Why Your Body Is Changing in Perimenopause

And why what worked in your 30s doesn’t work anymore

By Lizalet Oosthuizen APD, The Menopause Dietitian

If you are in your 40s and feel like your body has changed overnight even though nothing in your diet has changed, you are not imagining it, and it is not your fault.

Perimenopause is a genuine hormonal shift that changes how your body stores fat, builds and maintains muscle, handles blood sugar, and regulates hunger.

Understanding what is happening is the first step to working with your body instead of against it.

This post explains the key hormonal mechanisms behind body composition changes in perimenopause, what the latest research says about metabolism, and what the evidence supports in terms of practical next steps.

The hormone that was doing more than you realised

Most people think of oestrogen as a reproductive hormone. And it is. But oestrogen receptors are found throughout the body: in fat tissue, skeletal muscle, the liver, the brain, and the gut. Oestrogen plays a significant role in regulating where fat is stored, how muscle is maintained, and how the body handles blood sugar.

As you enter perimenopause, oestrogen does not simply disappear. It fluctuates, sometimes dramatically, before eventually declining. These fluctuations are what drive many of the symptoms women notice in their 40s, long before they reach menopause itself.

The Study of Women’s Health Across the Nation (SWAN), one of the most comprehensive longitudinal studies of women’s health, followed over 3,000 women through the menopausal transition and documented significant changes in body composition that were directly associated with hormonal changes, independent of ageing or lifestyle factors alone.

Fat moves, even when your weight does not

One of the most consistent findings in perimenopause research is a shift in where fat is stored. Research by Lovejoy et al., published in the Journal of Clinical Endocrinology and Metabolism, found that women who became postmenopausal gained significantly more visceral fat than women who remained premenopausal, even when total body weight was similar.

Visceral fat is the fat that accumulates around the internal organs rather than under the skin. It is metabolically active in ways that subcutaneous fat is not. It releases inflammatory compounds and free fatty acids directly into the portal circulation, the blood supply that flows straight to the liver. This drives a process called hepatic insulin resistance.

The consequence is a self-reinforcing cycle. More visceral fat drives more insulin resistance. More insulin resistance makes it harder to mobilise stored fat for energy. This is why fat in this area feels so stubborn, and why research by Tchernof and Despres (Physiological Reviews, 2013) and Despres and Lemieux (Nature, 2006) describes visceral fat accumulation as strongly associated with metabolic dysfunction and not just aesthetics.

Your body composition can change significantly in perimenopause even if the number on the scales barely moves. Fat shifting from the hips and thighs to the abdomen is a hormonal phenomenon, not a lifestyle failure.

Muscle is being lost faster than before

Oestrogen plays a direct role in maintaining skeletal muscle. As it declines, the rate of muscle loss,  a process called sarcopenia,  accelerates. Research published in the Journal of Clinical Endocrinology and Metabolism has documented this association clearly. Muscle is metabolically expensive tissue. It burns energy at rest simply by existing. When you lose muscle, your body burns fewer calories throughout the day even without any change in activity.

This is why many women find that the same food intake that maintained their weight in their 30s starts producing gradual weight gain in their 40s. The body composition has shifted underneath the surface. The muscle that was quietly burning energy is diminishing.

This also has implications beyond weight. Muscle loss is associated with reduced strength, increased fracture risk, impaired blood sugar regulation, and reduced quality of life as women age. Building and preserving muscle in perimenopause is not purely a body composition goal,  it is a long-term health investment.

The metabolism myth

One of the most widely repeated claims in the menopause wellness space is that menopause slows your metabolism. It is repeated so often, and feels so true to so many women, that most people accept it without question.

But the research tells a more nuanced story and getting this right matters for how women approach this life stage.

A landmark study published in the journal Science in 2021 by Pontzer and colleagues analysed total energy expenditure across the lifespan in over 6,400 people. The key finding was that when researchers accounted for body composition,  specifically fat mass and lean muscle mass,  there was no significant drop in metabolism at the menopause transition. The notable metabolic slowdown the study identified occurred from around age 60, not at perimenopause.

This finding was reinforced by a 2023 study by Ihalainen and colleagues published in the Journal of Clinical Endocrinology and Metabolism, which measured resting energy expenditure directly in women across different menopausal stages. Their conclusion was consistent: menopausal status was not independently linked to lower resting metabolic rate. Resting energy expenditure did not vary according to sex hormone concentrations once body composition was controlled for.

Two independent research groups, using different methodologies, reached the same conclusion: the metabolic changes women experience in perimenopause are driven by changes in body composition,  specifically the loss of muscle, not a direct hormonal braking of the metabolic engine.

This reframe is important and genuinely reassuring. It means your metabolism is not broken. But it does mean your body composition needs attention and that requires a different approach than simply eating less.

Blood sugar is handled differently now

Oestrogen supports the body’s sensitivity to insulin. Insulin is the hormone that moves glucose from the blood into cells. As oestrogen declines, insulin sensitivity can decrease. This does not automatically mean diabetes, but it does mean the body may handle carbohydrates differently than it did in your 30s.

Combined with the increase in visceral fat (which itself drives insulin resistance), blood sugar regulation becomes a more significant factor in body composition and energy levels during perimenopause. Women often notice more energy crashes, stronger carbohydrate cravings, and difficulty sustaining energy across the day, all consistent with shifts in blood sugar regulation.

Sleep disruption compounds everything

Vasomotor symptoms like hot flushes and night sweats are driven by the same hormonal fluctuations and frequently wreak havoc on sleep. This matters for body composition and weight because sleep quality directly affects appetite-regulating hormones.

Research by Spiegel and colleagues demonstrated that short or poor-quality sleep is associated with lower levels of leptin (the hormone that signals fullness) and higher levels of ghrelin (the hormone that signals hunger). This is not specific to menopause, but it compounds the other changes already occurring, creating a set of conditions where hunger is harder to regulate even when nothing in the diet has changed.

 

What the evidence actually supports

Understanding that the changes of perimenopause are driven primarily by body composition shifts rather than a broken metabolism changes the strategy entirely. The goal is not to eat less. It is to preserve and rebuild muscle, support blood sugar regulation, and work with the body’s changing needs rather than against them.

1. Resistance training

Lifting weights or using bodyweight exercises against load,  is the most evidence-supported intervention for preserving and rebuilding muscle during and after the menopause transition. Research by Daly and colleagues in the British Journal of Sports Medicine, and a substantial body of work on exercise and ageing, consistently shows that progressive resistance training attenuates muscle loss and supports metabolic health. Even two sessions per week produces meaningful benefit.

2. Adequate protein — not excessive protein

Protein is essential for muscle maintenance and repair. In perimenopause, when muscle is being lost faster, adequate protein intake becomes more important than at earlier life stages.

The PROT-AGE Study Group consensus, based on a systematic review of protein requirements in older adults, supports a target of 1.2 to 1.6 grams of protein per kilogram of ideal body weight per day, distributed across meals. This is meaningfully higher than the standard adult recommendation but is achievable through real food without the extreme amounts promoted by some fitness influencers.

It is also worth noting that protein does not work in isolation. The combination of protein and fibre at meals is more effective at supporting blood sugar stability, satiety, and body composition than protein alone.

3. Fibre and gut health

Dietary fibre is one of the most underemphasized tools in perimenopause nutrition. Fibre slows the absorption of glucose, feeds beneficial gut bacteria, supports hormone clearance, and contributes significantly to feelings of fullness. In a context where blood sugar regulation is already shifting, fibre is a practical and evidence-supported tool.

Emerging research is also pointing to a connection between oestrogen decline and changes in the gut microbiome. A less diverse gut microbiome has been associated with greater inflammation, impaired metabolic function, and mood changes. Eating a wide variety of plant foods, aiming for around 30 different plant foods per week, a target derived from the American Gut Project data, is a practical way to support gut health. It is worth noting that this area of research is still developing, and the gut-menopause connection is considered emerging rather than definitively established.

4. Prioritise sleep quality

Because poor sleep independently raises hunger hormones and lowers fullness signals, addressing sleep is a genuine nutritional and body composition strategy in perimenopause and not simply a wellness nice-to-have. Working with a healthcare provider to manage vasomotor symptoms that disrupt sleep is therefore also relevant to body composition outcomes. Talk to a menopause informed GP about your options and menopause hormone therapy.

5. Stop restricting calories severely

Given that the changes of perimenopause are driven by muscle loss, severe calorie restriction is counterproductive. It accelerates muscle loss, further reduces the calories burned at rest, and creates a cycle that is increasingly difficult to break. Eating enough to support muscle and particularly enough protein, is more important than eating less.

The bottom line

Perimenopause is a real, hormonal transition that changes body composition through multiple simultaneous pathways. Fat shifts to the abdomen. Muscle is lost faster. Blood sugar is regulated differently. Sleep suffers. Hunger increases. These changes are not imagined, and they are not a failure of willpower or effort.

The research is clear that the metabolic changes women experience are driven primarily by the loss of lean muscle tissue, not a direct hormonal slowdown of the metabolic engine. This is meaningful because it points toward a strategy: protect the muscle, support the gut, pair protein with fibre, prioritise sleep, and move in ways that build strength.

Your body is not broken. It is changing — and it deserves a strategy built around what is actually happening, not around eating less and pushing harder.

If you are navigating these changes and would like personalised support grounded in the evidence, I would love to help.

References

Pontzer H et al. Daily energy expenditure through the human life course. Science. 2021;373(6556):808-812.

Ihalainen JK et al. Age but not menopausal status is linked to lower resting energy expenditure. J Clin Endocrinol Metab. 2023;108(11):2789-2797.

Sowers M et al. SWAN: A multi-centre, multiethnic, community-based cohort study of women and the menopausal transition. Study of Women’s Health Across the Nation.

Lovejoy JC et al. Increased visceral fat and decreased energy expenditure during the menopausal transition. Int J Obes. 2008;32(6):949-958.

Tchernof A, Despres JP. Pathophysiology of human visceral obesity: an update. Physiol Rev. 2013;93(1):359-404.

Despres JP, Lemieux I. Abdominal obesity and metabolic syndrome. Nature. 2006;444(7121):881-887.

Bauer J et al. Evidence-based recommendations for optimal dietary protein intake in older people: a position paper from the PROT-AGE Study Group. J Am Med Dir Assoc. 2013;14(8):542-559.

Daly RM et al. Protein-enriched diet, with the use of lean red meat, combined with progressive resistance training enhances lean tissue mass and muscle strength and reduces circulating IL-6 concentrations in elderly women. Am J Clin Nutr. 2014.

Spiegel K et al. Sleep curtailment in healthy young men is associated with decreased leptin levels, elevated ghrelin levels, and increased hunger and appetite. Ann Intern Med. 2004;141(11):846-850.

Josse AR, Phillips SM. Impact of milk consumption and resistance training on body composition of female athletes. Med Sci Sports Exerc. 2012.

McDonald D et al. American Gut: an open platform for citizen science microbiome research. mSystems. 2018;3(3).