Leviticus 3 talks about the preparation of animal sacrifices and ends by saying, "All fat belongs to the LORD. This is a permanent rule for your future generations, wherever you live. You must not eat any fat or blood." (Lev. 3:16-17) What on earth is this about? Why would God single these things out?
Ah, you've stumbled on one of my favorite Bible verses! No fat shaming here--All fat is the Lord's!
Seriously, though, here's what I think is going on. There are probably more technical answers that can be provided about the sacrificial system in ancient Israel. But I am generally more interested in what larger Truths any given part of the Bible might be trying to teach us rather than specific facts about an ancient culture, even though they may be interesting. So here's how I find this weird detail to be relevant for anyone in any age, including our own.
The passage references both fat and blood and, to me at least, the blood seems more obvious. While it is God's spirit that gives life in the breath, the physical substance that makes sure the oxygen in the breath gets to all parts of the body and sustains life is the blood. Blood is literally the stuff of life and is therefore the most precious part of any offering. The blood is God's gift and should therefore be given back to its Maker once it is no longer needed.
Blood is still precious; we should not lust for blood. We should not shed the blood of others and, if someone lies bleeding before us, we should rush to stem the bleeding. Treating the blood with reverence is a symbolic way of reverencing all life. And refraining from eating blood is a way to symbolically recognize that we don't live by taking another life. Our lives are God's direct gift.
The fat is slightly different. Fat in an animal--human or otherwise--is stored energy that the body is not currently using. It's excess fuel that is conveniently stored for a time when food is less plentiful or when a person or animal is otherwise unable to eat for a time due to illness, hibernation, or other circumstance. You literally then live off the fat until you are able to eat again. In the Bible, fat signifies abundance. The Bible talks about fat times and lean times, like the dream Joseph interprets for Pharaoh in Genesis 41 and describes an area rich in resources as a fat land. So fat is generally a good concept in the Bible, bringing up images of a land that can produce an abundant harvest, where there is enough to feed everyone.
But the Bible also uses fat in a negative way, especially in the prophets. In the Bible, that abundance or fatness is good only when it is shared. If a king or someone of means has grown fat while others are left wanting, dire warnings abound. Read Ezekiel 34 if you want to have your hair blown back by God's anger about this kind of injustice. Ezekiel 34:20 sums up the warning with, "I will judge between the fat and the lean sheep." In God's economy, fatness represents taking more than we need, which creates scarcity for others. The need of the "lean sheep" brings God's wrath on those of us who have failed to carve off our own excess so that others might have enough.
So here in Leviticus, mandating that "All fat is the Lord's" is a way of creating a symbolic gesture that recognizes that any abundance belongs to God. We are not to take more than we need for ourselves. Fat is extra and that goes back to God. Always. God has provided plenty for all; but only if we return any fat.
But, biologically, don't we need at least some fat to live? Yes, we do. But the healthy fats for human bodies are the plant-based fats in things like nuts, seeds, avocados, etc. Animal-based fats are actually the problem fats that clog our arteries and cause health issues. So if you put the command to abstain from both blood and fat together, you get a diet that is both biologically and spiritually healthy.