I want to know if it is true that "raspberry ketones" product is useful for decreasing body fat?

No. Avoid fad diets.: Helpful weight loss diets have natural foods that take time to chew but contain few calories. Examples: carrots, celery, green leafy lettuce (little/no salad dressing), etc... When shopping for fruits, buy small ones no bigger than a tennis ball, because fruits have sugar. Drinks without calories include water and green tea. A calorie is a calorie, but surely avoid sugar ; simple starch calories.