Macronutrients

Macronutrients are basic sources of energy and nutrition for our bodies. They usually include proteins, carbohydrates and fats.

Proteins

Proteins provide 4 calories of energy per gram. They are essential nutrients, which means you can’t live without them. Their main function, however, is not to be used as energy, but to help us build and repair our bodies. They are the main building blocks of our organism. They are important for building muscles, and also for regeneration and healing of each and every organ and tissue of our bodies.
They consist of amino acids. Amino acids can be essential and non-essential. Complete proteins are only those which contain ALL the essential amino acids in adequate quantities. Incomplete proteins contain only some essential amino acids in adequate quantities, and they can’t be fully utilized by your body, unless they are combined with other proteins (complete or incomplete), so that the combination becomes complete (containing all the essential amino acids in adequate quantities).

Best sources of protein are: fish, seafood, meat, eggs, dairy products. These foods contain COMPLETE proteins. Plant foods contain incomplete proteins, but they can become complete if you combine them. For example, you get complete proteins by combining legumes with grains.
Good plant sources of protein are also: soy, peanuts, other nuts, mushrooms, etc. They all contain INCOMPLETE proteins, but you can get all the essential amino-acids if you eat diverse plant foods, and combine many different plant sources of protein.

Please note that you don’t need to always combine incomplete proteins inside the same meal in order to make them complete. It’s enough that you take proteins from variable sources each day so that your overall intake of all the amino acids is adequate.

Carbohydrates

Strictly speaking, carbohydrates are not essential nutrients, which means you can survive without them. (Body can use alternative fuels, such as ketones obtained from fat metabolism).
However, carbohydrates are THE BEST FUEL for our organism. Brain uses glucose, and unless you eat carbohydrates, you must produce glucose in your body from fat or proteins.
Carbohydrates can be divided into simple sugars (monosaccharides and disaccharides), mainly glucose, fructose and sucrose; and complex sugars, mainly starch.
Simple sugars cause fast increase in blood sugar levels followed by sharp drop which can cause craving for more sugar. Whatever tastes sweet contains simple sugars, including fruit. Overindulgence in simple sugars leads to obesity, can cause diabetes as well, and many other problems. So we need to be careful with sweet things and eat them in moderation. Fruit is better eaten whole than in juice, because whole fruit contains fibers as well, which slow down the absorption of sugar.
Complex sugars are generally slow and steady, reliable energy sources, but some of them can also cause sharp spikes in blood sugar, for example white rice and white bread, which is best to be avoided. Whole grains, on the other hand also contain fibers, which slow down absorption of starch, and they are excellent and healthy energy source. Carbohydrates, like proteins also contain 4 calories per gram.

Fats

Unlike carbohydrates, fats ARE essential nutrients, so you must eat them to survive. To be more precise, you must eat fats which contain essential fatty acids. Besides that, fats are most densly packed with energy: each gram of fats contains 9 calories.
There are only 2 essential fatty acids: Omega 3 and Omega 6.
Sources of Omega 6 and Omega 3 are: fish and shellfish, flaxseed (linseed), hemp seed, soya oil, canola (rapeseed) oil, chia seeds, pumpkin seeds, sunflower seeds, leafy vegetables, and walnuts.
Omega 3 to Omega 6 ratio is also important. In standard Western diet people usually get too much Omega 6 and too little Omega 3.
Best source of Omega 3 is fish and fish oil, plant sources must be further processed in body to provide satisfactory Omega 3. Walnuts are also good source of Omega 3.

Fats can be divided into: saturated fats, unsaturated fats, trans fats and cholesterol.
Generally unsaturated fats are good for you, saturated fats are controversial and potentially risky, but probably not as bad as many people think, at least when consumed moderately. Cholesterol from food sources generally does not have a very strong effect on blood cholesterol levels, so its impact has also been exaggerated. However if you already have high cholesterol, you should try to cut your cholesterol intake from food.

By far the worst fat is trans-fat, (so much that it’s already banned in some countries)… It is artificial partially hydrogenated vegetable oil, that’s artificially and partially saturated, so that it becomes solid. It is present in most margarines, shortenings, and industrial cookies. Ideally, they should be avoided completely. (Pay attention not to become orthorexic though)
Unsaturated fats are from plant sources generally (and some animal sources like some fish). They can also be problem if eaten too much, but they are generally very healthy.
Saturated fats are from meats and animal sources. They can cause increased blood cholesterol, heart disease, atherosclerosis, etc. However if eaten moderately, and combined with other healthy fat sources, such as nuts, olive oil, etc, their impact can be neutralized. It’s important not to overindulge in foods that contain saturated fats, eat them rarely and in small amounts, but there’s no need to avoid them completely.
Fats are also excellent energy source, and they are generally not fattening. Fat does not make you fat! The only thing that is fattening is consuming more energy than you spend. Eating fats also activates fat metabolism, so when you eat fats, you send your body signals to burn fat, which can even help with weight loss. So generally extra-low-fat diets, generally produce worse results, compared with other diets, when it comes to weight loss.