Step on a digital scale. Then, drink tons of water (not too fast) and don't go pee. Step on the scale again. You've gained weight without consuming any calories (or any fat, sugars, carbs, or cholesterol). Go pee and step on the scale again. No calories were either consumed or burned in the making of this weight change.

Salt is another food with zero calories, zero fat, zero sugars, zero carbs, and zero cholesterol that will still make you gain weight, but this one acts much more slowly and will raise your blood pressure too.

So, you don't need to consume any calories to gain weight or to make yourself unhealthy.

Furthermore, while a pound might be 3500 calories, those 3500 calories can take the shape of a massive and jiggly pound of fat or a tight and trim pound of muscle. If the 3500 calories turn into a jiggly pound of fat, future calorie burn rates will be lower than if those calories turn into a trim pound of muscle.

Furthermore, changes in eating and exercising habits can also change metabolism. Eating fewer calories might result in fewer calories being burned by the body.

This is why removing calories from junk food also doesn't make people healthy. It doesn't matter if they make the junk food no-fat, no-sugar, no-carb, and no-calorie. It's still junk food. (Remember salt and high blood pressure?)

Being healthy is not merely about losing weight or eating fewer calories. If you're only thinking about weight, maybe a pound is a pound, a calorie is a calorie, and math is simple. However, for overall health, a pound of muscle is different from a pound of visceral fat, a calorie of whole grains is very different from a calorie of high fructose corn syrup, and we can't even get the right numbers to use in the math formulas.