Written by Courtney Human RD(SA)

Somewhere between your mid-20s and your late 30s (and definitely into your 40s), something starts to feel… different. The same meals, the same workouts, the same routine, but suddenly your body is responding like it didn’t get the memo. Clothes fit differently, weight creeps up more easily. The same thing.” And you probably are. The problem is, your body isn’t. And just to add to the fun, this shift tends to arrive unannounced. No email, no calendar invite. Just vibes. Strong, slightly confusing vibes.

While the wellness world loves to torture us with the idea of “just try harder,” biology is quietly doing its own thing in the background. Let’s start with metabolism, because it tends to take the blame for everything. Metabolsim is the body’s internal engine that’s running 24/7 to turn the food and drinks we consume into energy needed to keep us alive. It powers every vital function. Interestingly, large-scale research has shown that when one accounts for body size and muscle mass, metabolic rate remains relatively stable through early and middle adulthood, with more noticeable declines only occurring later in life. So if your weight is changing in your 30s and 40s, it’s not because your metabolism suddenly “slowed down overnight” but rather because your body composition, hormones and lifestyle are shifting in more subtle ways.

One of the biggest drivers of this is the gradual loss of muscle mass, known as sarcopenia. From around midlife, adults can lose approximately 0.5 to 1% of muscle mass per year if it’s not actively maintained. Which doesn’t sound like much until you feel it adding up while you’re busy being a functioning adult. Muscle is metabolically active tissue, meaning it plays a role in how your body uses energy, manages blood glucose and supports overall metabolic health. As muscle slowly declines, the body becomes slightly more energy-efficient. Over time, that small shift can contribute to gradual weight gain, even if nothing obvious has changed in your routine. At the same time, fat mass tends to increase (great news), particularly in women (even better). Research shows that body fat percentage often rises between the ages of 40 and 50, even when overall weight remains relatively stable. That redistribution of fat, often toward the abdominal area, is not random. It’s influenced by both hormonal and metabolic changes. If you’ve ever wondered, “When did my jeans become personally offensive?” you’re not alone.

But, why do women tend to feel this punishment, I mean shift, more than men? Biologically, women start with lower muscle mass and higher essential fat stores compared to men, which already places them at a (crappier) different metabolic baseline. As ageing progresses, this difference becomes more pronounced, with hormonal fluctuations, adding another layer. Oestrogen levels begin to fluctuate, influencing fat distribution, appetite regulation and insulin sensitivity. This doesn’t mean weight gain is inevitable, but it does mean the body becomes more responsive to changes in stress, sleep and overall lifestyle. Men are not immune to these changes (amen). Testosterone gradually declines with age, contributing to muscle loss and increased fat storage. However, because men generally start with higher muscle mass, the changes can feel less abrupt initially. The underlying process is similar, but the timeline and perception often differ (it was too good to be true.)

Beyond physiology, there are quieter lifestyle shifts. We move less without realising it. Fewer steps, more time sitting, more time working, more time driving, more time saying “I’ll go for a walk later” and then not going. This reduction in non-exercise activity can significantly lower daily energy expenditure, often without any conscious awareness. Sleep also becomes a bigger player than we tend to acknowledge. Poor sleep has been shown to disrupt hunger hormones like ghrelin and leptin, increasing appetite and reducing satiety. Add stress into the mix and cortisol begins to influence both appetite and fat storage too. The body becomes slightly more protective of its energy stores, which from an evolutionary perspective makes sense. From a modern perspective, it feels like your body is hoarding snacks you didn’t even eat. Then there’s dieting history, which often gets overlooked. Many women, in particular, have spent years navigating cycles of restriction, dieting and “starting again on Monday.” Over time, the body adapts. Energy expenditure may decrease slightly, hunger signals may become stronger, and the body becomes more efficient at conserving energy.

But instead of responding by pushing harder, eating less, exercising more and adding more restriction, the more effective approach is to adjust with your body, not against it. And yes, this is the part where I gently remind you of what registered dietitians have been saying for years:

1. Support muscle mass as an important strategy with aging. Resistance training is no longer optional, it’s protective.
2. Prioritize protein intake as the body becomes slightly less efficient at using it for muscle repair and maintenance.
3. Move outside of structured exercise. Walking more, standing more, and generally increasing daily movement can help offset the gradual decline in energy expenditure.
4. Manage sleep and stress as these are not luxuries, they are part of the metabolic equation. Supporting these areas has a direct impact on appetite regulation, energy levels and overall health, even if it’s not the most exciting advice.
5. Monitor your emotional response to all of this. Stop comparing your current body to a previous version of yourself without accounting for everything that has changed in between.

This is where working with a registered dietitian can make a significant difference, not as a last resort, but much earlier in the process. Rather than waiting until frustration turns into burnout, let us help you understand what is actually changing in your body and how to respond in a way that is both realistic and effective. There is also value in identifying early signs that may need further investigation. Subtle changes in energy, unexplained weight fluctuations, increased fatigue or changes in appetite can sometimes be linked to factors like blood sugar regulation, thyroid function, iron status or vitamin D levels. Not everything needs to be tested, but knowing when to look deeper can prevent months, or years, of guessing and Googling at midnight – yes, I’m talking to you mom.

Ironically, after tooting my own horn loudly above, let a registered dietitian help you remove the noise. In a space filled with conflicting advice, having an evidence-based, individualised approach can prevent the cycle of trying everything and feeling like nothing works. Because the goal is not to “go back” to how your body used to be. The goal is to support the body you have now in a way that is sustainable.

Because, once again, your body hasn’t stopped working. It’s just working differently now, and with the right support, that can be something you work with, not against.