Why is it the calorie count of food, and not the mass, that determines weight gain?
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The calorie count of a food is a rough approximation of how much useable stuff is inside it. Your body will take that usable stuff to construct more of you and also to burn for energy to power your body.
If you eat a pound of sand (don’t) your body can’t use it. It will just pass through. Same if you eat chalk.
If you eat a pound of ice cream, well that has fat and sugar and protein. Your body can use all that. Special mini machines in your guts grab hold of all the sugar and fat and protein and move them out of your gut and into your blood.
Imagine that the proteins you eat are like already assembled Lego models. Your body takes the proteins apart into bricks (called amino acids) and then reassembles them to make new protiens– like new Lego structures. Those new proteins become part of the physical structure of your body. So just like you could take a Star Wars Lego fighter and disassemble it into pieces and build a little house, you can take a piece of chicken meat and disassemble it and turn it into human protiens. Just like lego constructions can be solid structures or cool machines with moving parts, protiens can be solid structures like the connective webbing that holds your organs together, or moving machines, like your muscle fibers.
Something similar can happen for fat. Fat is used to make the envelopes of your cells. Each cell in your body is like a tiny soap bubble, and the fats are the slimy stuff that encloses each one.
So you can gain weight by taking these plant and animal fats and proteins and breaking them into pieces and then reassembling them to make more of you.
Sometimes your body has plenty of fat and protein pieces, and it doesn’t need to grow, and so it takes the extra “Lego pieces” and actually burns them and extracts the energy from them.
Most of the time, your body prefers to get its energy from the sugar you eat. Sugar is mixed with oxygen inside your cells and that burns the sugar, just like if it was set on fire. Just like with a fire, oxygen gets consumed and carbon dioxide is released. Your cells have special rechargeable mini-batteries called ATP. One single molecule of sugar can charge 32 ATP batteries. Each mini battery can make one tiny machine in your body move.
Your body can store energy in the ATP mini batteries for short term, but for long term storage, the body makes special structures out of sugar molecules or fat molecules. These structures are made to be tightly packed into storage areas in your body. Later, if you are low on energy, the stored sugar or fat can be burned to charge the ATP mini-batteries.
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