Muscle Is Your Energy Reserve
Written by the Editorial Team.
What if muscle is the quiet organ behind steady energy, steady mood and a body that keeps pace with a full life? The editorial team follows the research on strength, hormones and GLP-1 medicines for women 40+.
Conversations about energy in midlife tend to begin with sleep and stress. A third subject is joining them: skeletal muscle. It stores and spends a large share of the body's fuel, it hosts a dense population of mitochondria, and it sends chemical signals to the rest of the body when it contracts.
The physician Gabrielle Lyon, founder of the Institute for Muscle-Centric Medicine and author of Forever Strong, argues that muscle sets the trajectory of health and ageing. This essay follows the research behind that idea and the newest questions around it.
Muscle is a reserve that can be built at any age, and the research gives the building a clear shape.
Muscle is a working organ
Lyon describes muscle as the body's largest endocrine organ. In conversation with Mary Claire Haver, author of The New Menopause and The New Perimenopause, she has pointed to myokines, signalling molecules released when muscle contracts, including BDNF, which supports brain health.
Haver has brought the muscle conversation to midlife women, and the two physicians frame strength training as part of how a woman protects her independence. These are clinicians' arguments, and the research below tests parts of them.
Training builds mitochondria
A 2025 meta-analysis of randomised trials found that endurance exercise significantly boosted PGC-1 alpha, the master regulator of new mitochondria production in muscle, whether training was continuous or interval-based.
A 2026 study in Experimental Physiology found that six weeks of cycling interval training, in sessions of just 20 minutes three times a week, improved fat-burning capacity and heart-rhythm complexity in active postmenopausal women.
The exercise physiologist Stacy Sims argues in Next Level that women in perimenopause and menopause respond well to short, hard efforts and strength work, and the emerging data fit her argument. The studies are small, and they show a consistent direction.
Hormones shape how mitochondria work
A 2026 review in the International Journal of Molecular Sciences gathered evidence that oestrogen, testosterone and progesterone each shape mitochondrial function differently, producing distinct mitochondrial profiles across pregnancy, menopause and other hormonal transitions.
The menopause transition therefore changes the hormonal environment in which a woman's mitochondria work. That gives researchers a reason to study strength training in this stage of life specifically.
GLP-1 medicines put the muscle question in front
GLP-1 medicines such as semaglutide and tirzepatide have become common, and a practical question follows them: how much of the weight lost is muscle?
A preprint on medRxiv analysed 670,422 people starting GLP-1 therapy for the first time, 456,742 on semaglutide and 213,680 on tirzepatide, using AI-based extraction of body-composition data from medical records. Tirzepatide users showed a greater decline in lean body mass than semaglutide users.
The study has not completed peer review, and its extraction method has not been independently validated at this scale, so it stands as an early, large signal.
Hormone therapy and tirzepatide
A Mayo Clinic study, published in The Lancet Obstetrics, Gynaecology, & Women's Health in 2026, reviewed the records of 120 postmenopausal women who had taken tirzepatide for 12 months or more. The 40 women who also used hormone therapy lost 19.2 percent of their body weight on average, against 14.0 percent in 80 closely matched women who did not, with additional improvements in blood pressure, triglycerides and liver enzymes.
The study was observational, so it cannot show that hormone therapy caused the difference. Women who choose hormone therapy may be more engaged with their health already, and relief from hot flushes and poor sleep may make lifestyle changes easier to sustain.
Several authors disclosed industry ties, and a randomised trial is planned. The study measured weight, and muscle remains the open question.
A practical weekly shape
The World Health Organization recommends muscle-strengthening activity on two or more days a week for adults, and that range fits the research above. A sample week might combine two strength sessions, regular walking and, for those who are ready, a short interval session once a week. Lyon argues for generous protein alongside training. The right amount depends on health, kidney function and medicines, so a GP or registered dietitian can set it. Anyone taking a GLP-1 medicine can ask the prescriber how to protect muscle during treatment.
Rebuilding the reserve
Calm Power is the steady strength that grows from Core Self, and muscle gives that steadiness a physical base. Each strength session adds to a reserve that serves a woman on an ordinary Tuesday as much as on a demanding one.
She can then lead from capacity she has built. Decisions come more clearly from a body that has energy to spare, and ambition draws on strength that regenerates.
Ready to understand your current recovery needs more precisely?
What's next
FAQ
Why does muscle matter for energy?
Skeletal muscle stores and spends a large share of the body's fuel and hosts many mitochondria. Training builds more of them, and the studies above link interval and endurance exercise to measurable metabolic changes in postmenopausal women.
Practical step: add two short strength sessions a week to your existing routine.
Do GLP-1 medicines cause muscle loss?
Weight lost on these medicines includes lean mass, and a large preprint found a greater decline with tirzepatide than with semaglutide. The preprint has not yet been peer reviewed, and the question is an active one in research.
Practical step: ask your prescriber about protein and resistance training while on treatment.
Does hormone therapy improve results on tirzepatide?
A Mayo Clinic observational study found greater weight loss in women who also used hormone therapy, and its authors note that causation cannot be confirmed. A randomised trial is planned.
Practical step: discuss hormone therapy and weight management together with a menopause-trained clinician.
How often should I do strength training?
The World Health Organization recommends muscle-strengthening activity on two or more days a week for adults. Starting lighter and building gradually suits most people.
Practical step: book two fixed slots in your diary this week and begin with a short routine.
When should I seek medical advice before starting?
Speak to a GP before starting vigorous exercise if you have a heart condition, high blood pressure, joint problems or a long break from activity. This article offers education and does not replace medical advice.
Practical step: ask whether any limits apply to you, and write down the answer.
Curious to explore more?
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