Why weight gain happens after 40 – and the truth about your changing metabolism

For years we’ve accepted that hitting middle age comes with an unavoidable metabolic slowdown: the waistline expands, energy levels dip and the weight that once disappeared after a few extra bike rides suddenly becomes harder to shift. But is ageing really to blame?

According to Professor Adam Collins, associate professor of metabolic nutrition at the University of Surrey, the answer is more complicated. The idea that your metabolism suddenly falls off a cliff at 40 simply isn’t supported by the evidence.

“People think that because they notice physical changes in middle age – the so-called ‘middle-age spread’, less energy or things becoming a bit harder – that their metabolism must be slowing down,” he says. “When we talk about metabolism, what people usually mean is metabolic rate: the number of calories your body burns at rest, known as your basal metabolic rate or resting energy expenditure.”

In reality, metabolism is a complex network of processes controlling how your body uses and stores energy. Resting metabolic rate is only one part of the picture and, according to Collins, the biggest influence isn’t your age, but your body composition.

Age is just a number

“The main thing that affects your metabolic rate is how much of you there is and what you’re made of,” says Collins. “The bigger you are, the higher your metabolic rate, because there’s more tissue to metabolise. But, body composition matters, too. The more active tissue you have, the higher your metabolic rate.”

That active tissue is known as fat-free mass, which includes skeletal muscle, but also the body’s organs. “People often assume muscle is the main driver because we associate it with movement, but your organs are incredibly metabolically active. They probably account for around two-thirds to three-quarters of your resting metabolic rate.”

The liver, brain, heart and kidneys all consume large amounts of energy, even while you’re sitting still. “So, when you compare younger and older people, once you account for body composition – particularly fat-free mass – metabolic rate becomes remarkably predictable.”

So why does the idea of a middle-aged metabolic collapse persist? Collins says much of the research has compared different age groups at a single point in time, rather than following the same people throughout their lives. “A lot of studies are cross-sectional, meaning you’re looking at different people of different ages rather than measuring the same person repeatedly as they get older.”

He points to the widely publicised research led by Herman Pontzer, which analysed metabolic data from thousands of people using the International Atomic Energy Agency’s Doubly Labelled Water Database. While groundbreaking, Collins notes the database combines data from many separate studies, including endurance athletes, and people with disease, making it less representative of the general population.

The key takeaway, however, remains clear: “Once body size and composition are taken into account, you don’t suddenly hit middle age and go off a cliff. Even when metabolic rate declines later in life, it’s gradual.”

Why weight gain happens after 40

If metabolism isn’t the main culprit, why do so many of us gain weight during middle age? “Lifestyle,” says Collins. “To blame it on metabolic rate is a bit misleading. In your 20s and 30s, you’re generally more active, you might go to the gym more often, you’re conscious of your appearance and your lifestyle naturally supports maintaining your weight.” Then, careers, families and busy schedules take over.

“You settle down, work generally becomes more settled, life is less frenetic, you have more disposable income and you naturally slow down a little.” The result isn’t a broken metabolism, but a gradual mismatch between calories consumed and calories burned.

One often-overlooked factor is non-exercise activity thermogenesis (NEAT) – the calories burned through everyday movement such as walking, standing, climbing stairs and household tasks. These small movements often decline with age, and can significantly reduce daily energy expenditure.

Hormones are also frequently blamed for middle-aged weight gain, particularly menopause in women and declining testosterone in men. Collins says the picture is more nuanced: “For women, studies measuring metabolic rate through the menopause transition haven’t consistently shown a reduction in resting metabolic rate.” Instead, hormonal changes may affect appetite, energy balance and where fat is stored.

“You might get changes in appetite regulation, making it easier to end up in a positive energy balance. It’s not necessarily because you’re burning fewer calories, but because you’re consuming more.” After menopause, fat also tends to be stored more centrally around the abdomen as hormone levels shift. “Metabolism is more than just how many calories you’re burning. It’s also about how your body manages the fuel in the system,” adds Collins.

Online, declining testosterone is often blamed for everything for men, from stubborn belly fat to sluggish metabolism. Collins says that’s an oversimplification: “Testosterone doesn’t directly drive metabolic rate. It may influence body composition by helping maintain muscle mass, but by itself it won’t do anything.”

He compares testosterone to protein: “Simply giving someone more protein doesn’t automatically build muscle. You need resistance training to provide the stimulus.” Likewise, testosterone helps preserve muscle only when combined with physical activity.

Collins also questions the widespread belief that modern men are experiencing dramatic testosterone declines. “Yes, testosterone generally falls with age, but it’s highly variable. Not everyone is guaranteed to have a significant drop.” Rather than chasing metabolism boosters or supplements, Collins believes the focus should be on preserving fat-free mass, particularly muscle.

“Exercise is too specific,” says Collins. “I’d rather talk about being physically active because it’s about doing things that positively influence body composition. You don’t have to be heading to the gym or running marathons, you just need to be active and adequately challenging your muscles and cardiovascular system each day.”

That advice is echoed by Sophie Bertrand, registered nutritionist and advisor to Abbott’s Lingo glucose monitoring system. “Weight gain in our 40s is rarely the result of one single factor or a dramatic slowdown in metabolism,” she says. “It’s usually the combined effect of changes in body composition, hormones, daily movement and lifestyle habits that gradually shift energy balance over time.”

One of the biggest physiological changes is the gradual loss of muscle mass, which begins in our 30s unless we actively maintain it. “Muscle supports glucose uptake, strength, mobility and healthy ageing, making it central to overall metabolic health,” says Bertrand.

Resistance training combined with sufficient protein intake remains the most effective strategy for maintaining muscle. At the same time, don’t underestimate everyday movement. “Walking, standing, gardening, taking the stairs and other daily activities all contribute to total energy expenditure,” Bertrand says. “Maintaining these habits can make a meaningful difference.”

Eat for the decade you’re in

Collins also believes many people also need to adjust their eating habits as they age. “Stop eating like you’re 25,” he says. “When you’re younger, you can often get away with blowout meals and nights drinking, and then burn it off at the gym. That ability to balance things out disappears in middle age.”

But, cutting calories too aggressively isn’t the answer either. “Restricting calories or eliminating food groups can make it difficult to meet protein and nutrient requirements, increasing the likelihood of losing muscle as well as fat,” Bertrand warns. Instead, she advises building meals around protein, fibre, healthy fats and quality carbohydrates while prioritising sleep, managing stress and staying active.

Remember, your metabolism hasn’t suddenly stopped working because you’ve turned 40. “The challenge isn’t fighting age itself but adapting your habits to match the body you have today,” adds Collins. Maintaining muscle, staying physically active and eating appropriately for your lifestyle will do far more for your long-term metabolic health than worrying about a mythical midlife metabolic crash.

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