Which diet works best for you may depend on whether or not you are secreting high levels of insulin. From September 2004 to December 2006, researchers monitored 73 obese young adults who ate either a low-fat diet (55 percent carbs and 20 percent fat) or a low-glycemic diet (40 percent carbs and 35 percent fat).
They found that when insulin concentration (measured thirty minutes after a dose of oral glucose) was above the median, the low glycemic diet produced a greater decrease in weight and body fat than the low-fat diet.
In fact, those people lost 12.8 pounds, versus only 2.6 pounds on the low-fat diet. For people with insulin secretion below the midpoint, there was no difference between the two diets as far as weight and fat loss.
In both low and high insulin groups, however, the low-glycemic diet produced more HDLs (healthy cholesterol) and lower triglyceride concentrations. The low-fat diet produced bigger improvements in LDL (bad) cholesterol in both low and high insulin groups.