As with most things in nutrition, there’s no simple answer. Your optimal daily protein intake depends on various factors, notably your health goal, body composition, and level of physical activity. And even taking all this into account, you’ll end up with a starting number, which you’ll need to adjust through self-experimentation.
Optimal daily protein intake in grams per kilogram of body weight (g/kg)
It is easy to assume that getting more daily protein than the Recommended Dietary Allowance (RDA) serves no purpose. Despite their name, though, RDAs do not represent optimal intakes; rather, they represent the minimum needed by healthy, sedentary adults to avoid deficiency-related health issues (such as scurvy from not enough vitamin C).
In the case of protein, the 0.8 g/kg RDA represents the minimal amount a healthy, sedentary adult needs daily to prevent muscle wasting when total caloric intake is sufficient. This number has been challenged, however: 1.2 g/kg has been suggested as a better number, by studies that used the Indicator Amino Acid Oxidation (IAAO) method to overcome many of the shortcomings of the nitrogen-balance studies used to establish the RDA.
For example, nitrogen-balance studies require that people eat experimental diets for weeks before measurements are taken. This provides ample time for the body to adapt to low protein intakes by downregulating processes that are not necessary for survival but are necessary for optimal health, such as protein turnover and immune function. By contrast, the IAAO technique takes just 24 hours to determine protein requirements.
Of course, if you are not sedentary, if you exercise regularly through work or leisure, then you need even more protein. The American College of Sports Medicine, the Academy of Nutrition and Dietetics, and the Dietitians of Canada recommend 1.2–2.0 g/kg to optimize recovery from training and promote the growth and maintenance of lean mass when total caloric intake is sufficient. This recommendation is similar to the 1.4–2.0 g/kg promoted by the International Society of Sports Nutrition (ISSN).
Importantly, it may be better to aim for the higher end of the above ranges. According to the most comprehensive meta-analysis to date on the effects of protein supplementation on muscle mass and strength, the average amount of protein required to maximize lean mass is about 1.6 g/kg, and some people need upward of 2.2 g/kg.
However, only 4 of the 49 included studies were conducted in people with resistance- training experience (the 45 others were in beginners). IAAO studies in athletes found different numbers: on training days, female athletes required 1.4–1.7 g/kg; the day following a regular training session, male endurance athletes required 2.1–2.7 g/kg; two days after their last resistance-training session, amateur male bodybuilders required 1.7–2.2 g/kg. And this is when calories are sufficient. An early review concluded that, to optimize body composition, dieting athletes should consume 1.8–2.7 g/kg.18 Later studies have argued that, to minimize lean-mass loss, dieting athletes should consume 2.3–3.1 g/kg (closer to the higher end of the range as leanness and caloric deficit increase).This latter recommendation has been upheld by the ISSN and by a review article on bodybuilding contest preparation.
There may also be a reason to eat a little more protein when bulking up. Although gains in lean mass and strength are unlikely to benefit from more than 1.8–2.6 g/kg, a few studies suggest you’ll gain less fat if you consume 3.3 g/kg when eating a mildly hypercaloric diet (370–800 kcal above maintenance) and providing a progressive resistance-overload stimulus.
It should be noted, however, that people who are overweight or obese and looking to lose weight don’t need as much protein as their leaner peers, regardless of activity level. Several meta-analyses involving people with overweightness or obesity suggest that 1.2–1.5 g/kg is an appropriate daily protein intake range to maximize fat loss. It is important to realize that this range is based on actual body weight, not on lean mass or ideal body weight.
Finally, IAAO studies have suggested that the RDA for pregnant women should be about 1.66 g/kg during early gestation (weeks 11–20) and 1.77 g/kg during late gestation (weeks 32–38) When you finally give birth and start lactating, protein requirements are at least 1.5 g/kg.
The protein RDA, 0.8 g/kg, represents the minimal amount a healthy, sedentary adult needs daily to prevent muscle wasting. Not only does this number not represent an optimal intake, but it is based on older assessment techniques. Newer data suggest that 1.2 g/kg is required to avoid deregulation of protein turnover and immune function. And of course, your requirements can increase based on your specific situation (genetics, age, level of physical activity).