Very simply, macronutrients are the nutrients which the body requires in large amounts. These nutrients are fats, carbohydrates, and protein. Carbohydrates are used for fueling high intensity exercise and can be thought of as the generic fuel your body runs on. Carbs are pretty much the least specific nutrient you will take in and have the most common uses. If you aren’t much of an athlete, you probably don’t need a ton of them. If you are an athlete, you probably should start eating lots of pasta, fruits, and whole grains.
Protein is an extremely important micronutrient, as it is what the body essentially uses to repair itself. From your hair to the cell membranes in your body, protein is involved in repairing everything. Once again, if you’re a sedentary person, you most likely don’t need a ton of protein, probably 0.4 to 0.5 grams per pound of body weight or target body weight. If you’re an athlete, bump that up to 0.8 to 1.1 grams.
Fats are also extremely important. Fats function both as energy reserves and as means of distributing fat-soluble vitamins throughout the body. While you don’t need an extreme abundance of fat, it is safe to say 30-40 grams is a minimum amount depending on where you are weight-wise. It is also important to get nutritious fats from things like eggs, fish, avocados, nuts, and other regularly occurring fats. While things like butter are not necessarily bad, it do not add much micronutritional value to your meal and it is extremely calorically dense (meaning just a little bit of butter packs a large amount of fat). Thus, it’s generally a better rule of thumb to stick to foods that have both fat and other vital micronutrients.
The next important thing to understand is how macronutrients correspond with calories. The relationship is simple: Protein contains 3.4 calories per 1 gram, carbohydrates contain 4 calories per 1 gram, and fat contains 9 calories per 1 gram. As you can see, high protein diets will likely be low calorie diets, and high fat diets will likely be high calorie diets. In order to build a complete diet, one that takes into account your caloric needs and micronutrient needs, it’s important to do a bit of a balancing act. You must get a healthy amount of protein, fat, and carbohydrates. As a general rule of thumb, the way you decide how much you get of what follows a priority order: Protein, fat, carbohydrates. First attend to protein needs, then factor in how much fat you need (or want) in your diet, and then fill the rest of the calories up with carbohydrates.
That being said, it is possible to do low carb diets, low protein diets, or even low fat diets. It depends on what your goal or situation is. You never need to do these kinds of diets. The point is to say that if you have celiacs, can only eat meat, or want to go vegan and don’t want to eat a ton trying to get protein, all of those options are viable depending on the goal. If you want to be a bodybuilder, you will need loads of protein and carbs. If you don’t want to be a professional athlete or trainer, then those considerations don’t matter.