You try on last year’s jeans and have to pull your stomach in to do up the button. When you step on the scales, you’re a few kilos heavier than you remember. Around thirty, it’s easy to put that down to age. “I eat much the same as I used to. My metabolism must be slowing down.”
By the time the jeans feel tight, your days may have looked different for months. A new job might mean driving where you used to walk. Dinner gets later, and nights get shorter. After a while, that feels like a normal week.
Does your body really start using much less energy around thirty?
Energy expenditure through adulthood
In a large comparison of people at different ages, average daily energy expenditure held fairly steady from roughly 20 to 60 once the researchers took body composition into account. Relative to their body composition, young children used more energy and gradually approached adult levels as they grew. Average expenditure fell in older adults. 1
Your own energy needs can shift during those decades. You might weigh the same at 35 as you did at 25 and carry a different amount of muscle or fat. How much time you spend moving also affects how much energy you use over a day. 2
The movement you get outside the gym
Walking to work, doing chores and working out all count towards the energy you use in a day. Your daily total also includes the energy needed to process food and keep your organs working. Under standard conditions of rest and fasting, the energy your body uses to maintain its basic functions is called basal energy expenditure. 2
Suppose you still lift weights three times a week, but now drive to a job you used to walk to. The training sessions are easy to count. The walking you have lost may barely register, especially if the change came with a new job and a whole new routine. The energy you use over the week can fall even if your workouts and basal expenditure stay about the same. 2
Researchers use the term NEAT for energy spent on everyday activity outside exercise. It includes walking to the shops and the small movements you make as you stand or shift position. In a small study that gave participants the same extra calories, the increase in expenditure from everyday movement varied. The people whose expenditure rose more tended to store less fat during the study. 3
Why weight can rise slowly and then level off
When energy intake stays above expenditure, some of the surplus can accumulate as fat. That difference can be small enough to pass unnoticed in any given day. You may be eating about as much as before and using less energy across the week. 2 4
As weight rises, a larger body generally uses more energy at rest and can use more to walk the same distance. The gap between intake and expenditure can narrow. If the two come back into balance, weight may settle at a higher level while food intake stays the same. A calculation that multiplies a daily calorie surplus by the number of days misses those changes in expenditure. 4
Daily expenditure varies between people, and repeated measurements in adults showed that these differences can persist. Among adults aged 20 to 60, the researchers found no link between initial daily expenditure, adjusted for age, sex and body composition, and changes in body fat percentage. The measurements in the main analysis were about seven weeks apart on average. 5
Two people can use different amounts of energy and both maintain their weight, provided each takes in roughly as much as they use. Changes in sleep can affect how much they eat. 2 6
Sleep, appetite and the food within reach
You may feel hungrier after a short night or find a high-calorie snack more appealing. When short nights become part of the working week, eating habits can shift along with them. Work and family commitments also shape when you can sit down for a meal and what is available when you do. 6
In a two-week randomised trial, adults who were overweight and usually slept less than six and a half hours were given advice on how to sleep longer. They slept about an hour more each night, and their energy intake fell by an average of 270 kcal a day compared with the group that kept its usual routine. The researchers found no significant difference in the change in daily energy expenditure. 7
After a late shift, the places still open nearby can shape what you have for dinner. A meal that looks like your usual portion may contain more calories. Two servings that weigh the same can have very different energy contents, depending on how many calories each gram provides. 6 8
In a hospital study, participants could eat as much as they wanted from each of two menus served for two weeks apiece. They ate about 500 calories more a day and gained weight on the menu high in ultra-processed foods. They also ate faster, and the food on that menu contained more calories per gram when drinks were left out of the calculation. Both differences may have contributed to the amount eaten on the menus tested. 8
Moving in together
Moving in with someone can change what you keep in the fridge. When you share a kitchen, one person’s shopping habits can shape what both of you eat. You may sit down to dinner together more often, and the portions you serve may change. Cooking for two can make it easier to eat at home. A different commute may give you fewer chances to walk. The effect on weight depends on how those routines change.
In one analysis of US data, married men had a BMI about 0.44 points higher than unmarried men. At a height of 180 cm, that is a difference of about 1.4 kg. The estimate compares relationship statuses using the heights and weights participants reported. 9
German researchers also found that people who lived with a partner tended to see their BMI rise over time. That pattern held for men and women after the researchers accounted for reported exercise, attention to healthy eating and smoking. The survey covered particular habits, leaving open how much changes in food intake and everyday movement contributed. If your weight changed after you moved in with someone, the date can help you compare your routines before and after the move. 10
Putting a change in weight in context
Weight, waist and BMI
If you have earlier measurements, look at how your weight and waist have changed over several months.
Body mass index uses weight and height, and needs careful interpretation in someone with a lot of muscle. WHO uses a BMI of 25 as the threshold for overweight and 30 for obesity in adults. Obesity is a chronic disease with several contributing factors, including biology and the conditions in which people live. 11 12
NICE also recommends looking at waist size relative to height in adults with a BMI below 35. A waist that measures half your height or more suggests increased health risk associated with fat around the abdomen. For someone who is 180 cm tall, that starts at 90 cm. A clinician can consider the measurement alongside the rest of your health information. 12
To measure your waist, place the tape halfway between your lowest rib and the top of your hips. Take the reading after breathing out normally. 12
Daily routine and activity
For a gradual change, a short record of a normal week can give you something concrete to work with. Write down roughly how long you sleep, how much you walk or train, and what you eat and drink. If the walk to work disappeared, you might look for a way to bring some walking back into the day. If meals at work are the main difficulty, start with food you can realistically keep there or bring with you.
Being active benefits your health even when your weight stays steady. For adults, WHO recommends at least 150 minutes of moderate activity or 75 minutes of vigorous activity a week, or an equivalent mix. It also recommends working the major muscle groups on at least two days a week. 13 14
When to talk to your doctor
Some medicines and health conditions can contribute to weight gain. Talk to your doctor if you've gained weight quickly, can't see a reason for it or have other symptoms. A note of when it started, any measurements you have and a list of medicines can help the doctor decide what to investigate. 6 15
Weight and metabolism after thirty
Average daily expenditure shows no sharp drop around thirty once body composition is taken into account. If last year’s jeans feel tight, think back to the days when they still fitted. Your walk to work, dinner and bedtime may have changed long before you noticed the difference on the scales. 1 6
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Levine JA, Eberhardt NL, Jensen MD. Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science. 1999;283:212–214. PubMed.
Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet. 2011;378:826–837. PubMed.
Rimbach R, et al. Total energy expenditure is repeatable in adults but not associated with short-term changes in body composition. Nature Communications. 2022;13:99. DOI.
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Tasali E, et al. Effect of sleep extension on objectively assessed energy intake among adults with overweight in real-life settings: A randomized clinical trial. JAMA Internal Medicine. 2022;182:365–374. PubMed.
Hall KD, et al. Ultra-processed diets cause excess calorie intake and weight gain: An inpatient randomized controlled trial of ad libitum food intake. Cell Metabolism. 2019;30:67–77.e3. Full text.
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