Weight gain and a growing waistline in midlife are often blamed on a slowing metabolism. Research suggests a more complex explanation: daily energy expenditure, resting energy use and body composition can change in different ways.
There is no established metabolic turning point at 40. However, this does not mean that energy needs and body shape remain unchanged until 60.
Metabolism and Calorie Expenditure Are Different
Metabolism includes all the chemical processes that keep the body functioning. When discussing weight, the more precise term is energy expenditure: how much energy the body uses.
Daily energy expenditure includes energy used at rest, energy needed to process food, and energy spent on exercise and everyday movement. These components can change independently. A woman who moves less may use less energy overall even if her resting energy expenditure remains unchanged. [1, 3]
In simple terms: Your daily calorie needs depend on both your body and how much you move. A change in either can affect them.
Does Energy Expenditure Decline After 40?
A large study published in Science in 2021 analysed total daily energy expenditure in 6,421 people. After accounting for body size and composition, expenditure was relatively stable between approximately 20 and 60 years of age. [1]
This was a population finding, rather than evidence that every woman needs the same calories throughout adulthood. Changes in body composition and activity can still affect individual energy requirements.
A smaller 2023 study compared 120 women across different age groups. Women aged 41–58 had resting energy expenditure approximately 88 kcal per day lower than women aged 22–38 after adjustment for fat and fat-free mass. Because different women were compared at one point in time, the study cannot show how an individual woman’s expenditure changes as she ages. [2]
Together, these studies challenge a universal slowdown at 40 while leaving room for differences in resting energy expenditure during midlife.
In simple terms: Turning 40 does not automatically mean your metabolism slows down. Your calorie needs may still change, and women do not all follow the same pattern.
Does Menopause Reduce Resting Energy Expenditure?
Studies have not consistently found that menopause independently lowers resting energy expenditure.
In a 2023 study, resting energy expenditure adjusted for body composition did not differ significantly by menopausal status. However, this comparison involved only 59 middle-aged women, limiting the conclusions that can be drawn. [2]
The five-year MONET study found that estimated total daily energy expenditure declined among women who became postmenopausal, mainly alongside lower expenditure from physical activity. Average resting energy expenditure remained stable in the five-year group comparison. A separate analysis, corrected in 2014, found higher resting expenditure one and two years after the final menstrual period compared with the year of that period. [3]
These findings show why changes in total daily expenditure should be distinguished from changes in energy use at rest.
In simple terms: Menopause does not produce a predictable drop in resting calorie use in every woman. Moving less can reduce daily calorie needs even when resting expenditure stays stable.
Why Can Body Shape Change Without a Large Weight Gain?
Body weight and body composition describe different things. Weight includes fat, lean tissue, bone and water; it does not show how their proportions change.
A SWAN analysis following 1,246 women found that average fat gain accelerated and lean mass began declining approximately two years before the final menstrual period. These changes continued into early postmenopause. Weight continued to rise, but its rate of increase did not significantly accelerate. [4]
The average rate of fat gain increased from approximately 0.25 to 0.45 kg per year. These were group averages, with differences between women. Lean mass measured in this study included more than skeletal muscle, so its decline should not be described entirely as muscle loss. [4]
Separate SWAN research also linked the menopausal transition with changes in regional fat distribution, including greater central fat accumulation. [5]
In simple terms: You can gain fat and lose some lean tissue while the scales change only a little. Your waist and how your clothes fit may change more noticeably than your weight.
What Does This Mean for Women After 40?
The evidence supports a more individual explanation than “metabolism slows after 40.” Changes in activity, body composition and resting energy expenditure can all matter, and their contribution varies.
Muscle contributes to resting calorie expenditure, although it uses substantially less energy per kilogram at rest than organs such as the brain, liver, heart and kidneys. Its importance therefore extends beyond how many calories it burns. [6]
When interpreting midlife changes, it helps to consider daily movement and changes in body shape alongside weight, rather than treating the scales as a complete picture.
In simple terms: A changing body does not automatically mean a failing metabolism. Look at movement, body shape and weight together to understand what is changing.
References
Pontzer H, et al. (2021). Daily energy expenditure through the human life course. Science, 373(6556), 808–812.
https://doi.org/10.1126/science.abe5017Karppinen JE, et al. (2023). Age But Not Menopausal Status Is Linked to Lower Resting Energy Expenditure. The Journal of Clinical Endocrinology & Metabolism, 108(11), 2789–2797.
https://doi.org/10.1210/clinem/dgad321Duval K, et al. (2013). Effects of the menopausal transition on energy expenditure: a MONET Group Study. European Journal of Clinical Nutrition, 67, 407–411.
https://doi.org/10.1038/ejcn.2013.33
With correction: Duval K, et al. (2014). Erratum: Effects of the menopausal transition on energy expenditure: a MONET group study. European Journal of Clinical Nutrition, 68, 142.
https://doi.org/10.1038/ejcn.2013.246Greendale GA, et al. (2019). Changes in body composition and weight during the menopause transition. JCI Insight, 4(5), e124865.
https://doi.org/10.1172/jci.insight.124865Greendale GA, et al. (2021). Changes in Regional Fat Distribution and Anthropometric Measures Across the Menopause Transition. The Journal of Clinical Endocrinology & Metabolism, 106(9), 2520–2534.
https://doi.org/10.1210/clinem/dgab389Wang Z, et al. (2010). Specific metabolic rates of major organs and tissues across adulthood: evaluation by mechanistic model of resting energy expenditure. The American Journal of Clinical Nutrition, 92(6), 1369–1377.
https://pubmed.ncbi.nlm.nih.gov/20962155/
