β-Hydroxy β-methylbutyric acid (HMB) is a metabolite from leucine and is considered to be a very promising anabolic agent.
Leucine is first converted in the body to α-ketoisocaproic acid and then to HMB.
As seen in pigs, about 5% of leucine is converted to HMB, and the HMB concentrations increase significantly with the consumption of leucine. This has yet to be determined in humans.
If extrapolated to humans, and assumed that humans have similar enzyme actions (to pigs), a human would produce:
- 0.45g HMB from 100g protein from steak, which contains 8.9g leucine
- 0.67g HMB from 150g protein from steak, which contains 13.4g leucine
- 0.88g HMB from 200g protein from steak, which contains 17.6g leucine
So it could be assumed that supplementing with 3g+ HMB daily could potentially have a very anabolic effect.
How does HMB work?
HMB has been studied for more than 20 years now; it works through a few mechanisms that boosts muscle protein synthesis, reduces muscle damage, increases strength, athletic performance and even anabolic hormones such as testosterone and IGF-1 while reducing catabolic ones such as cortisol (1).
- Stimulates muscle protein synthesis by increasing PI3K-Akt-mTOR and IGF-1
- Decreases fat mass by activating AMPK, thus promoting mitochondrial biogenesis, higher oxygen consumption, improved efficiency of carbohydrate and fat metabolism, as well as increased lipolysis. More efficient oxidative metabolism is also associated with increased muscle strength generation.
- Inhibits proteolysis (muscle breakdown), by suppression of mitochondrial-caspase signaling pathway, modulation of the autophagy-lysosomal system, and downregulation of the ubiquitin-proteasome proteolytic pathway (lowering FoxO, MurF1, Atrogin 1 and proteolysis-inducing factor).
- Lowers circulating pro-inflammatory cytokines (TNFα, IL-6, which prevent catabolism, contractile dysfunction, and disruption of myogenesis)
- Promotes muscle fiber cholesterol synthesis, augmenting the cholesterol concentration in the cellular matrix, which would reduce proteolysis
So basically, HMB increases muscle protein synthesis, inhibits muscle catabolism, improves energy metabolism, strength and athletic performance, reduces inflammation and DOMS and increases the synthesis of cholesterol, which can be used to increase androgen synthesis, such as testosterone.
Many studies do find HMB to be effective at boosting hypertrophy, strength athletic performance and testosterone, while also reducing fat mass, cortisol, inflammation, and DOMS. Why did I say steroid like results?
According to this study by Wilson et al. (2), 3g HMB supplementation for 12 weeks in already trained men resulted in an increase in lean body mass by 7.4kg vs. only 2.1kg and a decrease in fat mass by 5.4kg vs 1.7kg in the placebo group. Plus the HMB group put on an extra 28kg, 9kg, and 14kg on their squat, bench press and deadlift respectively vs the placebo group. The researchers claimed that the groups consumed only 22% protein and didn’t specify how much calories they consumed. So if we assume that they consumed 2,500 calories, they would have eaten around 137g of protein daily.
Sounds like steroid like results to me.
But how effective is it really?
I looked at studies to see the training status of the individuals, protein intake and the training programs, and here’s what I found:
HMB was effective at increasing strength, GH, IGF-1, while eating only 120g of protein daily in untrained individuals (3).
3g/6g HMB increased hypertrophy but didn’t reduce fat mass or improve strength in untrained men while eating 100g of protein daily (4).
3g HMB improved aerobic capacity, testosterone and body composition in highly trained men (who did sport) while eating 1.5g/kg of protein (5).
3g/6g HMB for 28 days did not reduce muscle catabolism or affect training-induced changes in body composition and strength in experienced resistance-trained
men, while consuming 2.2-2.4g/kg/day of protein (6).
3g HMB didn’t increase strength or affect body recomposition in trained men after 6 weeks while eating 2.4g/kg/day of protein (7).
3g HMB didn’t affect testosterone or cortisol after 6 weeks in trained men; this study did not measure body recomposition, strength or protein intake (8).
3g HMB improved body composition and increased aerobic and anaerobic capacity after 12 weeks in trained combat sports athletes while eating 1.7g/kg/day (140g) of protein (9).
This recent meta-analysis also found no effect of HMB supplementation on strength and body composition in trained and competitive athletes (10).
It would seem that if you’re relatively healthy, have a little bit of training experience and are eating enough protein daily, HMB would give you no benefit. It would seem HMB still gave some benefits while only eating 140-150g of protein, but not when eating >2.2g/kg. HMB might be more beneficial for people who aren’t that healthy, really struggle to build muscle and/or cannot eat lots of protein.
Plus, HMB is a little expensive so it might be a good idea to rather get some protein/protein rich food instead which will also contain other beneficial nutrients, vitamins, and minerals that HMB doesn’t have. But if you truly don’t have time for food, or can’t eat a high protein diet, HMB might really help you out.