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First, let me say that I am generally on the side of "My roof, my rules." If you look at my posts you will see a pattern of supporting the parents as they are the ones that need to make decisions for their household.
Now the reason I say this is because you are "dead wrong", both in your goal and your way of achieving your goal. Some of the things on your agenda at least border on child abuse. I'm pretty sure that at least one thing is technically child abuse, and if it's not, then it's certainly enough to get your local version of DCF involved in your family situation.
I will be negative towards your goals, but I will try to explain alternatives. First, let me start by saying keeping your daughter healthy is important. The fact you're willing to tackle this health issue is a great thing. You just need to be very careful that you're tackling the health issue and not an image issue.
Let's start here. In the US and most of the western world ideal weights, in the public sense, are not based on health or science, but instead on unobtainable body images that very few people have, and even fewer people have naturally.
What's worse is that an ideal weight concern will mask any real concerns about health. Losing 25 pounds is not a health concern on its own. It's a cosmetic one. Only when in context (BMI) is there actually a health concern. When speaking to your daughter about her health never use weight, only BMI. Doing that will place you, and hopefully her, in the correct mind set.
She is a teen. They do that. You can stop buying junk food and replace it with healthy foods, that might work. But to be honest, you should probably give up on this one. In fact, most adults don't eat healthy as a "first choice"; they give up the garbage foods in order to have a better diet when the diet is the goal. For example, people may not eat a doughnut when they are counting calories, not because they don't want the unhealthy doughnut, but because using half of today's calories on one snack is bad.
Unless she has her own motivation to monitor her intake, the best you can really do is make her hide food, or purchase it outside the house.
Withholding food or making less
This is the that can get you in trouble for child abuse. You can not, not feed your daughter. If she says she is hungry after the 15th rotisserie chicken, then give her a 16th. Under no circumstances are you allowed to withhold or deny food unless it's medically required to do so.
Now the laws are a bit odd when it comes to eating disorders. For example, you can say no to someone that is binge eating, but only after you can prove that your way over their needed calories, and usually only with the assistance/advice of a doctor.
What you can do instead, is limit the types of food you cook. And you can limit, reasonably (I know, but almost all child protection laws are fuzzy), parts of a meal. So for example, "You can eat all the salad you want, but you can only have so much ranch dressing". Another great example is not cooking rotisserie chickens but instead making stuffed squash.
Remember the axiom "It's our job to provide the food and your job to eat it." In this case, you can choose what to provide, but how much needs to be as much as she asks for.
My best advice here is to teach her the correct ways to limit herself. You should not be limiting her in any other way. As a bonus, if she learns how to limit herself, she will be less likely to spend $90 on kit-kats at the local 7-11 on her way to school. If you force her, she will just eat what she wants when you're not around.
Locking her out of her phone, etc.
This one is borderline child abuse. I know it's odd to think so and you certainly can use removal of privileges as a discipline technique, but by removing her Internet access your are restricting her from "normal social interactions". What's worse is that you're giving her a punishment (removal of privileges) for a natural state and not to STOP a behavior. Remember punishments only work to stop behaviors; they don't teach good ones. Instead, reward for good behaviors. Give her money. Teens love money. Agree to buy her a new phone if she paints the house, etc. Removing her access to her friends, entertainment, and communication, will not teach her anything except that you don't understand her, and that you are trying to isolate her.
Remember the removal of privileges is a punishment, and punishments should not be given out to START behaviors, but to STOP them. Also removing cell/Internet access is the modern equivalent of locking children in a room or closet. Keep in mind that cell phone access is considered a basic entitlement in the US as is Internet access. Currently, if you don't make enough money to have a cell phone or Internet access the federal government will provide both to you for free.
Level of activity
Best line of your OP:
I'd like to get her to a gym, but none of her friends want to go, and none of the gyms will take her without an adult present.
This is probably 99% of your entire issue. You have shown, through example, that getting to the gym is just not that important. That it comes "after" everything else. I mean think about it. You're essentially saying "It's super important for you to get to the gym, but I'm not going. I have better things to do."
If you want your daughter to prioritize going to the gym, then you have to. You mention logistics, that's fine; it's another excuse to not go. I am great at making these excuses. But the bottom line, if the gym is a priority then you need to be prepared to say and act like it.
"I want to go to band", "sorry, right now is gym time, band will have to wait." Until you can do that, no gym for you.
What about her goals
Like many things in life, you can't force her to do what she doesn't want to do. She has to decide that this is important to her. You can not force her to care. It is certainly possible to be happy and overweight. It's even possible to be healthy and overweight. Being overweight increases risks of other health issues, but if your ok with those risks then you likely won't choose to put down the twinkie.
Make sure to ask her, and truly consider her goals. If she feels that school is too much stress and she doesn't want to worry about weight control right now, then you should consider that. Maybe you could point out how to make dieting less stressful. Maybe you could find ways to reduce school-based stresses.
Whatever her goals are, you need to get her to put this on the list, and you can't force it. If she really wants to she can just walk out the front door, head to the nearest 7-11 and eat every old hotdog on the roller thingy. She has to decide to make this a goal. You can't force her to.
What you can do/general advice
- Don't talk to her about her weight! EVER!
- Don't withhold food
- Don't punish her for a natural state
Don't expect her to have more commitment to this than you do.
Do see a doctor, and have them discuss the health issues
- Do accept the doctor's stance. If he suggests it's fine, then it's fine
- Do focus on health issues and concerns.
- Do set an example by prioritizing these issues higher on your priorities list.
- Do understand that her goals may not be your goals. You need to focus on her goals.
- Do make adjustments as a family, and not just her.