How much muscle do you lose while prepping for a show?

Short answer

About 1/4 of the weight lost. 19 natural competitors dropped 7.1 kg over about 21 weeks, of which 1.7 kg was lean mass, and most of it came back within 6 weeks after the show.

How much muscle do you lose while prepping for a show?
'Contest prep = muscle loss' — does this formula hold?
Prepping for a show means months of eating less and doing more cardio. So many say you pay for the stage with the muscle you built. We checked studies that measured people prepping without drugs.
About 1/4 of the weight lost
7 kg out of 7.1 kg
19 natural competitors were measured from about 21 weeks out. They lost 7.1 kg: 5.8 kg of fat and 1.7 kg of lean mass (muscle and everything except fat). About 80% of the weight lost was fat.
Women lost almost none
Body weight down 12%, lean mass unchanged
27 women prepping for fitness competitions lost about 12% of body weight and 35-50% of their fat over 4 months. Lean mass measured by a precise scan (DXA) did not change; only the front-thigh muscle shrank slightly in cross-section.
The bigger losses came in men who got leaner
One man lost 3.4 kg
Pooling case reports of 15 people tracked through more than 3 months of prep, the men, who reached lower body fat for the stage, tended to lose muscle, while women kept or slightly gained it. One man lost 3.4 kg.
Going to extremes shook the whole body
1 man: body fat 14.8% → 4.5%, testosterone down to a quarter
In a case report following 1 man for 12 months around a show, body fat fell from 14.8% to 4.5%. Meanwhile his testosterone fell as if 100 became 25, and his strength dropped. Hormones recovered after the show, but strength was not fully back even 6 months later.
Losing slowly meant losing less
At 0.7% of body weight a week, lean mass actually rose
24 people lifted 4 times a week while losing 0.7% or 1.0% of body weight per week. The slow group gained 2.1% lean mass; the fast group stayed the same. For a 70 kg (154 lb) person, that is about 0.5 kg vs 0.7 kg a week.
Cutting more than 500 kcal a day stopped muscle gain
Strength held, but muscle growth stalled
An analysis pooling studies of lifting in a calorie deficit found strength gains no different from groups not in a deficit, but smaller muscle gains. Around a 500 kcal daily deficit, muscle gain disappeared.
Bottom line: you lose 1/4, and 6 weeks brings it back
'Contest prep = muscle loss' is only half true
The 19 natural competitors who lost 1.7 kg regained 2.7 kg within 6 weeks after the show, and at 12 weeks were 1.6 kg above where they started. Women lost almost nothing, and those losing slowly, about 0.7% of body weight a week, lost less.