Although there are some fruits that contain a decent amount of protein (like guavas, avocados, jackfruit, and kiwi), fruits can’t be considered a good source of protein in and of themselves.
The problem with fruitarian diets is the significant lack of protein in them. Not only does protein have a higher thermic effect than carbs and fat (and is therefore beneficial for boosting metabolism), but it also contributes toward increasing muscle mass (via Healthline). When you’re not getting sufficient protein, your muscle strength and mass will suffer. This can slow down your metabolism too, according to Dr. Caroline Apovian, director of the Nutrition and Weight Management Center at Boston Medical Center (per The Healthy). “Your resting energy expenditure (REE) will go down because [it] is directly related to how much muscle mass you have,” explained the expert.
Resting energy expenditure, also known as resting metabolic rate, refers to the rate at which your body burns energy while it’s resting. Functions like breathing, cell repair and growth, blood circulation, regulating body temperature, and eating all fall under everyday things your body needs energy for, even when you’re not actively exerting yourself. That being said, this doesn’t mean that you shouldn’t be eating fruit every day.