"My metabolism stopped working after 40." I hear this sentence from new clients almost every other day. When we go through a normal week, the food is about the same as ten years ago. What changed is something else: how much you walk, how much you can lift, and how much muscle is left underneath.
Short answer: Energy use per kg of fat-free mass stays stable between ages 20 and 60 (Pontzer, 2021), so your metabolism does not crash at 40. What changes is the amount of muscle and the amount of movement, and when muscle goes, energy use goes with it. After 40 the answer is strength training, enough protein and a milder deficit.
Your metabolism did not crash at 40
The biggest database so far of total daily energy expenditure, measured with doubly labeled water, covers people from 8 days to 95 years old. Energy use per kg of fat-free mass (muscle and organs) stays stable in adulthood between 20 and 60. It starts to drop only in older people (Pontzer, 2021, Science, PMID 34385400).
Age 40 is not a break point in that data. What does change is the fat-free mass itself. Energy use is calculated from it, so less muscle means less energy use. The difference is in the cause: age itself, or what you stopped doing in the last ten years. You can still change the second one today. I go deeper in the article on the slow metabolism myth.
What changes after 40
One MRI study measured muscle mass in 468 men and women aged 18 to 88. The share of muscle in body weight went down from the third decade of life (the 20s). A clear drop in the absolute amount of muscle showed up only toward the end of the fifth decade (the late 40s), mostly in the lower half of the body (Janssen, 2000, PMID 10904038).
This matches what I see in clients. The legs go first. Stairs, cycling and longer walks slowly disappear from a normal day, and a car plus a home office finishes the job. Then the whole deficit has to come from food, because on the burning side there is almost nothing left to add.
The second thing I see all the time after 40 is sleep. With kids, work and a phone in the evening it gets shorter, and for most people evening appetite gets worse with it. Sleep alone does not rewrite thermodynamics, but it decides whether you can stick to the plan. More in sleep and fat loss, and steps in how many steps a day.
Food: a milder deficit and more protein
A Dutch study put 100 people with overweight or obesity, aged 55 to 80, on a calorie deficit for ten weeks. They were split into four groups: higher protein (target 1.3 g per kg of body weight) or not, and weight training three times a week or not. In the main analysis, protein alone or training alone did not change fat-free mass in a significant way. The only group whose fat-free mass clearly went up during weight loss did both, by 0.6 kg (p = 0.011) (Verreijen, 2017, PMID 28166780).
To be fair: the study was done on people older than most readers of this text, and the result was inside one group, not the main effect of the study. I take the direction from it. Protein and weights work together, and each one alone pulls less.
In practice, for clients over 40, I set the deficit at about 15 to 20% below estimated energy use. That is a pace of roughly 0.3 to 0.7 kg per week. I aim protein at 1.6 to 2.2 g per kg of body weight per day and want it in every main meal. To estimate your own numbers, use the calculator and read calorie deficit: how much to eat. Portions and sources are in how much protein, and the protein tool ranks foods by protein per 100 kcal (calories).
A crash diet at 1,200 kcal has an even worse risk-to-reward ratio after 40 than at 20. More muscle goes with it, and at 50 muscle comes back more slowly than it left.
Strength training is the main lever
If I have to choose one thing to start after 40, it is lifting weights. It protects exactly what the measurements show disappears first, and it keeps your strength for the next twenty years.
To start, two strength sessions a week of 40 to 50 minutes are enough. Use compound lifts for the lower body (squat, deadlift, lunge), a push and a pull for the upper body, and above all add load step by step. If you have never lifted, start with your body weight and one dumbbell. You can grow on that for the first months. The bigger picture is in strength training in one hour a week.
Women and menopause: the scale can lie
In the SWAN study, women had their body composition measured repeatedly with a DXA scanner. At the start of menopause the rate of fat gain doubled and fat-free mass began to drop. This lasted until about two years after the last period, then the curves leveled off. One interesting detail: body weight itself had been rising steadily before menopause and did not speed up at its start (Greendale, 2019, PMID 30843880).
In practice this means one thing. The number on the scale can look the same for months while the ratio of fat to muscle changes under it. That is why in this period I also measure waist circumference and check whether the load in the gym goes up. Talk to your doctor about hormone therapy. I don't give advice on that.
Frequently asked questions
Do I have to eat less after 40 than at 30?
Mostly you don't need to eat much less. You need to build the day differently. But if you lost muscle and movement over those ten years, your energy use is lower than before, and your intake has to respect that. I would first bring steps back and add strength, and only then cut from the plate.
Does the gym work if I have never trained?
Yes. A beginner over 40 often makes the fastest progress in the first months, because he starts from zero. Regular training and adding load step by step matter more than which exercises you pick.
How fast can I lose weight after 40?
For most clients a pace of about 0.5% of body weight per week makes sense. That is roughly 1.5 to 3 kg a month, depending on how much you weigh. A faster pace is possible, but you pay with more muscle loss and a plan that is harder to keep.
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Apply for coaching Get the free Fat Loss KitSources, so you can check:
1) Energy expenditure across life: Pontzer H, et al. Daily energy expenditure through the human life course. Science. 2021;373(6556):808-812. DOI: 10.1126/science.abe5017 | PMID: 34385400
2) Muscle mass and age: Janssen I, et al. Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr. J Appl Physiol (1985). 2000;89(1):81-88. DOI: 10.1152/jappl.2000.89.1.81 | PMID: 10904038
3) Protein and lifting during weight loss in older adults: Verreijen AM, et al. Effect of a high protein diet and/or resistance exercise on the preservation of fat free mass during weight loss in overweight and obese older adults: a randomized controlled trial. Nutr J. 2017;16(1):10. DOI: 10.1186/s12937-017-0229-6 | PMID: 28166780
4) Body composition in menopause: Greendale GA, et al. Changes in body composition and weight during the menopause transition. JCI Insight. 2019;4(5):e124865. DOI: 10.1172/jci.insight.124865 | PMID: 30843880
Martin Barna
Online nutrition and fitness coach. Since 2013 I have helped 600+ people lose fat and keep the result. Science, not crash diets.
Apply for coachingThis article is for information only. It is not medical advice. If you have a health condition, talk to your doctor.