HMB supplementation has no significant effect on muscle hypertrophy in individuals consuming adequate protein, but may produce minor effects under conditions of high energy deficit or extreme training volume.

Study: A double-blind placebo controlled trial into the impacts of HMB supplementation and exercise on free-living muscle protein synthesis, muscle mass and function, in older adults

HMB didn’t make older men much stronger or bigger than exercise alone, but it did help them gain a little more muscle over time—so it’s not useless, just not a huge boost.

Study: Effects of beta-hydroxy-beta-methylbutyrate supplementation on strength and body composition in trained and competitive athletes: A meta-analysis of randomized controlled trials.

This study looked at whether HMB supplements help athletes build muscle and found no real benefit, even in people training hard or eating lots of protein. So, it supports the idea that HMB doesn’t do much for muscle growth.

Study: The Influence of Acute Beta-Hydroxy Beta-Methylbutyrate (HMB) Ingestion on the Human Skeletal Muscle Transcriptome

This study shows that HMB triggers genes in muscles that help build and repair muscle tissue, similar to how protein does. It doesn’t prove HMB makes muscles bigger, but it does show HMB is doing something helpful at the cellular level.

Study: Does HMB Enhance Body Composition in Athletes? A Systematic Review and Meta-analysis

This study found that HMB supplements don’t really help build muscle if you’re already eating enough protein, but might help a tiny bit if you’re training super hard or not eating enough calories.