If you've spent any time around the health and fitness world, you've probably heard of macronutrients. But what, exactly, is a macronutrient? More importantly why do they matter so much? The Broad Definition Put simply, a macronutrient is a nutrient that your body requires in large quantities – typically because these substances are used for fuel. Generally, fats, protein and carbohydrates are listed as macros but some experts add water to the list as well. This inclusion is a little controversial, though, since water doesn't actually bring any calories with it. Still the ever-present liquid is required for every chemical reaction that your body uses to release the energy from the food you eat, making it a vital part of the refueling process. Similarly, some sources will classify fiber as a macronutient. And, technically, it is. But fiber is also a form of carbohydrate. So, making it its own category is a littler redundant and not commonly done. For our purposes, then, we're just going to focus on the main three macronutrients that everybody can agree on: protein, carbs and fats. Protein Often called “the king of macronutrients,” protein plays a wide variety of important functions within your body. Although we generally talk about protein as if it's just one substance, the truth is that there are tons of different types of protein that all varying in form and function. To understand this – and why proteins are so important – though, it's important to talk about amino acids. The smaller substances not only make up proteins, but they make up just about every tissue, cell and chemical in your body. Each protein that you ingest is built of amino acids, combined in a specific structure and order. Unless your body has an immediate need for that particular protein, however, most proteins are rapidly deconstructed. Their amino acids are then reordered, repurposed and used to fill some other need within your system. While each gram of protein contains about 4 calories, most of that energy is needed to take apart the complex structure of the protein itself and don't really produce much fuel. Plus, your body needs those amino acids for so many other things that it would rather not burn them for energy. Although there is quite a bit of disagreement about exactly how much protein people should be eating, the truth is that many people simply are not getting enough. Generally, the modern diet – particularly one that consists mainly of processed foods – is high in both fats and carbohydrates. To ensure adequate protein intake, protein powders are convenient nutritional tool. Carbohydrates Carbs, on the other hand, are exactly the sort of thing your body wants to use for fuel. Every gram of carbohydrate that you ingest contains 4 calories that can be quickly absorbed and put to use. In order to really benefit your body, however, these carbohydrates must first be converted into glucose – which is actually a fairly simple process. Any glucose that isn't immediately needed is then turned into glycogen and tucked away in your liver and muscles for future use. It's unusual but glycogen that exceeds even the vast storage capacity of your liver and muscles, can be turned into adipose tissue (AKA body fat). As mentioned, the umbrella-term “carbohydrates” also include fiber – which is not actually broken down by your body. Still this nutrient is vital in maintain both a health cardiovascular and digestive system. Fats Finally, we come to the oft-maligned and sorely misunderstood fats. Packing a whopping 9 calories in each gram, fats are a powerful source of energy and are generally used in a 50/50 mix with carbohydrates. Because fats are a little more complicated and take longer to breakdown, however, your body tends to rely on them more for longer-duration, lower-intensity activities. Besides their role in energy production, though, fats are also vital for the processing and absorption of many vitamins and minerals – which can only be dissolved in fat, rather than water. Some fats have also been shown to exert powerful benefits on the function of the immune system, heart and brain. Read More: The Bioavailability of Whey Protein Isolate High Protein, Low Carb Diet and Why You Need Whey Protein Plant Based Protein Benefits