Can you eat less protein on rest days?

Short answer

Barely. The protein need measured on a rest day was about 119 g a day for a 70 kg (154 lb) person, 85% of the 140 g measured right after training. That amount was already more than the roughly 113 g a day at which muscle gains stopped rising.

Can you eat less protein on rest days?
'No training → no muscle building → less protein': does this formula hold?
Some say muscle is only built on training days, so protein can be cut on rest days. We checked how long muscle building lasts after a workout and how much protein is needed on a rest day.
How long does muscle building last after a workout?
Untrained muscle was still higher after 48 hours
When 8 people who did not lift did one bout of strength training, muscle protein building was 65% higher after 24 hours and 34% higher after 48 hours. When 10 people trained only one leg for 8 weeks and then exercised again, the trained leg was back to resting levels after 28 hours, while the untrained leg was still 70% higher.
That is where the 'eat less on rest days' idea comes from
But protein eaten the next day was put to more use in muscle
15 people did strength training with one leg and had 15 g of whey protein 24 hours later. After training to failure, the same 15 g raised muscle building more than in the untrained state. After lighter training not taken to failure, this difference did not appear.
Measuring the need directly on a rest day
About 119 g a day for a 70 kg person
Researchers measured the protein need of 8 people with 3 or more years of strength training on a non-training day. For a 70 kg (154 lb) person, the average need works out to about 119 g a day and the generous recommended amount to about 154 g, about 2.6 times the general adult recommendation.
Compared with a training day?
The rest-day value was 85% of the post-workout value
Measured the same way right after a workout, 7 people who already lifted needed about 140 g a day at 70 kg. The rest-day value of 119 g is 85% of that. A 2024 study comparing days with a 10 km or 20 km run and a rest day found that, on the same intake, the body's protein balance was lowest on the rest day.
At what daily amount does muscle stop growing more?
It stopped at about 113 g a day for a 70 kg person
An analysis that pooled 49 studies with 1,863 people found that during strength training, once daily protein went past about 113 g for a 70 kg person, lean mass (muscle and everything except fat) grew no further. To get the most growth, the researchers suggested about 154 g a day at 70 kg.
Summary: eat nearly the same amount on rest days
The rest-day need was already above the 'stopping point'
Muscle building after a workout returned to normal sooner in trained muscle, but the need measured on a rest day was 85% of the post-workout value. That amount (about 119 g) was already more than the daily amount at which muscle gains stopped (about 113 g). At 70 kg, about 119-154 g a day on rest days is enough.