Disclosure
This article contains an affiliate link. Mizuno, discussed near the end, runs an affiliate programme we participate in, and we may earn a commission if you buy through our link. We do not accept payment for favourable coverage — and you will see below that we say plainly that footwear is not the thing that solves the problem this article is about. Read our full affiliate disclosure.
This page is informational only and is not medical advice.
The short version
In the semaglutide trials, somewhere between a quarter and two-fifths of the weight lost was lean mass. In STEP 1 and SUSTAIN 8 the figure was roughly 38–40%.
Resistance training plus 1.2–1.6g of protein per kilogram of body weight per day cuts that to near zero. That is the whole intervention. It is unglamorous, it is well evidenced, and almost nobody sells it to you.
The Numbers
Body-composition data from the major trials is more consistent about the direction than the magnitude, so it is worth seeing the spread rather than a single headline figure.
| Trial | Medication | Share of weight lost as lean mass |
|---|---|---|
| STEP 1 | Semaglutide | About 38–39%. 1,961 adults with obesity, about 15% body weight lost over 68 weeks. |
| SUSTAIN 8 | Semaglutide | In the same 39–40% range. |
| REDEFINE 1 | Semaglutide arm | About 28%. |
| SURMOUNT-1 | Tirzepatide | About 25% on DXA — close to what dieting alone produces. |
Two things follow. First, the range is wide enough that no one should quote a single number as settled — measurement method and population both move it. Second, even the lowest figures mean a meaningful share of what you lose is not fat.
The important qualifier
In STEP 1 and SUSTAIN 8, the proportion of lean mass to total body mass still went up. Body composition improved overall, even as absolute lean mass fell. Losing some lean tissue is normal in any substantial weight loss — a smaller body needs less structural tissue.
So this is not a reason to avoid treatment. It is a reason to do the two cheap things that change the ratio in your favour.
What Actually Prevents It
The evidence here is unusually clean, and it points at two interventions rather than a product.
- Resistance training. In obese older adults undergoing caloric restriction, resistance training preserved nearly all lean mass. Combined with adequate protein, the research indicates lean-mass loss can be cut to near zero. This is loading muscle against resistance — weights, machines, bands, bodyweight — not cardio.
- Protein at 1.2–1.6g per kg per day during active weight loss. For an 80kg person that is roughly 96 to 128 grams daily, which is more than most people eat without deliberately trying.
- Doing both together. Protein without loading gives the body no signal to keep the muscle; loading without protein gives it nothing to build from. The near-zero result comes from the combination.
The practical difficulty is obvious and worth naming: a GLP-1 suppresses appetite, and you are being asked to eat more of something at exactly the moment eating is least appealing. Eating protein first at every meal, favouring dense sources over bulky ones, and using liquid protein when solid food is unappealing are the usual workarounds. Our guide to protein and satiety covers why protein also helps with fullness, which makes it the rare intervention that works on both problems at once.
Who Should Take This Most Seriously
- Adults over 40, and especially over 60. Age-related muscle loss is already underway, so you start from a lower baseline and the functional consequences arrive sooner.
- Women in perimenopause and after. Falling oestrogen accelerates muscle loss independently — covered in our piece on women, hormones and GLP-1 weight loss.
- Anyone planning to stop treatment. Muscle is metabolically active, so losing it lowers resting energy expenditure and makes maintenance harder — which matters given how commonly weight returns after stopping.
On Footwear — and What It Does and Does Not Solve
A brief, honest section. If reading the above has you thinking about starting to move more, that is a good instinct and footwear is a reasonable thing to get right — particularly during weight loss, when joint loading, gait and even shoe fit change as your body does. Many people find their shoe size shifts.
Mizuno is a long-established athletic brand with a well-regarded running range. Its everyday Wave Rider 29 sits at $119.95, and its max-cushioned Neo Vista super trainer at $180, with Wave Sky and Wave Horizon models using its Enerzy foam. Running shoes carry a one-year limited warranty against defects in workmanship and materials from the date of purchase.
But be clear about what this fixes
Running shoes address cardio. The muscle-preservation evidence in this article is about resistance training — loading muscle against resistance — and protein intake. Cardio is genuinely good for your heart, your mood and your general activity level. It is not what stops you losing lean mass.
If you buy one thing after reading this page, a set of adjustable dumbbells or a gym membership does more for the problem described above than any shoe will. Buy the shoes because you intend to walk or run, not because you think they address muscle loss.
An established brand with a genuinely well-reviewed running range at competitive prices and a one-year warranty against defects. Scored on its merits as footwear, not as a solution to muscle loss — which it is not. Note that returns must be sent within two weeks of receiving an RMA and shipping costs are not refunded.
Footwear · Resistance training is what preserves muscle
Frequently Asked Questions
How much muscle do you actually lose on a GLP-1?
More than most people expect, and it varies by trial. In STEP 1 and SUSTAIN 8, roughly 38–40% of the weight lost on semaglutide registered as lean mass on body-composition scans — STEP 1 enrolled 1,961 adults with obesity who lost about 15% of body weight over 68 weeks. Other datasets are lower: SURMOUNT-1 with tirzepatide showed about 25% lean-mass loss on DXA, and the semaglutide arm of REDEFINE 1 came in at 28%. For context, roughly 25% is what dieting alone typically produces, so the higher figures are worth taking seriously without treating them as unique to these drugs.
Is losing lean mass automatically bad?
Not automatically, and the framing matters. In STEP 1 and SUSTAIN 8 the proportion of lean mass to total body mass still increased, meaning overall body composition improved even as absolute lean mass fell. Some loss of lean tissue is expected in any substantial weight loss — the body needs less structural tissue to move a smaller frame. The concern is about the amount, and about who is losing it: older adults and postmenopausal women start from a lower muscle baseline and have more to lose functionally.
What actually prevents muscle loss during weight loss?
Two things, and the evidence for both is strong. Resistance training combined with adequate protein has been shown to cut lean-mass loss to near zero — in obese older adults undergoing caloric restriction, resistance training preserved nearly all lean mass. The protein target used in this research is roughly 1.2 to 1.6 grams per kilogram of body weight per day during active weight loss. Neither is glamorous and neither is sold to you in an ad, which is largely why the supplement aisle is louder than the evidence.
How do I eat 1.2–1.6g of protein per kg while my appetite is suppressed?
This is the genuine practical difficulty, and it is worth planning rather than improvising. GLP-1s suppress appetite, which makes hitting a protein target harder precisely when it matters most. The usual approach is to eat protein first in every meal, prioritise dense sources over bulky ones, and use liquid protein when solid food is unappealing — a shake is often tolerable when a chicken breast is not. Our guide to protein and satiety covers the mechanism, and protein does double duty here by supporting fullness as well as muscle.
Is cardio enough, or do I need weights?
The muscle-preservation evidence is specifically about resistance training. Walking and running are excellent for cardiovascular health, mood, and general activity, and if you are going to do one thing then moving more is a good thing to do. But the studies showing lean mass preserved during caloric restriction used resistance training — loading muscle against resistance — not aerobic exercise. If muscle retention is the goal, that distinction is the whole point.
Should I worry about this if I am young and healthy?
Less than an older adult should, but it is still worth addressing. Muscle is metabolically active tissue, so losing it lowers resting energy expenditure and makes maintenance harder later — which matters given how many people regain weight after stopping. It also relates to strength and function decades out. The countermeasures cost little and have no downside, which makes this an easy thing to get right rather than an emergency.
Where to Go Next
- Protein and satiety — the half of this you can start today
- Weight set point — why maintenance is its own problem, and why muscle matters for it
- Women, hormones and GLP-1 weight loss — why this matters more after 40
Medical Disclaimer
We're not doctors, and this article is for education only — it isn't medical advice. GLP-1 medications are prescription-only and require evaluation by a licensed healthcare provider. Individual results vary widely, and only a clinician who knows your history can determine what is safe and appropriate for you. See our full medical disclaimer.
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