Why "Boosting Metabolism" Is All Wrong

Good morning!

So "I have a slow metabolism" is probably a phrase you’ve heard quite a lot. And yeah, you can throw in your two cents if someone tells you this, but… what actually is metabolism?

Take this scenario, where your coworker or friend eats pizza every day and stays thin. You eat a salad and gain weight. The conclusion seems obvious: your metabolism must be slower than hers. She's lucky. You're not.

Except metabolism doesn't work that way. Here's what metabolism is, how it works, and what you should do about it.

Key Takeaways

• Metabolism makes up the majority of daily energy expenditure and is influenced primarily by body size, muscle mass, age, gender, and thyroid function.

• A “healthy” metabolism is not defined by being fast or slow, but by the body’s ability to appropriately adapt to changes in activity, food intake, stress, sleep, and recovery.

• Muscle mass, protein intake, non-exercise movement, sleep quality, stress levels, and thyroid health are the major factors that influence metabolism.

CORE

What is Metabolism?

Metabolism is the sum of all chemical processes your body uses to convert food into energy and… well, maintain life. When people say "metabolism", they usually mean basal metabolic rate (BMR), the number of calories your body burns at rest just to keep you alive.

Your BMR accounts for about 60-70% of your total daily energy expenditure. It powers breathing, circulation, cell production, temperature regulation, and every other automatic function happening inside you right now. Your brain alone uses about 20% of your resting energy. Your liver uses another 20%. Skeletal muscle uses roughly 20% even when you're sitting still.

The remaining 30-40% of daily energy expenditure comes from the thermic effect of food (the energy required to digest, absorb, and process what you eat, about 10% of total expenditure) and physical activity, which includes both intentional exercise and non-exercise movement like fidgeting, standing, and walking around your house.

Your BMR is determined primarily by body size and composition. Larger bodies require more energy to maintain. Muscle tissue is metabolically more active than fat tissue, meaning it burns more calories at rest, though the difference is smaller than most people think. Men generally have higher metabolic rates than women because they're typically larger and carry more muscle mass.

Age also affects metabolism, but not as dramatically as commonly believed. BMR declines about 1-2% per decade after age 30, largely due to loss of muscle mass rather than aging itself.

Is There Such a Thing as a “Good” Metabolism?

People talk about a “fast metabolism” like it’s a superpower and a “slow metabolism” like it’s plain bad. But the reality is more nuanced than that.

From a purely survival standpoint, a slower metabolism is actually incredibly efficient. A body that can maintain itself on fewer calories has an evolutionary advantage during famine or food scarcity. Historically, conserving energy meant surviving longer when food was unpredictable. In that sense, a slow metabolism is adaptive.

A higher metabolism, on the other hand, means your body burns through more energy to maintain itself. That can make staying lean easier in modern environments where calories are abundant, but it also means you require more food just to maintain normal function. People with very high metabolic demands often feel hungrier, lose weight unintentionally during stress or illness more easily, and may struggle to maintain muscle or body weight.

So is one healthier? Not inherently.

A healthy metabolism is less about being “fast” or “slow” and more about being responsive and appropriate for your body’s needs. Your metabolism should adapt when you exercise more, eat less, gain muscle, get sick, recover, sleep poorly, or experience stress. That flexibility, not raw speed, is what matters most metabolically.

The problem is that modern culture treats metabolism as a moral trait. Thinness gets interpreted as “good metabolism,” while weight gain gets blamed on a “bad” one. 

And importantly: a chronically suppressed metabolism isn’t ideal either. Severe dieting, under-eating, excessive stress, and poor sleep can reduce energy expenditure as a protective mechanism.

Moral of the story, neither extreme is healthy.

What Actually Affects Metabolism

Muscle mass. Muscle burns more calories at rest than fat tissue. Gaining muscle through resistance training increases your BMR. The effect is real but modest, each pound of muscle burns roughly 6 calories per day at rest compared to 2 calories per pound of fat. Ten pounds of added muscle increases resting metabolism by about 40 calories daily, equivalent to half an apple. Significant, but not transformative.

However, the real metabolic benefit of strength training isn't just resting burn, it's the energy expended during and after exercise. Resistance training creates an "afterburn effect" where metabolism remains elevated for hours post-workout as your body repairs muscle tissue.

Protein intake. Of all macronutrients, protein requires the most energy to digest and metabolize. The thermic effect of protein is 20-30%, meaning you burn 20-30% of protein calories just processing them. A high-protein diet can increase daily energy expenditure by 80-100 calories through this mechanism alone. Protein also preserves muscle mass during weight loss, preventing the metabolic slowdown that comes from losing muscle alongside fat.

Thyroid function. Your thyroid produces hormones that directly regulate metabolic rate. Hypothyroidism (underactive thyroid) slows metabolism significantly, some people with untreated hypothyroidism burn 30-40% fewer calories than expected for their size, and hyperthyroidism (overactive thyroid) speeds it up.

NEAT (Non-Exercise Activity Thermogenesis). This is all the non-exercise exercise. Actions like fidgeting, standing, etc. NEAT varies dramatically between individuals, sometimes by 2,000 calories per day.

Sleep and stress. Chronic sleep deprivation and elevated cortisol from stress both negatively affect metabolism. Poor sleep disrupts hunger hormones (ghrelin and leptin), increases cravings for high-calorie foods, and can reduce insulin sensitivity. Chronic stress raises cortisol, which promotes fat storage.

Temperature exposure. Your body burns calories maintaining core temperature. Exposure to cold temperatures provokes your body to generate heat by burning calories. Regular cold exposure, cold showers, lower indoor temperatures, outdoor winter activities, can increase daily energy expenditure, though the effect is modest for most people.

MAGNESIUM & METABOLISM

Did you know that magnesium is needed for the processes that convert carbohydrates, fats, and protein into usable cellular energy (ATP)? Without enough magnesium, energy production becomes less efficient, which is one reason deficiency is often associated with fatigue, weakness, poor exercise performance, and muscle dysfunction.

Did you also know that nearly half of Americans fail to meet recommended magnesium intake levels? With OmniBlue Original, you get your entire day’s worth of magnesium in a clean, all natural and bioavailable form.

Get 10% off with OmniBlue Original with Sub & Save, or get 20% off with OmniBlue Original 3-pack!

ENDNOTE

Final Thoughts

Thanks again for making it all the way to the end of the newsletter! As always, if there’s a topic you’d love to see us cover, feel free to pitch it by replying to this email. Quite a few of our favorite editions have come directly from readers, so don’t be afraid to throw your hat in the ring.

Until next week!

Adrian Macdonald | Team Dietitian | The 8% Newsletter Author