Slow Metabolism: Is It Really Why You Are Gaining Weight?

You eat pretty much the same meals you did in your twenties. Dal, rice and roti for lunch, a cup of chai with biscuits in the evening. Yet somewhere in your thirties, the weight starts creeping up. Your clothes feel tighter, and losing those extra kilos seems harder than before. Someone tells you it's because of a slow metabolism. After all, doesn't your metabolism naturally slow down once you turn 30?

Interestingly, research suggests otherwise. A large study found that the amount of energy your body burns, after accounting for differences in lean body mass, stays fairly stable between the ages of 20 and 60. So, if your metabolism isn't slowing down as much as you thought, what's causing the weight gain? Changes in muscle mass, daily activity, sleep and sometimes underlying health conditions can all play a part. Understanding how metabolism works, what actually affects it and which everyday habits can help is a better starting point than simply blaming your age.

What is metabolism, in plain words?

Metabolism is the process your body uses to turn food into energy and keep everything running, from breathing and thinking to walking and exercising. When someone says they have a fast or slow metabolism, they're usually talking about how many calories their body burns each day.

Your daily calorie burn comes from three main things:

1. Basal metabolic rate (BMR): This is the energy your body uses even when you're completely at rest. Your heart needs to beat, your lungs need to breathe and your brain needs to function. All of this burns calories and makes up the largest part of your daily energy use.

2. Physical activity: This includes exercise, but also everyday movements like walking to work, climbing stairs, cooking, cleaning or carrying groceries. Even small movements throughout the day contribute to how much energy you burn.

3. Digesting food: Your body also uses energy to break down and absorb what you eat. This is called the thermic effect of food. Protein generally takes more energy to digest than carbohydrates or fats.

Your BMR depends largely on your body size and the amount of lean tissue you have, including muscles and organs. In general, a larger body with more lean tissue uses more energy at rest. So, your metabolism isn't just about how quickly you burn food. It's also influenced by your body composition and how active you are throughout the day.

Does metabolism really slow down after 30?

This is the question everyone at the dinner table assumed they knew the answer to. A large study published in Science in 2021 tested it properly. Researchers pooled measurements of daily energy use from people aged 8 days to 95 years, using a precise method called doubly labelled water. After adjusting for lean body mass, they found that energy use declines slowly to adult levels by about 20 years, remains stable in adulthood (20 to 60 years), even during pregnancy, then declines in older adults. Between 20 and 60, for the same amount of lean tissue, the rate does not meaningfully drop. Your body at 36 is not a worn engine running slower than it did at 26. So if the engine has not changed, something around it has.

Why Do You Gain Weight as You Get Older?

If your metabolism stays fairly stable through adulthood, why does gaining weight seem easier in your thirties and forties? The answer often lies in small changes to your everyday routine and body composition. You may be moving less, losing some muscle or getting less sleep than you used to. These changes may seem minor, but over months and years, they can make a difference to your weight. Here are three common reasons why this happens.

You Gradually Lose Muscle

Muscle uses energy even when you're resting. The more muscle you have, the more calories your body generally needs to maintain itself. However, we tend to lose muscle as we get older, especially if we aren't physically active. A review published in Current Sports Medicine Reports found that inactive adults can lose around 3% to 8% of their muscle mass per decade. This can reduce the calories they burn at rest and contribute to fat gain.

So, when someone says their metabolism has slowed down, part of what they're noticing may be a gradual loss of muscle rather than ageing alone. The encouraging part is that regular strength training and enough protein can help you maintain muscle as you age.

Also read: Why Men Lose Muscle After 30: Causes and What Helps. The advice on maintaining muscle applies to women too.

You may move less than you used to

Scientists have a name for the energy spent on everything that is not sleep, eating or formal exercise: non-exercise activity thermogenesis, or NEAT. It includes standing, pacing during phone calls, household work and even fidgeting. NEAT varies enormously between people. In a classic overfeeding experiment, 16 volunteers ate 1,000 extra calories a day for 8 weeks. Changes in NEAT accounted for the 10-fold differences in fat storage that occurred between them. Those whose NEAT rose more stored less fat.

Now think about life at 36 compared with 22. The college years meant walking between classes, taking the stairs, running errands on foot. Today there is a car or a cab, a desk job, long meetings, a sofa and a phone in the evening. The daily step count may have halved without you ever deciding to move less. The dal and roti did not change, but the movement around them did.

Sleep gets squeezed

Work deadlines, a small child or a late-night series cut into sleep for many people in this age group. In a small controlled crossover study, 12 healthy adults spent two weeks with only 4 hours in bed and two weeks with normal sleep. During the short sleep phase, they ate more calories while their energy expenditure was unchanged, so in this study short sleep seemed to work through appetite rather than metabolism. They gained more weight than during normal sleep. Their abdominal fat increased only during the sleep restriction phase.

So the extra biscuit at 11 pm may be a sleep problem dressed up as a willpower problem. Our guide on how much deep sleep adults need explains what good sleep looks like.

When is it worth getting tested?

Sometimes there really is a medical reason behind steady weight gain. It is worth ruling these out with your doctor rather than guessing.

• Underactive thyroid (hypothyroidism). When the thyroid gland makes too little hormone, the body does work more slowly. The NHS lists tiredness, feeling cold, weight gain and low mood among the common symptoms. A simple blood test for TSH and T4 confirms or rules it out. It is easy to add to a routine health check.

• PCOS. MedlinePlus lists polycystic ovary syndrome among the hormone conditions that can cause unintentional weight gain. If you think it might apply to you, raise it with a gynaecologist. Our article on PCOS and insulin resistance covers the tests and diet in detail.

• Medicines. MedlinePlus lists birth control pills, corticosteroids, some medicines used to treat diabetes and some medicines used to treat bipolar disorder, schizophrenia and depression among drugs that can cause weight gain. Never stop a prescribed medication on your own. Ask your doctor whether an alternative is possible.

MedlinePlus advises contacting your doctor if weight gain comes with constipation, hair loss, feeling cold more often than before, swollen feet and shortness of breath, uncontrollable hunger with palpitations, tremor and sweating, or vision changes. The same applies if you gain a lot of weight without a known cause.

Do metabolism boosters work?

Shop shelves and social media are full of drinks, powders and fat burner capsules that promise to "boost your metabolism". Be cautious with these claims. A capsule cannot rebuild the muscle you have lost or put back the walking your routine has removed. Those are changes that can drive weight creep in midlife. Some stimulant products can also raise heart rate or interact with medication, so speak to your doctor before trying one. The habits below work on those likely causes.

What actually helps a slow metabolism

You don't need an expensive gym membership or a strict diet to support your metabolism. Small changes to your everyday routine can make a difference, especially when you stick with them.

1. Make time for strength training. Try simple exercises like squats, wall push-ups, lunges or lifting dumbbells two to three times a week. A review of studies found that resistance training increased resting calorie burn by around 96 calories a day on average compared with control groups. However, the studies had limitations, so individual results may vary.

2. Include protein in every meal. Protein helps your body build and maintain muscle. Try adding dal, curd, paneer, eggs, sprouts, chana, fish or chicken to your meals rather than eating most of your protein at dinner.

3. Move more throughout the day. Take the stairs, walk to nearby shops or stand up between long hours of sitting. A short walk after meals can also help manage blood sugar. These small movements add to your daily calorie burn, even when you're not exercising.

4. Get enough sleep. Try to maintain a regular bedtime and reduce screen time before sleeping. Poor sleep can affect hunger and make it harder to manage your weight.

5. Notice the small extras in your diet. Biscuits with every cup of chai, sugary drinks and frequent snacks can add more calories than you realise. You don't have to give them up completely, but being mindful of portions can help.

6. Speak to a doctor if something feels unusual. If you're gaining weight unexpectedly or experiencing persistent tiredness, your doctor may recommend checking your thyroid or looking for other possible causes.

Give these changes time to work. Building muscle and improving your fitness happen gradually. A review published in Current Sports Medicine Reports reported that ten weeks of resistance training may increase resting metabolic rate by around 7%, although results vary. You may notice improvements in strength, stamina or waist size before seeing a big change on the scale. Focus on steady progress rather than expecting your metabolism to change overnight.

Also read: What Are Macronutrients? and Walking After Meals: How It Helps Your Blood Sugar.

Back to that family dinner

The next time someone says your metabolism has slowed, you will know a little more than they do. At 36 your body burns energy at much the same rate per kilo of lean tissue as it did at 26. What changed is the muscle you carry, the steps you take, the sleep you get and perhaps a thyroid or hormone level worth testing. Each of those is within reach. Keep the dal, rice and roti. Add a few squats before your evening chai and a walk after dinner. The kurta may fit again by the next Diwali.

Sources

• Daily energy expenditure through the human life course (Science, 2021)

• Resistance training is medicine: effects of strength training on health (Current Sports Medicine Reports, 2012)

• Role of nonexercise activity thermogenesis in resistance to fat gain in humans (Science, 1999)

• Effects of experimental sleep restriction on energy intake, energy expenditure and visceral obesity (Journal of the American College of Cardiology, 2022)

• The effect of exercise interventions on resting metabolic rate: a systematic review and meta-analysis (Journal of Sports Sciences, 2020)

• Underactive thyroid (hypothyroidism) symptoms (NHS)

• Weight gain, unintentional (MedlinePlus)

Medical Disclaimer: This article is for general information and is not a substitute for advice from your doctor. Speak to a qualified healthcare professional before starting any supplement, especially if you are pregnant, breastfeeding or taking medication.