system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do our bodies metabolize carbs first and not lipids or protein?
gpt
Our bodies use carbohydrates (i.e. glucose) as a quick energy source because glucose has the potential to be used by any of your cells.  Think of glucose as CASH--your cells can spend that shit anywhere in your body.  Your cells don't always have cash on hand though, unless you spend 24 hours a day eating and ingesting glucose. Your body also stores glucose and there are a lot of other competing metabolic processes involved, but this is ELI5!

Lipids (Fatty Acids) represent the most efficient energy storage molecules in the body.  Lipids can be stored in our fat cells as triglycerides.  They also pack together tightly, which is convenient for storage.  Think of lipids like your savings account.  You will tap into your savings account when you don't have cash to spend.  Similarly, we break down lipids in order to make energy for our cells when we haven't eaten in awhile and we're low on glucose.  If you eat too much food too often, you will always have a lot of $ (gluclose) lying around to spend.  Your lipids will stack up and your bank account will get FAT.  You will also get fat.

As for proteins -- your body doesn't like to use proteins for energy because they cost a lot of energy to make.  Think of proteins like your watch or your gold chain or whatever.  You can pawn your watch or sell your chain, but man do you really want to do that?  You worked hard for that fucking chain.  But, if you starve yourself for long enough, your body will use proteins to make energy.