Food and nutrition are constant fascinations of mine. I need to summarize what I've learned about the basics over the past couple years, because it really shouldn't be as hard as most folks make it.
There are 4 kinds of nutrition- Protein, Carbs, Fats, and Micros (vitamins and minerals). The Micros help the body use the three Macro nutrients effectively. The brain functions primarily and best on glucose (sugars.) Muscles and tissues at the cellular level are built out of proteins and fats while being fueled by carbs. Water and Fiber are also key components of food, but are primarily modes of transport for the other components.
Plants have the most Micros and Carbs. Animals have the most Protein and Fat. Humans seem to be able to assimilate the protein and fat that are in plants more efficiently than those found in animal sources, meaning less energy consumption and consequent stress on the body overall. The total diet should include more plants than animals, for optimal nutrtion and digestive health.
Refined foods are higher in calories and lower in nutrients. They also frequently contain high levels of salt or added fats for preservation and flavor, respectively. Both are harmful to the body in the amounts used in standard food stuffs.
Weight management is simply a balance of calories in and calories out. Food eaten vs. activity and exercise.
Different parts of the GI tract break down different types of foods. Protein is broken down in the acidic environment of the stomach while sugars are broken down in the mouth and small intestine. Fiber and water exchange happens in the colon. The liver and gallbladder break down fats and the pancreas secretes numerous enzymes and hormones to digest carbs.
Application of heat to food, better known as cooking, breaks down nutrients, enzymes, chi and fiber. Foods that can be consumed raw and whole contain the most nutrients, enzymes, chi and fiber (found only in plants). The body digests one food at a time best- aka monoeating. Monoeating whole, raw foods as soon as they are picked from their source plant are the most vital, potent, and nutritive. There are animal derived foods that can be eaten raw, whale fat for the Inuits, for example. In those cases, the freshest meat from the kill, while it's still pumping blood through its tissues, would be "healthiest."
Balance is the key. American's typically eat way too much protein and fat, from animal sources, and put undue stress on our bodies in the process. We also eat complicated foods with multiple macros that don't combine well, creating an overall acidic, malnourished, overfed, and overworked state of chronic disease.
Once a person understands these basics about what is happening, it's all a matter of finding the appropriate balance of foods of the right types, in the right proportions, with the right preparation, for the lifestyle of the eater. That's where science of nutrition becomes art.