Why Your Body Changes After 40 …and How to Work with it
By Jamie Bussin, featuring Dr. Colleen Hartwick ND
For most of my life, I’ve described myself as having a “brutal metabolism.” A slow burner. Someone who had to work harder than most to keep things in balance. But as I’ve gotten older, especially approaching midlife, it’s felt like the rules changed entirely. What worked in my 20s and 30s suddenly stopped working. And judging by the conversations I’m having, I know I’m not alone.
People in their 40s and older are asking the same question: Is my metabolism actually changing, or is this all in my head? The answer, according to the science and clinical experience, is clear. This isn’t perception. It’s physiology. I discussed this with naturopath, Dr. Colleen Hartwick in Episode #418 of The Tonic Talk Show/Podcast. This is a digest of that conversation, which I like to call the Midlife Metabolic Reset.
What Happens to Metabolism After 40?
Around the age of 40, give or take a few years, our metabolism really does begin to shift. This is true for both men and women, although the mechanisms differ slightly.
For women, this stage often coincides with perimenopause, a time when estrogen, progesterone, and even testosterone begin to decline. For men, there’s andropause, marked by a gradual drop in testosterone that becomes more noticeable around midlife.
Why does this matter? Because these hormones play a major role in:
- Maintaining muscle mass
- Regulating insulin sensitivity
- Supporting overall metabolic rate
According to Dr. Hartwick, as hormone levels change, so does our ability to use carbohydrates efficiently, build muscle, and burn energy at rest. The result? Weight gain (often 10–15 pounds), lower energy, and the frustrating feeling that “nothing works anymore.”
Why Eating Less and Exercising More Stops Working
One of the most common frustrations I hear is this: “I’m doing everything right, but I’m still gaining weight.”
Says Dr. Hartwick:
Here’s what’s happening.
As estrogen and testosterone decline, insulin sensitivity often declines with them. That means carbohydrates are less likely to be used as fuel and more likely to be stored as fat. At the same time, it becomes harder to build and maintain muscle, the most metabolically active tissue in the body.
For men, declining testosterone contributes to muscle loss and fat gain, especially if strength training isn’t part of the routine. For women, reduced estrogen has an even stronger effect on carbohydrate metabolism and fat storage.
The old formula of “eat less, move more” doesn’t fail because of willpower, it fails because the body has changed.
The Midlife Metabolic Reset: A New Way of Thinking
From a naturopathic perspective, according to Dr. Hartwick, a midlife metabolic reset starts with a mindset shift.
The body you have at 40 is not the body you had at 25. And fighting that reality only leads to frustration. The reset is about working with your physiology instead of against it.
That also means shifting the goal away from weight loss alone. Weight is a volatile metric, especially for women and it can undermine motivation. Instead, focusing on:
- Strength
- Energy
- Endurance
- Metabolic health
leads to more consistency and better long-term outcomes.
Ironically, when people stop obsessing over weight, their bodies often respond more favorably.
Nutrition After 40: What Needs to Change
Does nutrition need to change in midlife? Yup.
Because insulin resistance becomes more common, many people benefit from reducing (but not necessarily eliminating) carbohydrates, especially refined and starchy ones. This doesn’t mean going carb-free, but it does mean being more intentional.
The bigger nutritional priority? Protein.
As we age, our muscles become less sensitive to dietary protein. That means we need more protein—not less—to maintain and build muscle.
Says Dr. Hartwick. “One of the most common mistakes I see, especially in women, is under-consuming protein. A solid rule of thumb is aiming for 25–30 grams of protein per meal, whether from animal or plant sources. Think of a generous serving like a heaping cup of Greek yogurt.”
Protein isn’t just about strength. It’s foundational for metabolic health.
Intermittent Fasting in Midlife: Does It Work?
Intermittent fasting can be helpful. …but context matters.
Men generally tolerate fasting better than women, largely because the female hypothalamus (the brain’s hormone control centre) is more sensitive to calorie restriction. That doesn’t mean women can’t fast, but it does mean the approach should be more cautious.
For many people, narrowing the eating window: about 6–8 hours for men and 10–12 hours for women, can improve insulin sensitivity, support fat metabolism, and allow time for cellular repair. As always, individualization is key.
Why Strength Training Is Non-Negotiable After 40
Dr. Hartwick thinks that if there’s one form of exercise to prioritize during a midlife metabolic reset, it’s strength training.
Hormonal changes and increased sensitivity to the stress hormone cortisol accelerate muscle breakdown. Resistance training helps counteract this by preserving and building skeletal muscle, a true metabolic powerhouse.
Muscle:
- Burns energy at rest
- Stores glucose efficiently
- Improves insulin sensitivity
- Protects against cognitive decline
Dr. Hartwick’s best advice: Cardio, mobility, and movement still matter. But if you’re lifting heavy things a few times a week, you’re directly investing in your metabolism.
Stress, Sleep, and the Cortisol Connection
Chronic stress plays a massive role in midlife metabolism. It elevates cortisol, which raises blood sugar, increases inflammation, promotes muscle wasting, and encourages fat storage, especially around the midsection.
One of the most underestimated tools for metabolic health? Sleep.
Poor sleep increases stress hormones, disrupts appetite regulation, and impairs muscle recovery. If you’re training but not sleeping, you’re missing the benefits. Seven to nine hours of quality sleep isn’t a luxury…it’s metabolic medicine.
Supplements That May Support Metabolic Health
While supplements aren’t magic, a few can be supportive depending on individual needs:
- Protein powder to help meet daily protein requirements
- B-complex vitamins to support carbohydrate metabolism
- Niacin derivatives (like NMN) for mitochondrial health
- Adaptogens (ashwagandha, rhodiola, holy basil) to buffer stress
- Magnesium, which supports stress tolerance, sleep quality, and hormone balance
Most people are magnesium deficient, and correcting that alone can improve both sleep and stress resilience.
The One Mindset Shift That Changes Everything
If there’s one takeaway from the idea of a midlife metabolic reset, it’s this:
Work with the body you have instead of resisting it.
That shift alone reduces frustration, lowers stress, and makes sustainable change possible. Midlife isn’t a metabolic dead end. It’s an invitation to recalibrate, adapt, and reset.
And when you do, your body often responds better than you expect.
