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What even is your metabolism?

At its core, metabolism is how your body extracts energy from food and converts it into the fuel that powers everything you do—from basic survival functions to intense workouts. Think of it like a car: putting gas in the tank doesn't start the engine on its own. Something has to turn that fuel into usable energy, and that conversion process is your metabolism.

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The Biggest Contributors to Metabolism

To understand your metabolism, it helps to look at your Total Daily Energy Expenditure (TDEE)—the total number of calories your body burns in a 24-hour period. TDEE is made up of three main components: physical activity, the thermic effect of food, and its largest foundation, your Basal Metabolic Rate (BMR).

Your BMR represents the energy your body needs just to carry out essential life functions at rest—like breathing, circulating blood, repairing cells, and keeping your organs running. When it comes to weight loss, BMR is crucial because it sets the baseline for how many calories you burn without moving a muscle. The higher your BMR, the more energy you burn at rest, making it easier to create the calorie deficit needed for fat loss.

Basal metabolic rate (BMR) accounts for roughly 50–70% of your total daily energy burn. Most of your BMR is driven by vital organs—like your brain, liver, heart, and kidneys—while other tissues like fat, skin, and intestines contribute the rest. Although these organs are incredibly calorie-hungry, their metabolic rates are largely out of our direct control; you can’t "train" your liver or brain to burn extra calories. However, there is one major component of BMR that you can actively build and preserve: your skeletal muscle. More on this as you read on.

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Hormones: The Behind-the-Scenes Regulators of BMR

While skeletal muscle is the main metabolic lever you can control, your resting metabolism is continuously managed behind the scenes by your hormone network. For example, thyroid hormones (T3 and T4) act as your metabolic thermostat. When active T3 drops, your cells burn fewer calories at rest, while elevated Reverse T3 (rT3) can block active T3 and slow energy expenditure further. Similarly, estrogen keeps your cells' energy generators (mitochondria) functioning efficiently and supports muscle repair. Because hormones play such a central role, conditions like hypothyroidism or the transition into menopause can cause a noticeable drop in your baseline calorie burn. While these hormonal shifts make body recomposition more challenging, they don't make it impossible—you can still optimize your metabolism through key, modifiable lifestyle factors as discussed below.

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Beyond BMR: Movement, Food, and Adaptation

The rest of your daily energy use comes from a mix of lifestyle-driven factors:

• Physical Activity (20–45% of TDEE): Includes Exercise Activity Thermogenesis (EAT)—calories burned from structured exercise like running, weightlifting, or cycling and Non-Exercise Activity Thermogenesis (NEAT), which covers calories burned through everyday, unconscious movements, like fidgeting or pacing.
• Thermic Effect of Food (TEF, ~5–15% of TDEE): Represents the energy required to digest, absorb, and process nutrients. The thermic effect varies by macronutrient: Protein: 20–30% of calories TEF; Carbohydrates: 5–10% as TEF; Fat: 0–3% as TEF.

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The Modifiable Factors — Exercise and protein

Lifestyle factors such as muscle mass, daily activity, and nutrition can influence your metabolism in meaningful ways. Muscle is the most powerful adjustable lever — it contributes about 20% of BMR, and unlike your organs, you can build more of it. At rest, muscle burns ~6–10 calories per pound per day. Muscle's daily calorie burn is modest compared to organs, but it still burns more energy than body fat (~ 2 calories per pound per day). Every extra pound of muscle not only burns calories at rest but also makes walking, climbing stairs, or carrying groceries more metabolically costly.

Therefore, incorporating exercise that challenges the skeletal muscles would contribute to a higher overall BMR, and therefore, more calories burned while engaging in daily activities and at rest. Two days of strength training, at the very least, can make a difference. Incorporating aerobic exercise in conjunction with a strength training regimen will only support further calories burned throughout the day. From a body recomposition standpoint, at least 50 minutes of moderate activity (i.e., risk walking, cycling, swimming), 5 days a week OR 30 minutes of vigorous activity (i.e., running, HIIT, fast cycling), 5 days a week is recommended.

A great starting point for improving your physical activity is understanding your baseline. Tracking your physical activity, like your steps, for example, gives you clear feedback on whether you're hitting movement targets. Many people consistently go to the gym two or three days a week but overlook the rest of their daily activity. By assessing your starting point, you can build a realistic plan to progressively step up your overall movement.

Choosing a diet richer in protein leverages the Thermic Effect of Food (TEF)—boosting your daily calorie burn through digestion while preserving lean muscle and promoting satiety (fullness after meals). However, a high-protein target is not one-size-fits-all, as optimal intake depends on individual factors like height and biological sex. Unfortunately, many people calculate their protein targets incorrectly, setting goals that are unrealistic and unsustainable. Working with a registered dietitian can help you establish an accurate, personalized protein target that fits seamlessly into your lifestyle.

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Going too far with Calorie Restriction

Calorie intake strongly influences your overall energy expenditure. While cutting calories is standard advice for improving body composition, taking it too far can backfire. Dropping below general safety thresholds—typically 1,200 calories per day for women and 1,500 for men—signals your body to conserve energy for long-term survival. This metabolic adaptation causes Non-Exercise Activity Thermogenesis (NEAT) to decline as subtle, everyday movements slow down. Additionally, eating significantly less food inherently lowers the Thermic Effect of Food (TEF), since there is less volume to digest. Instead of cutting further, recovery often requires strategically raising intake (with an individualized calorie upper limit in mind), prioritizing protein, and focusing on resistance training to preserve lean mass.

So if you want to boost your metabolism, focus on the levers you can actually pull:

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1. Build and maintain muscle.
2. Eat protein-rich foods.
3. Eat enough calories.
4. Stay active throughout the week.

These facts are not as sexy or as easy as taking an expensive “metabolism-boosting supplement,” but it really is the best information we have based on solid evidence. At Hygeia, we design programs around these modifiable factors — helping you build lean mass, fuel smartly, and stay active in ways that elevate both your metabolism and long-term health.

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