There are six major classes of nutrients: carbohydrates, fats, minerals, protein, vitamins, and water.
These nutrient classes can be categorized as either macronutrients (needed in relatively large amounts), including carbohydrates (including fiber), fats, protein, and water or micronutrients (needed in smaller quantities), minerals and vitamins.
The macronutrients (excluding fiber and water) provide structural material (amino acids from which proteins are built, and lipids from which cell membranes and some signaling molecules are built) and energy. Some of the structural material can be used to generate energy internally, and in either case it is measured in Joules or kilocalories (often called “Calories” and written with a capital C to distinguish them from little ‘c’ calories). Carbohydrates and proteins provide 17 kJ approximately (4 kcal) of energy per gram, while fats provide 37 kJ (9 kcal) per gram., though the net energy from either depends on such factors as absorption and digestive effort, which vary substantially from instance to instance. Vitamins, minerals, fiber, and water do not provide energy, but are required for other reasons. A third class of dietary material, fiber (i.e., non-digestible material such as cellulose), is also required, for both mechanical and biochemical reasons, although the exact reasons remain unclear.
Other micronutrients include antioxidants and phytochemicals, which are said to influence (or protect) some body systems. Their necessity is not as well established as in the case of, for instance, vitamins.
Most foods contain a mix of some or all of the nutrient classes, together with other substances, such as toxins of various sorts. Some nutrients can be stored internally (e.g., the fat soluble vitamins), while others are required more or less continuously. Poor health can be caused by a lack of required nutrients or, in extreme cases, too much of a required nutrient. For example, both salt and water (both absolutely required) will cause illness or even death in excessive amounts.2.1.1. CARBOHYDRATES 2.1.2. PROTEINS 2.1.3. FATS 2.1.4. VITAMINS 2.1.5. MINERALS 2.1.6. WATER