Do you lose muscle more easily the leaner you get?

Short answer

Yes. The lower the body fat, the bigger the share of weight loss that came from muscle. Still, people who lost slowly at 0.7% of body weight a week and ate plenty of protein kept or even gained lean mass.

Do you lose muscle more easily the leaner you get?
'Less fat → more muscle loss' — does this formula hold?
People often say muscle goes too in the final weeks of contest prep. We checked step by step whether muscle really goes more easily as fat gets lower, and whether it can be prevented.
Less fat, a bigger share from muscle
The leaner the body, the more muscle went
A review of body-weight changes found that the lower the starting body fat, the bigger the share of weight loss that came from lean mass (muscle and everything except fat). In twin pairs with different weights, half of the gap was lean mass in lean twins, versus 1/4 in twins with more fat.
The same in 32 people on a semi-starvation diet
More starting fat meant more fat lost
A re-analysis of an experiment where 32 people were semi-starved for 24 weeks found that the higher their starting body fat, the smaller the share of lost energy that came from protein (the muscle side). In reverse, the leaner they started, the more came from muscle.
What about real contest prep?
34-43% of the weight lost was lean mass
One person who prepped for 26 weeks went from 88.6 kg to 73.3 kg and from 17.5% to 7.4% body fat, losing about 5.2 kg of lean mass (34% of the weight lost). Another who prepped for 14 weeks lost 11.7 kg, of which 5.0 kg (43%) was lean mass. Each is a single-person case (1 each).
Measuring men and women together
Men lost more lean mass
Among 13 men and 10 women preparing for a contest over 23 weeks, both lost fat, but men lost more lean mass. In 27 women who cut 12% of body weight over 4 months, lean mass measured by DXA held steady, but thigh muscle size dropped slightly.
So can it be prevented?
The slow group actually gained
24 people lost weight while lifting. Those who lost 0.7% a week gained 2.1% lean mass, while those who lost 1.4% a week did not gain (-0.2%). An analysis pooling studies of training while eating less found that a shortfall of about 500 kcal a day stopped muscle gains.
The leaner you are, the more protein you need
Lean mass loss: 1.6 kg vs 0.3 kg
20 people ate only 60% of their usual calories for 2 weeks. Those eating about 70 g of protein a day (for a 70 kg (154 lb) person) lost 1.6 kg of lean mass, while those eating about 161 g lost 0.3 kg. A review of 6 studies concluded that protein needs rise the leaner you are and the less you eat.
Bottom line: the leaner you get, the easier muscle goes
So go slower near the end, and keep protein high
The lower the body fat, the bigger the share of weight loss from muscle, and in 2 contest-prep cases 34-43% of the loss was lean mass. Losing slowly at 0.7% a week and eating plenty of protein protected muscle. The contest-prep guideline is 0.5-1% of body weight a week (0.4-0.8 kg for an 80 kg person).