How can I help my preteen lose weight without damaging her self-image?
Girls at this transitional age are very vulnerable to body image and self-esteem issues, and seeds planted here—both positive and negative—can have long-term effects. So, it's wise to proceed with caution. Indeed, the emphasis should not be on your daughter's need to "lose weight," but rather for her to "get healthy" and physically feel better by making lifestyle modifications. If executed correctly and consistently, weight loss will be a sure-fire by-product of this endeavor.
Start with a trip to her pediatrician to ensure there are no immediate and problems—such as blood pressure, blood glucose and cholesterol levels—associated with her overweight condition.
Assuming her pediatrician indicates that your daughter needs only to make some lifestyle modifications, have a frank discussion with her about specific health risks often associated with childhood obesity or an overweight condition—even in children—such as type 2 diabetes. Also discuss any physical limitations she has experienced due to being overweight, such as an inability to ride a bike for any length of time, if at all, getting winded climbing up stairs, feeling hot and sweaty when others in the room are perfectly comfortable, etc.
Lead with a discussion about the problems she is facing due to her poor state of health, and then segue into a brainstorming session of relatively small actions that, if taken consistently, will help her get on track. For instance, you can suggest swapping out whole milk for skim, white bread for whole grain. This will give her new perspective with far less shame and defensiveness than if the discussion were centered on her physical appearance.
This approach will underscore that, although her poor health condition has serious consequences and needs to be addressed, it is not a "life sentence" unless she allows it to be. She should understand how and why she is in complete control of changing the state of her health through a consistent sequence of relatively simple actions, if she so chooses. It's OK to mention that she also will lose weight with these lifestyle changes, which surely will provide her with an extra dose of motivation. But that should not be the point of focus for discussion, nor the primary objective of the changes being made. This discussion is about vitality—not vanity.