One tablespoon of fresh lemongrass provides about five calories, most of which come from carbohydrate (fiber) and protein according to USDA data.4
Lemongrass also provides certain vitamins (in very small amounts) including, vitamin A, vitamin C, folate, and niacin. However, consuming lemongrass won’t have a significant impact on your daily vitamin needs.
Keep in mind that lemongrass flavored oil provides significantly more calories because it is usually a combination of a cooking oil (like canola oil) and lemongrass extract. For example, one popular brand of lemongrass flavored spray on oil provides 40 calories per serving (1 teaspoon) and 4.5 grams of fat.