Adults in general need at least 1,500 calories a day to keep their bodies functioning, because even when you're lying down, your body needs energy to maintain body temperature, cardiorespiratory fitness, and brain function. Basal metabolic expenditure can vary depending on height, weight, age and gender among individuals. Humans need energy to survive and get that energy from food. The calorie content of a food is a measure of how much potential energy that food produces. 1 gram of carbohydrate contains 4 calories, 1 gram of protein contains 4 calories, and 1 gram of fat contains 9 calories.
So consuming 400 calories doesn't count for anything, because people just burn energy sitting down.
What you're asking about is how many calories burned must be "energy burned through the exercise itself" in order to be truly healthy and beneficial.