Does contest prep weaken your bones?

Short answer

Not from one contest prep. Bone mass fell 1.3% and came back over 3–4 months of recovery. Women, though, also saw hormone drops and more irregular periods.

Does contest prep weaken your bones?
'Extreme cutting → lower hormones → weaker bones' — does this formula hold?
Contest prep means months of eating less, moving more and stripping body fat as low as possible. A common worry is that hormones drop and bones weaken. We gathered studies that actually measured bone during contest prep.
What did case studies find?
All 4 people whose bone density was measured kept it
An analysis of 11 case studies (15 people) tracking contest prep for over 3 months found 3 studies, covering 4 people, that measured bone density with precise scans. None of the 4 lost bone density. One person who cut body fat by 55% over 32 weeks also kept whole-body bone density unchanged.
What happens in a larger group?
27 people, 4 months of cutting, 1.3% less bone
When 27 people preparing for a contest cut body weight by about 12% over 4 months, whole-body bone mass measured by precise scans fell 1.3% on average. Over the same period, 23 people who did not diet gained 0.3%.
The lost bone came back
After 3–4 months of recovery, no difference from before prep
Over 3–4 months after the contest, with more food and less cardio, bone mass rose 1.5% again, ending 0.1% above the pre-prep level. Body weight and most hormones also recovered in that period.
What else changed during prep?
Hormones dropped and irregular periods rose
During prep, the same 27 people saw estrogen, testosterone, thyroid hormone and the appetite hormone leptin all fall, and more of them had irregular periods. The drop in bone mass came at the same time as the drop in hormones.
Differences between women and men
In 5 days, only women's bone markers changed
In a 5-day experiment of eating less and running daily, 11 women showed a fall in bone-building markers and a rise in bone-breakdown markers. In 11 men, neither marker changed. In 1 male bodybuilder, testosterone fell during prep from 100 to about 25, then recovered after the contest.
Summary: one prep did not weaken bones
At most 1.3% lost, and it came back during recovery
The 4 case-study athletes kept their bone density, and the 27-person group lost 1.3% and regained it over 3–4 months of recovery. Women, though, also saw hormone drops and irregular periods, and bone markers shifted within 5 days. The post-contest recovery period with more food was when bone came back.