HMB
A metabolite of leucine that's especially good at one thing: stopping your body from breaking down muscle. Most useful when you're dieting, aging, or on a GLP-1 drug.
HMB (beta-hydroxy beta-methylbutyrate) is a substance your body naturally makes from leucine — but only about 5% of the leucine you eat gets converted. Supplementing gives you a concentrated anti-catabolic signal that tells your muscles "don't break down."
Good for you if: You're cutting weight, over 40 and losing muscle, on a GLP-1 drug like Ozempic/Mounjaro, recovering from illness, or doing intense training with insufficient recovery.
Dive deeper into the researchCommon side effects
- No significant side effects reported in clinical trials
- Mild GI discomfort possible if taken on an empty stomach
- Rare: mild nausea in the first few days
What does HMB do?
Your body is constantly building up and breaking down muscle protein. When you're in a calorie deficit, not training enough, or stressed, the breakdown side wins. HMB works primarily by slowing down muscle protein breakdown — it's anti-catabolic rather than anabolic.
Think of it this way: creatine helps you perform better in the gym. Protein helps you build. HMB helps you keep what you've built, especially when conditions aren't ideal.
What can you expect?
- Less muscle loss during dieting — preserves lean mass when calories are restricted
- Faster recovery — reduces muscle damage markers after intense training
- Better outcomes for older adults — especially combined with resistance training
- Muscle sparing on GLP-1 drugs — helps counteract the lean mass loss that comes with rapid weight loss
How to take it
3 g per day, taken in 1 g doses with meals. Or take all 3 g at once — both work. HMB-CA (calcium salt) is the most common and affordable form.
If using HMB-FA (free acid), take it 30 minutes before training for faster absorption.
Who benefits most?
HMB doesn't help everyone equally. Here's who gets the most out of it:
| Scenario | Benefit level |
|---|---|
| Calorie deficit / cutting | High — significant muscle sparing |
| Older adults (50+) | High — combats sarcopenia |
| GLP-1 drug users | High — reduces lean mass loss |
| Beginners / untrained | Moderate — some muscle and strength gains |
| Trained lifters (maintenance) | Low — minimal added benefit |
Losing weight on a GLP-1? Track what you're keeping.
eterni monitors your lean mass markers so you can see if HMB and training are protecting your muscle.
Get early accessFrequently Asked Questions
Does HMB build muscle?
HMB is better at preserving muscle than building it. In trained lifters with adequate protein, HMB doesn't add much for muscle growth. But during calorie restriction, aging-related muscle loss, or GLP-1 drug use, HMB shines — it significantly reduces the muscle you'd otherwise lose.
HMB-CA vs HMB-FA — which form is better?
HMB-FA (free acid) absorbs faster and peaks in about 30 minutes vs 1–2 hours for HMB-CA (calcium salt). HMB-FA may be better for timing around workouts, but HMB-CA has more long-term research behind it. Both work at 3 g/day. HMB-CA is cheaper and more widely available.
Should I take HMB on Ozempic or Mounjaro?
It's worth considering. GLP-1 drugs cause significant weight loss, and 20–40% of that loss can be muscle. HMB at 3 g/day, combined with resistance training and adequate protein, can help preserve lean mass during rapid weight loss. It's one of the few supplements with direct evidence for this use case.
Can I take HMB with creatine?
Yes. They work through completely different mechanisms — creatine boosts performance and power output, while HMB reduces muscle protein breakdown. Stacking them is safe and may be synergistic, especially during a cut or for older adults.
How it works in your body
HMB reduces muscle protein breakdown through two main pathways. First, it inhibits the ubiquitin-proteasome pathway — the main system your body uses to tag and destroy muscle proteins. Second, it stabilises muscle cell membranes by contributing to cholesterol synthesis within muscle cells, making them more resistant to damage.
It also has a mild anabolic effect by stimulating mTOR (the same pathway leucine activates), but this effect is much smaller than leucine itself. The real value is on the anti-catabolic side.
What the studies show
- Calorie restriction: In a 2013 study, HMB preserved lean mass in athletes on a 20% calorie deficit while the placebo group lost significant muscle
- Older adults: Meta-analyses show HMB + resistance training increases lean mass by 0.5–1 kg over 12 weeks in adults over 60
- Recovery: Reduced creatine kinase (CK) levels — a marker of muscle damage — by 40–50% after eccentric exercise
- Trained athletes: Minimal additional benefit in well-fed, well-trained individuals — confirming HMB's primary role is anti-catabolic
Side effects & safety
HMB has one of the cleanest safety profiles of any sports supplement:
- No significant adverse effects in any published clinical trial at 3 g/day
- No hormonal effects — doesn't affect testosterone, estrogen, or cortisol
- No liver or kidney concerns — safe in multiple long-term studies (up to 12 months)
- Mild GI discomfort — occasionally reported, usually resolves by taking with food
Who should check with a doctor: People with kidney disease (as with any amino acid-derived supplement) or those on blood pressure medication (HMB-CA contains calcium).
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