In weight training and bodybuilding, protein intake recommendations of up to 4.0 g/kg of body weight and per day were propagated. Such enormous quantities were never based on evidence and are also not recommended in serious scientific sources.

As indicated by various scientific evidence, normal weight people who want to build up muscle mass should follow a daily protein intake of 1.4-3.3 g/kg of body weight. Sports science institutions recommend a protein supply of 1.3-1.8 g/kg of body weight for adults who are intensively exercising.⁽¹⁾ Noadditional muscle growth was observed with intakes of more than 2.6 g/kg of body weight, but fat accumulation could be reduced during a high calorie diet (mass phase/bulking).^{(2, 3)}

If, for some reasons, the total energy intake is reduced (calorie deficit), it is advisable to proportionally increase the protein intake to 1.8-2.7 g/kg of body weight (see also definition & diet or weight reduction).

Literature:
1) Hot Topic Protein im Sport Version 3.1, ed. Swiss Sports Nutrition Society
2) Leaf A., Antonio J.: The effects of overfeeding on body composition: the role of macronutrient composition - a narrative review, Int J Exerc Sci. (2017)
3) Antonio J et al.: A high protein diet (3.4 g/kg/d) combined with a heavy resistance training program improves body composition in healthy trained men and women - a follow-up investigation, J Int Soc Sports Nutr. (2015)

When the body's energy balance is negative, proteins are also used as energy source. Therefore, the proportional increase of protein intake is particularly important in such a situation. Consequently, in case of a low energy intake, slightly higher quantities of 1.8-2.7 g protein per kg of body weight are recommended. Dietary proteins have an effect on satiety and body composition. The intake of protein leads to a higher satiety than equal amounts of energy from carbohydrates or fat. Protein requires also more energy for digestion, which means less net calories. In addition, the fat-free body mass (lean muscle mass) is better preserved with a simultaneous decrease of body fat. (1, 2) 

Literature:
1) Hot Topic Protein im Sport Version 3.1, ed. Swiss Sports Nutrition Society
2) Proteine in der Ernährung des Menschen (2011), hrsg. von Eidgenössische Ernährungskommission, May 2011

As proteins are also used as an energy source by the body when the total energy supply is insufficient, an increase of protein intake during a calorie restriction diet is important. Individuals with a vegetarian or vegan diet must particularly pay attention to cover their needs for essential amino acids, which cannot be produced by the body itself. The intake of protein leads to a higher satiety than equal amounts of energy from carbohydrates or fat. Protein requires also more energy for digestion, which means less net calories. In addition, the fat-free body mass (lean muscle mass) is better preserved with a simultaneous decrease of body fat. (1)
In case of an intended weight loss (calorie deficit) of physically active persons, a protein amount of 1.8-2.7 g/kg of body weight per day is recommended.(2)
For overweight or obese people with no physical activity and normal renal function, a protein intake of 1.2-1.5 g/kg of body weight per day is recommended.(1)

Literature:
1) Proteine in der Ernährung des Menschen (2011), hrsg. von Eidgenössische Ernährungskommission, May 2011
2) Hot Topic Protein im Sport Version 3.1, ed. Swiss Sports Nutrition Society

Protein not only supports muscle growth, but also the maintenance and the regeneration of the muscles after exercising, which is often neglected, especially by endurance athletes. As a matter of fact, protein requirements are highly dependant on daily exercise volume and intensity. However, different protein recommendations depending on type of sport hardly makes sense. Also whether muscle build-up or endurance is trained is only of limited importance. Protein recommendations are rather about supporting specific training phases and targets. Accordingly, there is nowadays a uniform, sport-independent recommendation for protein intake. For physically active adults, this represents 1.3-1.8 g/kg of body weight per day.(1)

Literatur:
1) Hot Topic Protein im Sport Version 3.1, ed. Swiss Sports Nutrition Society

As elderly people have an increased risk of disease and malnutrition, sufficient protein supply and energy intake is of great importance for this population group. If illness, injury or being bedridden are added, an adequate protein supply becomes even more important. With regard to bone health in age, especially to fracture prevention, a protein intake of 1.2 g/kg of body weight per day is recommended – provided a sufficient energy intake (!) - to prevent the reduction of body proteins and muscles.

If the energy supply gets short due to illness or other reasons (e.g. lack of appetite), a proportionately increased protein intake becomes all the more important.⁽¹⁾ In severe, muscle-wasting illnesses but still stable energy balance, even 1.2-1.5 g/kg of body weight daily are recommended. Additional physical activity increases the protein needs to 1.3-1.8 g/kg of body weight.

For aged people with wasting illnesses it is particularly important to consider relatively larger portion sizes of protein, namely 25-30 g.⁽²⁾

Literature:
1) Proteine in der Ernährung des Menschen (2011), ed. Eidgenössische Ernährungskommission, May 2011
2) Paddon-Jones D., Rasmussen B. B.: Dietary protein recommendations and the prevention of sarcopenia, Curr Opin Clin Nutr Metab Care (2009)