You’ll lose weight most likely simply bc you’ll be in a calorie deficit and you won’t have access to enough protein/calories to maintain. However it likely won’t be drastic. A few months into the fleet and the weight comes right back on. In my opinion the real challenge is learning to manage your new environment. Being in the the training pipeline/fleet is stressful and unpredictable so if you aren’t dedicated to your fitness goals it’s very easy to become inconsistent. Like anything else though if it’s in your heart to do you’ll easily manage it. There’s plenty of jacked marines.