ANYONE WHO SPENDS significant time in the gym is probably willing to admit that muscle building isn't easy. It takes a ton of dedication, which is difficult to muster up day in and day out. So, it's only natural for people to look for quick fixes. That often takes form in supplements—and hydroxymethylbutyrate, better known as HMB, is popping up everywhere these days, from peeking out of your gym pal’s bag to that TikTok video.
“Anyone who's ever worked out knows that building muscle takes time and consistency. Recently, fitness influencers have been promoting HMB, claiming that supplementing with it can make it easier to build muscle,” says Kim Yawitz, R.D., a registered dietitian and gym owner in St. Louis, Mo.
HMB might play a role in muscle growth, that doesn't automatically mean it's worth adding into your supplement routine. Experts explain why, below.
What is HMB?
Scientifically known as beta-hydroxy-beta-methylbutyrate and the aforementioned hydroxymethylbutyrate, HMB is the colloquial term that you may see if you're scrolling TikTok or Instagram.
“HMB is a substance that's derived from leucine—the most potent amino acid for muscle building,” says Yawitz.
Maelee Wells Sutton, R.D.N., C.P.T., dietitian at nexEndo in New York City, says that this supplement is typically taken with the goal of muscle preservation, growth, and performance. But (spoiler alert) it's not exactly the pill you'd hope would replace hours in the gym.
“HMB is not a new supplement for athletes seeking to improve muscle mass and performance; however, there is also another facet behind the rising popularity of HMB,” says Wells Sutton. “The metabolite is being tested for effectiveness in muscle preservation, which can be greatly impactful among older adults, those suffering from chronic illnesses, and individuals on weight loss drugs.” (While weight loss drugs such as Ozempic and Mounjaro are certainly effective with weight loss assistance, people are seeing a loss in muscle mass as well as fat mass.)
Understanding HMB means understanding leucine, first. It's an amino acid that creates proteins, which are used to carry out tons of bodily processes, like muscle building. Leucine is considered an essential amino acid—meaning your body cannot make it, and must be consumed via food such as eggs, tofu, lentils, beans, nuts, seeds, and animal products. It is also consumed for performance enhancement and is frequently included in BCAA supplements on the market today.
HMB derives out of leucine. “However, in digestion, approximately only 5% of the leucine consumed will be broken down by the body into HMB. Since HMB itself can only be found in trace amounts in catfish, grapefruit, dairy, and alfalfa, not enough HMB can be practically consumed via food for tangible differences," Wells Sutton says.
Can HMB supplements really help build muscle?
So far, the findings are mixed.
Here’s a little Biology 101: Muscle growth occurs when the body makes new muscle proteins faster than it breaks down old ones, says Yawitz. HMB appears to help the body with both sides of this equation.
“In human and animal studies, HMB has been shown to activate mTOR, a protein that tells the body to make more muscle,” she says. “At the same time, HMB suppresses certain systems that break down muscle proteins, which can help protect against muscle loss.”
That said, Yawitz says it’s unclear whether HMB supplements have much muscle-building potential for active adults. Studies show they may be beneficial for conditions that accelerate muscle loss, including cancer, AIDS, and aging. And, there are some studies suggest that HMB may support muscle growth and post-exercise muscle recovery, she say—but they're all done on advanced athletes on highly demanding physical challenges (like an ultramarathon or a multi-event CrossFit competition). Studies have also found that HMB can help with muscle growth, but only when blood levels reach a certain minimum threshold. That threshold is difficult to reach with food alone.
“Traces of HMB can be found in a handful of foods, including catfish, grapefruit, avocado, asparagus, alfalfa, and cauliflower,” says Yawitz. You can also get it by eating salmon, beef, chickpeas, and other leucine-rich foods. Remember, though—only five to ten percent of leucine is converted into HMB. Accordingly, supplementation may help you meet that threshold. “Supplementing with HMB can help you achieve the minimum levels you need to support muscle growth, especially if you're an older adult or have chronic medical conditions associated with muscle loss,” says Yawitz.
Don't get too excited, though.
“Largely, HMB does not appear to be effective in increasing hypertrophy significantly,” says Wells Sutton. “For athletes, the evidence indicates its most effective use is in the prevention of muscle breakdown and maintaining strength/performance in a [calorie] deficit."
Meaning: it's not likely to make or break your gains.
Are there any other health benefits to taking HMB supplements?
As you’ve probably gathered by now, there’s a lot of interesting research in the realm of HMB, but there are no definitive take-away conclusions… yet.
As we touched on above, HMB has applications for those with cancer and AIDS. “Multiple studies, including [one] from the American Journal of Surgery, indicate a positive effect of HMB when supplemented with arginine and glutamine in slowing/halting cancer and AIDS-related fat-free body tissue wasting,” says Wells Sutton. “Many studies examined HMB in correlation with other supplements, but research on HMB alone in this context is still more sparse.”
Another realm of inquiry for HMB is its potential benefits for seniors. As Yawitz explains, leucine metabolism declines with age, making it more difficult for older adults to build and maintain muscle mass. Some preliminary studies suggest that HMB could help offset age-related muscle loss, especially when paired with exercise—but more research needs to be done.
What to Know About HMB Supplementation
If you’re considering taking HMB, talk to your doctor or dietitian about it first. There are a few different kinds at a few different dosages—so it's important to discuss with a health care provider.
There are two forms of HMB as supplements available on the market: Beta-hydroxy beta-methylbutyric free acid (HMB-FA) and Beta-hydroxy beta-methylbutyrate monohydrate (HMB-Ca or Ca-HMB).
“Most studies have been done with HMB-Ca. As of now, there is not a known difference in efficacy between the two when taken at their respective recommended times and dosages,” says Wells Sutton.
International Society of Sports Nutrition generally recommends taking one to two grams of HMB-Ca, 60 to 120 minutes before exercise; or one to two grams of HMB-FA, 30 to 60 minutes before exercise.
However, Wells Sutton says that supplementation of three grams per day is recommended for at least 2 weeks ahead of high-intensity training to maximize results, according to this ISSN study.
What are there risks of taking HMB supplements?
From current research, Wells Sutton says there are no known risks or negative side effects from taking the tested dosage of HMB, which was three grams per day in most studies. We'll say it again, though: talk to your doctor or another trusted healthcare professional before incorporating it into your routine—they'll know best about how the supplement might effect you personally.
As with supplements of any kind, when selecting a HMB supplement, Wells Sutton recommends only buying those that are third-party tested.
“Supplements are not regulated by the FDA, and as such, some are found to contain inaccurate ingredients and/or quantities. Third-party testing helps by verifying the supplements’ contents to the claims of the company and label, ” she says. “One of the main labels to look for is NSF, which confirms the label is accurate to the contents, as well as tests for toxins and contaminants. For athletes competing in regulated leagues, NSF also has a Certified for Sport label, which screens for banned substances."
