I started yesterday with an explanation of complete vs. incomplete protein, and together with a discussion I had with my oldest about nutrition and poverty in the US, reminded me that people in the US actually know very little about nutrition. When I was a kid, they used the 4 food groups and/or a pie chart; more recently they've used a food pyramid, and generally the schools seem to think that's enough information. I remember even as a kid being a bit offended when my teacher told me that a glass of milk could either count as a dairy or a protein food for the day, but not both. That implied that if the body used the calcium in milk, it couldn't use the protein and vice versa, which is absurd.
Let me start by reminding you that I'm not a trained nutritionist, just a self-educated one. A better simplified explanation is that foods provide three main things: protein, carbohydrates, and fats. They tend to be classified by what they provide the most of. Lean meat is mostly a protein, but it usually has some fat. The same with dairy. Whole wheat bread supplies a lot of carbohydrates, but it does have some protein and fat. Some fruits and vegetables supply almost entirely carbohydrates, but most of them are very high in vitamins and minerals that you don't get from the high-protein or high-fat foods.
The estimates of how much protein you need in a day varies, but despite what writers say, it's the food people are mostly likely to skimp on, particularly women. I like to aim for 60 grams in a day, which isn't as hard as you might think, but I focus particularly on protein at breakfast. My favorite nutrition writer (one who wasn't afraid to REALLY explain nutrition in detail) suggested that studies supported 22 grams of protein at breakfast as an ideal, along with a small amount of carbohydrate and fat, preferably including some fruit.
Why? This level of protein, when supported by the other foods, after "fasting" all night, raises your blood sugar to a good level (lots of energy) and keeps it high through lunch. Less protein, especially if replaced with sweetened foods, tends to give you a high level for a short time, then drops below what it was before you ate, making you very tired. If that happens, even a high protein lunch won't get that energy level back. But if you had that good breakfast, then a light lunch with at least a little of each, you should keep that energy level all day. A high fat breakfast, on the other hand, will raise your energy some and sustain it, but not nearly as much as the high protein breakfast.
Think of the carbohydrates like twigs for a fire. Fat turns the twigs into longer-burning logs, and protein is like a draft control, supplying just enough air to burn well without so much that it burns out too fast. Not a very good analogy, but you get the idea...