But what I don't see ever is any accounting for the duration/intensity of the bodybuilding routine. It seems like that would be an important factor.
I agree, it seems like that should matter! But it appears not to:
We again found a cut-off point at exactly 1.6g/kg/d beyond which no further benefits for muscle growth or strength development are seen [...]
Another frequently heard objection is that people need more protein because they are more experienced than the studied populations. Well, Tarnopolsky et al. (1988) used elite bodybuilders and found that less protein was needed than in novice bodybuilders.
In fact, the finding that the more experienced you are, the less protein you need, has been replicated in several studies (Rennie & Tipton, 2000; Hartman et al., 2006; Moore et al., 2007).
There are some exceptions, though:
The only people that may actually need more protein than 0.82g/lb [=1.8g/kg] are trainees on performance enhancing drugs, concurrent athletes with a high volume of both endurance and strength training and trainees with substantial muscle memory to allow for very rapid muscle growth. Adolescents that are still growing may require more too. There’s no good research on these scenarios. Those rare individuals with amazing bodybuilding genetics could also qualify, but unless your father happens to be a silverback gorilla, you probably don’t need more than the usual 1.8g/kg/d.
Edit: So it turns out I misunderstood the question. It's not "do you need to scale up if you lift a lot", it's "can you scale down if you don't lift like a professional bodybuilder". Very interesting. I can't find any studies on the subject, and I can't recall hearing about any such studies. I'm just some dude on the Internet, not a professional, so there may well be studies I don't know of, but still the answer might well be "no, there are no formulae accounting for this" (and might even be "no, there's no need to account for this").
So while I'm afraid I won't be able to produce a proper answer to the original question, let's aim for second best and do some counting to see if those 1.6 g are too much - is there any reason to aim lower than 1.6 g/kg if you're not a bodybuilder?
Well, no, not for health reasons, it seems. According to EFSA:
Considering the 97.5th percentile of the distribution of the requirement and assuming an efficiency of utilisation of dietary protein for maintenance of 47 %, the PRI for adults of all ages was estimated to be 0.83 g protein/kg body weight per day and is applicable both to high quality protein and to protein in mixed diets. [...] Data are insufficient to establish a Tolerable Upper Intake
Level (UL) for protein. Intakes up to twice the PRI are regularly
consumed from mixed diets by some physically active and healthy adults
in Europe and are considered safe.
So the PRI ("the intake of a nutrient that is likely to meets the needs of almost all healthy people in a population") for protein is 0.83g/kg bw/day; up to 1.66g/kg bw/day is considered safe; and there's no known upper limit at which protein is known to become harmful.
So eating bodybuilder-like quantities of protein isn't going to hurt us, per se, unless we're forsaking something else. Will we, at 1.6g/kg?
I played around with EFSA's DRV Finder (unfortunately you can't link to a specific configuration but they link to sources; numbers are for adults):
Adding up the carb/fat ranges, we can see that protein intake is expected to be in the range 5-35 E% (but above 0.83 g/kg bw/day), so there's quite a lot of wiggle room in here. Exactly how much wiggle room depends on how much energy you need in a day, but looking at EFSA's reference values for energy, 5-35 E% translated into grams of protein (with 1 g protein = 4 kcal) yields the following result:
For adult women:
Sedentary: 23.5-164.3g (5-35% of 1878 kcal)
Moderately active: 26.8-187.9g (5-35% of 2147 kcal)
Active: 30.2-211.3g (5-35% of 2415 kcal)
Very active: 33.5-234.8g (5-35% of 2683 kcal)
For adult men:
Sedentary: 29.2-204.6g (5-35% of 2338 kcal)
Moderately active: 33.4-233.8g (5-35% of 2672 kcal)
Active: 37.6-263.0g (5-35% of 3006 kcal)
Very active: 41.8-292.3g (5-35% of 3340 kcal)
I'm not sure which bodyweight was assumed for these numbers (for finding out what you need yourself, you'd probably get better numbers from a calculator), but according to EFSA's recommendations eating 1.6 g protein/kg bw/day is within the recommended span even for sedentary women up to 102 kg bodyweight, and sedentary men up to 127 kg (although individuals at those bodyweights will almost certainly need more energy than detailed above unless they are very short).
So it seems that for most people, there's little to no harm to be had from 1.6 g protein/kg bw/day. Not from the intake itself, and not because you're missing out on other nutrients (as long as you are eating a varied diet and not just chugging protein shakes).
Finally, the economic incentives to cut down on proteins. Yes, proteins tend to be more expensive than carbs or fat, but remember that all protein sources count. It's not just meat, eggs and dairy, but also grains, vegetables, nuts, seeds, etc, so make sure you account for these when you calculate your protein consumption. And if you still feel the need to increase your protein intake, instead of jsut adding more chicken to the chicken-and-rice lunchbox, you can replace some of the rice with lentils or beans (which actually is a good idea since the amino acids in rice and beans complement each other pretty well, plus you get some high-quality slow carbs from the beans). Beans still cost more than rice, but it's still a lot cheaper than meat or chicken.
In conclusion, I'm sorry for the wall-of-text mostly-non-answer, but this was really interesting to figure out! I'm a bit surprised at the amount of protein you can fit into a public-authority-approved diet.
