Chao ga is about as cheap as calories get on a Vietnamese street: a 400g bowl for 250 calories, and 18g of that is protein. The rice porridge itself is 60 to 80 calories per 100g depending on how thick the quan makes it, so around 150 for the bowl. The chicken is usually 50g of shredded boiled ga ta, 75 calories. Spring onion, coriander, pepper and a small spoon of fried shallots make up the rest.
The reason it works for weight loss is volume. 400g of warm liquid stretches the stomach the same way a bowl of pho does, for half the calories. The catch is you will be hungry in two hours because 32g of carbs and 18g of protein is not a lot of food. Ask for extra chicken, 100g instead of 50, and it becomes a 325 calorie bowl with 30g of protein, which holds you much longer.
The trap is the side order. Quay, the fried dough sticks, are 130 to 150 each and every chao quan sells them. Two quay torn into the bowl is 280 extra, more than the chao itself. The other one is chao long, the offal version, where the fried intestine and liver take a 250 bowl to 400 or more. And a raw egg cracked in, common at late night stalls, adds 75.