Canh chua is the lowest calorie thing on a Vietnamese table that still tastes like a meal. A full bowl is 150 calories. The broth is water, tamarind, fish sauce and about a teaspoon of sugar per bowl, so 30 calories. The vegetables, 100g of pineapple, tomato, okra and bac ha stem, are another 30. The piece of fish, usually ca loc (snakehead) or ca dieu hong (tilapia), is 65 for 60g and it is lean.
The published range is 90 to 180 per bowl and the fish is the variable. Canh chua with ca hu or ca basa, the fatty catfish cuts with the belly on, is 250 to 280 because catfish belly is 15 percent fat. With prawns it is about 120. Vegetarian canh chua with tofu is 100. The fried garlic on top is the one thing to watch: a teaspoon of toi phi in oil is 25 calories and some quan use a tablespoon.
At a com binh dan quan the soup comes as part of a set: a plate of rice at 300, a clay pot of ca kho at 300, and canh chua at 150. The soup is the part you should eat more of. Ask for a second bowl and the meal is 900 calories with an extra 14g of protein and a lot more volume, versus adding a second helping of rice at 150 for nothing.