood of developing type 2 diabetes.