When you say a fruit/vegetarian diet are you implying that you are purely having fruits and vegetables?
Are you implying that you are a vegetarian?
No issues, let me answer this question for both the scenarios.
In the former case you WILL lose muscle.
Fruits and vegetables have almost no protein in them. Ok broccoli, spinach have around 5g/cup but you need to eat a LOT of these to even reach close to 20–30g from them.
In the latter case, there are lot of vegetarian food options that can help you cross over the 100g protein mark even without supplements. Although careful planning and effort is required.
Many people on a vegan diet will say that they get adequate protein, but there’s another aspect that is often overlooked.
You specifically mentioned muscle. Leucine is the most important amino acid for protein synthesis and muscle growth. Many vegan sources of protein lack sufficient leucine for protein synthesis.
If you’re working out and getting adequate protein in your weight loss diet, you won’t lose muscle.
Muscles are build up of proteins which are obtained through different food sources. In your case, they’d be all kinds of pulses (legumes), soyabean, hemp seeds and flax seeds, quinoa, nutritional yeast and walnuts and almonds. As long as you’re taking these things in your diet with proper exercise, you need not worry about muscle loss.
Maybe, maybe not. What fruits and vegetables. Anything that causes you to make insulin will prevent fat from getting used for fuel. Insulin also generally protects muscle. That means it will get your metabolic rate down. Ideally you want low insulin and that lets fat be used and muscle stay protected. Remember sugarcane is also a vegetable, so is corn. Can’t eat that and say you’re eating veggies.