Do probiotics and digestive enzymes boost protein absorption?

Short answer

Absorption went up a little. Taken with protein, blood amino acids rose 14-23% more, but mostly in the first 1-2 hours, and lean mass did not increase.

Do probiotics and digestive enzymes boost protein absorption?
'More absorption → more muscle' — does this formula hold?
Many ads say that mixing probiotics or digestive enzymes into your protein shake boosts absorption and builds more muscle. We checked two things separately: does absorption really go up, and does that turn into muscle?
Probiotics: blood amino acids went up
Leucine from pea protein rose 23% more
When 15 people took 20 g of pea protein with probiotics for 2 weeks, blood leucine over the next 3 hours was 23% higher and total essential amino acids 16% higher. When 30 people added a different probiotic to 25 g of milk protein for 2 weeks, peak essential amino acids were also higher.
Digestive enzymes did much the same
Essential amino acids 14% higher within 1 hour
When 24 people drank 25 g of whey protein with protein-digesting enzymes, essential amino acids rose 14% more within 1 hour. In a trial of 24 people using 20 g of pea protein, amino acids also rose more in the first 2 hours.
But the gap showed only in the first 1-2 hours
Over the full period, leucine was no different
In the whey trial, total amino acids and leucine were no different with the enzymes. In the pea trial, leucine was no different over the full 5 hours. In 15 people who took an enzyme-probiotic blend for 15 days, absorption showed no meaningful difference.
So did muscles grow more?
Lean mass stayed the same
An analysis that pooled 20 trials and 679 people taking protein plus probiotics found body composition change close to 0. Another analysis of 12 trials of probiotics alone also found no difference in lean mass (muscle and everything except fat).
Strength went up a little more
Muscle mass stayed put; strength and endurance rose a bit
In the same 679-person analysis, strength and muscular endurance improved a little more with added probiotics. In a 60-day training trial adding a probiotic to whey protein, blood amino acids rose 16.1% more and the heaviest leg press rose 16.61% more.
Bottom line: absorption rises, muscle mass doesn't
The gain was blood amino acids in the first 1-2 hours
Probiotics and digestive enzymes raised blood amino acids 14-23% more, mostly in the first 1-2 hours after eating. Lean mass did not increase, and strength rose a little more. There is no real need to mix them in to build muscle.