The ALS worry did not show up in people's blood beforehand
Across five cohorts, 275 people who went on to develop ALS had blood drawn before any diagnosis. None of the three branched-chain amino acids was linked to higher risk.
What the study found
Blood measured years before diagnosis showed no raised ALS risk
The way to test this is to measure people before they get ill. Five cohorts, 275 people who went on to develop amyotrophic lateral sclerosis, blood drawn before any diagnosis: none of the three branched-chain amino acids was linked to higher risk, with every point estimate below 1.
The caveat · Blood levels, not supplement use
This measures what was in the blood, not whether anyone took a supplement, so a powder started after the draw would be invisible. The intervals are wide enough to include modest risk either way, leucine running from 0.57 to 1.33. Estimates below 1 are not evidence of protection.