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Recently, four men and sixteen women with metabolic syndrome, weighing an average of 200 pounds, were put on the low carb South Beach diet for three months.
Although they were told to reduce their carbs down to only ten percent of total calories in the first two weeks and to keep them down to 27 percent in the remaining ten weeks, they didn't manage to follow the diet that strictly.
In fact, they ended up eating 25 percent carbs during the first phase and 35 percent carbs during the second phase. Nevertheless, they lost an average of nearly ten pounds, and half of them no longer had metabolic syndrome by the end of the study.
To find out why and how this occurred, the researchers measured the production of hormones associated with appetite, including insulin, leptin, ghrelin, and cholecystokinin (CCK). CCK is a gastrointestinal hormone that stimulates the digestion of fat and protein and acts as a hunger suppressant.
Leptin is produced by adipose (fat) tissue; when it binds to brain receptors, it creates a sense of fullness. Ghrelin, produced by the stomach lining, increases before meals and decreases after meals; it stimulates appetite, working counter to leptin.
By the end of the first two weeks, insulin and leptin levels had both decreased. By the end of the second phase, insulin levels approached baseline again. Leptin levels, although they also rose, did not return to baseline.
By phase 2, fasting levels of ghrelin had also increased significantly from baseline. After-meal increases in CCK, however, were significantly greater than at the start of the study.
The researchers, led by Matthew Hayes of Pennsylvania State University, speculate that those ups and downs in hormone levels work synergistically, allowing the low-carb diet to create a sense of satisfaction in spite of fewer calories.
As usual, they note that further research is needed to underpin their speculations, especially when it comes to people without metabolic syndrome.
Sources: Journal of Nutrition, August 2007; Medline Plus
Aug 17, 2007
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