Canadian researchers reported that dieters with the most organochlorines (pollutants from pesticides, which are stored in fat cells) experienced a greater than normal dip in metabolism as they lost weight, perhaps because the toxins interfere with the energy-burning process.
In other words, pesticides make it harder to lose pounds. Other research hints that pesticides can trigger weight gain. Of course, it's not always easy to find—or to afford—a whole bunch of organic produce. So you need to know when organic counts, and when it's not that important. Organic onions, avocados, grapefruit? Not necessary. But choose organic when buying celery, peaches, strawberries, apples, blueberries, nectarines, bell peppers, spinach, kale or collard greens, cherries, potatoes and imported grapes; they tend to have the highest levels of pesticides.
A simple rule of thumb: If you can eat the skin, go organic.