What is HMB?HMB (beta-Hydroxy beta-Methylbutyric acid, try saying that three times fast!) is a metabolite of the branched chain amino acid Leucine that is naturally synthesized by the human body and involved with muscle protein synthesis (MPS).
HMB can be obtained through a normal diet, but it's only found in relatively minor amounts in grapefruit, catfish, and alfalfa sprouts, to name a few. More commonly, you'll encounter HMB in capsule form, sold as a dietary supplement.
Proposed Benefits of HMBHMB supports point to three major benefits when touting the benefits of this Leucine-derivative:
- Increased Muscle Mass
- Decreased Catabolism (muscle breakdown)
- Accelerated recovery
HMB ResearchThe purported benefits of HMB seem to be can't miss, but before you rush down to the store to buy a year's supply of it, let's see what the research says in regards to validating or disproving these claims.
When HMB first gained prominence in the sports science arena, a select number of studies did in fact show significant increases in both strength and muscle mass:
One of the earliest studies conducted on HMB occurred in 2000. The study, published in the research journal Nutrition, recruited a group of 36 men and women, ages 20-40, and gave half the group 3g/day HMB while the other half received a placebo. Following a 4 week resistance training program,the HMB group increased upper body strength and fat free mass compared to the placebo group. 
Another double-blind, placebo-controlled study, also published in Nutrition, investigated the effects of creatine, HMB, and a combination of the two compounds on individuals for a period of 3 weeks. Overall, all 3 groups gained strength and there was also a reduction in creatine phosphokinase (marker of muscle cell damage) compared to the placebo group.  Researchers concluded both HMB and creatine induce strength gains, but via different physiological mechanisms.
Note: Doses used for the various groups were 3g/day HMB for HMB only group, 20g/day creatine for the first 7 days followed by 10g/day for the following 14 days for creatine only group, and the combination group received the cumulative dose of both ingredients.
So far, it seems to be going well in favor of HMB supplementation. However, when you look a little bit more into those studies, you'll see that they were conducted by Steven Nissen, the owner of the patent on HMB. While this doesn't definitively invalidate the study's' findings, they do lend a touch of skepticism to results.
Now, let's take a look at some of the unbiased trials:
A 2009 double-blind, placebo-controlled study in the Journal of Strength and Conditioning Research concluded that although HMB did enhance lower body strength, it did not significantly alter body composition in resistance-trained men. 
A 6 week trial conducted in 2001 investigated if there were any differences between an instant release capsule vs. a time-release capsule of 3g HMB on body composition, 3-rep max, and markers of muscular damage. 
At the conclusion of the trial, researchers concluded that, "...HMB administration had no influence on these variables. Likewise, biochemical markers of muscle protein turnover and muscle damage were also unaffected by HMB supplementation."
The University of Memphis conducted a 4 week study on the effects of 3g vs. 6g doses of HMB on indicators of training-induced body composition, strength changes, and catabolism in resistance trained men. 
At the conclusion of the 4 weeks, researchers noted, "28 d of HMB supplementation (3 to 6 g x d(-1)) during resistance-training does not reduce catabolism or affect training-induced changes in body composition and strength in experienced resistance-trained males."
Much like you would have presumed, the non-biased research doesn't quite yield the same jaw-dropping benefits as the potentially biased research does.
To further drive the point home, a 2000 systematic review of all the HMB studies to date surmised:
"Of the literature reviewed relating to HMB administration during resistance training, only 2 papers are full manuscripts appearing in peer reviewed journals. The remaining 8 papers are published as abstracts only, making it difficult to critically review the research. There is clearly a need for more tightly controlled, longer duration studies to verify if HMB enhances strength and muscular hypertrophy development associated with resistance training across a range of groups, including resistance trained individuals." 
Recent HMB ResearchAs you may have noticed, the most recent of the above studies concluded in 2009. Surely there has been some more recent research on HMB, right?!
In fact there is!
Several studies have investigated a few different forms of HMB. Two in particular have been studied: one binds HMB to calcium salts (CA-HMB) and the other free acid HMB (HMB-FA). Both have found to be better absorbed by the body than isolated HMB and more effective.  But researchers in the group concluded that the HMB-FA was the superior form.
However, these are both relatively small studies in both population size and duration, which leaves a lot of room for error.
Still, a 2015 study on CA-HMB conducted on 20 elite canoeists concluded "supplementation might decrease the damage to skeletal muscle."  Additionally, a 2016 study on the calcium salt form of HMB on a group of "58 highly trained males" documented "advantageous changes in body composition and stimulates an increase in aerobic capacity." 
So, where does that leave us?
HMB-Ca or HMB-FA?!
The vast amount of research on HMB shows that it may help with lean mass changes and improving recovery, but overall adding HMB to your regimen would only warrant benefit if you are significantly increasing volume and/or frequency of workouts (i.e. an "overreaching" period in training). Otherwise, you really won't see much benefit at all.
One last note is that the Ca form of HMB is MUCH cheaper than the FA form, and equally as effective based on the most recent research, so if you do have some extra cash to spend on supplements, go for the cheaper calcium salt and dose accordingly.