Nothing special will happen, the person will not become a hulk or superman. A billion calories = a million kilocalories, which is roughly equal to 4.18 megajoules. Or 1.15 kWh, which is not so much if converted into conventional energy. But if you count the body's costs in calories, then according to any online calculators, the energy supply will be enough for you to play badminton for 4 months)) This is such an economical human body)
A bit of arithmetic:
Given 10 ^ 9 kcal.
Conventionally: 1 kg of fat ~ 7000 kcal.
10 ^ 9 / (7 * 10 ^ 3) ~ 1.43 * 10 ^ 5
143000 kg of fat ~ 159 cubic meters. Meters of fat.
Of the available foods, fat is the most nutritious.
And it is precisely this amount that represents the energy value of a billion kcal.
It can be assumed that the question does not make sense.
I guess the question is what would be the result if it were possible (for example, a "pill" with a nutritional value of a billion calories). Then it would be necessary to find out what exactly it contains: proteins, fats or carbohydrates, and build on this information.
But most likely, of course, death. Because, whatever it may be in an abnormally large amount, in the body, as a rule, leads to death.
This is possible only if a person drinks oil (900 kcal per 100 g), but the body is much smarter than we think, and therefore, soon after a large consumption of oil, a person will vomit. And in order to drink 11111 servings of oil at a time, that is, 111110 liters, it will take a very long time, given that our stomach cannot hold such a number of liters. And if we talk about the fact that a person does not care, even if he does not vomit, tries to do it, then there is only one outcome - rupture of the stomach and death.