Why Weight Gain Happens in Midlife (And What to Do About It)
Have you reached midlife and noticed that your weight is creeping up, despite not having changed your diet? Perhaps you have tried some of the things that used to work, like upping your cardio, restricting your meals or skipping breakfast, but they are just not making a difference in the same way? This is a common picture I see in clinic and you’re not imagining it: there’s a biological reason why this happens and there is something you can do about it. What’s more, it’s not about deprivation, battling your lack of willpower or toughing it out. The answer lies in steady blood sugar, satisfying and delicious meals, movement and a nervous system that feel safe rather than constantly on high alert. From a biological point of view, it’s about something called metabolic flexibility, which relates to your body’s ability to switch between burning carbohydrates and fats as fuel. When that flexibility slips, weight gain (especially around your middle), stubborn fat, low energy and even brain fog can follow.
What Is Metabolic Flexibility?
Think of your metabolism as a smart hybrid engine. A flexible metabolism will burn glucose from carbs after you eat, then switch to burning stored fat when you rest. This adaptability keeps your energy steady, keeps your blood sugar balanced and allows your mitochondria, the tiny power-houses inside every cell, to produce energy efficiently.
But as we get into our 40s and beyond, especially through perimenopause and menopause, hormonal changes can disrupt this system. Oestrogen levels fluctuate far more than in our reproductive years and progesterone levels are lower. This is normal and natural, but it does affect the enzymes that help your mitochondria to switch fuels, creating a kind of metabolic brake that makes it harder for your body to burn fat and easier to store it. This leads to weight gain even when your intake of calories hasn’t changed. It’s actually a protective mechanism, as fat around the middle is metabolically active.
Why Midlife Matters
What’s really key here is that it’s not just about oestrogen (which means that hormone therapy isn’t a magic bullet for everyone). A combination of factors in midlife can reduce metabolic flexibility:
Insulin insensitivity — your cells don’t respond to insulin as well, making it easier to store fat. This can be the case even if your blood glucose and HbA1c markers are in range, as is explained in more detail here.
High-sugar, high-fat diets — these push your metabolism to rely on glucose instead of fat.
Circadian rhythm disruption (poor sleep or irregular eating patterns) — this affects metabolic signalling.
Sedentary lifestyle — physical inactivity is linked to metabolic inflexibility.
Inflammation or gut imbalances — these make fuel switching less efficient.
So rather than your metabolism slowing with age, what’s often happening is that your body can’t switch fuel sources as easily: it’s stuck oxidising glucose and so stores more fat. Weight gain with age isn’t inevitable: it can be a signal from your body that your metabolic regulation can be improved. It might be that you are happy with your new size and shape, but if you do want to work on your metabolic health then I’ve shared some strategies below.
What You Can Do
The good news? Metabolic flexibility can be supported with the right nutrition and lifestyle strategies (and it doesn’t require extreme dieting).
Here’s where to focus:
1. Balanced, nutrient-rich eating
Prioritise quality protein, healthy fats (especially omega-3s) and fibre-rich carbohydrates such as vegetables, beans, lentils, chickpeas and berries. This steadies blood sugar and reduces insulin spikes that promote fat storage.
2. Smart movement
Resistance training and even a walk around the block can improve insulin sensitivity and support mitochondrial health.
3. Meal timing and circadian support
Time-restricted eating (like a 12-hour overnight fast), consistent mealtimes and consistent sleep rhythms help support your body’s metabolic rhythm. I’m also a great advocate of leading Alzeihmer’s researcher Dr Dale Bredesen’s advice to try to leave a three-hour gap between your last meal of the day and going to bed.
4. Reduce inflammation and stress
Sleep, stress management and anti-inflammatory foods (e.g. oily fish, a rainbow of fresh vegetables) help support metabolic flexibility.
If you’re frustrated by midlife weight gain, it’s important to remember: it’s not a personal failure. It’s a biological shift that you can understand and work with, not against. The goal isn’t a fast metabolism, it’s a flexible one that supports energy, clarity and a healthier body composition for the long haul.
How to Find Out More…
I offer a personalised, evidence-based approach to nutrition that means you don’t have to do it alone. I can be on your side to help you create healthy habits that are not only effective but are based on joy, not restriction. I offer a completely free 20-minute chat so you can find out more about how I work and if it would be the right fit for you. Click here to book now.