Metabolism 101: What Controls Your Body's Energy Use

"Metabolism" is one of the most used, and most misunderstood, words in the wellness space. It is used to explain why some people seem to eat freely without gaining weight, why energy levels drop as we age, and why certain supplements are marketed as "metabolism boosters." But what does metabolism actually mean, and what does the science tell us about what controls it? This article cuts through the noise.

What Is Metabolism?

In its most precise sense, metabolism refers to the entirety of chemical reactions that occur in living organisms to sustain life. These reactions include catabolism (breaking down complex molecules to release energy) and anabolism (building complex molecules from simpler ones). In everyday health and wellness contexts, "metabolism" is typically used to mean the rate at which the body burns calories, more formally called total daily energy expenditure (TDEE).

The Four Components of Energy Expenditure

Total daily energy expenditure is made up of several distinct components, each representing a different category of energy use:

• Basal Metabolic Rate (BMR), the energy required to keep the body alive at rest: breathing, circulation, temperature regulation, cell repair, and organ function. BMR accounts for roughly 60–70% of TDEE in sedentary individuals.
• Non-Exercise Activity Thermogenesis (NEAT), the calories burned through all physical activity that is not deliberate exercise: fidgeting, walking, standing, gesturing. NEAT varies enormously between individuals and is one of the most significant modifiable components of TDEE.
• Thermic Effect of Food (TEF), the energy cost of digesting, absorbing, and processing food. TEF accounts for approximately 10% of TDEE, with protein having the highest thermic effect (20–30%) compared to carbohydrates (5–10%) and fats (0–3%).
• Exercise Activity Thermogenesis (EAT), the calories burned during intentional physical exercise. This component varies the most between individuals depending on activity level.

Factors That Influence Metabolic Rate

Metabolic rate is not fixed. Several factors influence how quickly the body burns energy:

• Age, BMR tends to decline with age, partly due to hormonal changes and partly due to the gradual loss of lean muscle mass that occurs from the fourth decade onwards
• Muscle mass, muscle tissue is metabolically active, burning more calories at rest than fat tissue. Individuals with greater lean body mass have higher BMRs
• Body size, larger bodies require more energy to maintain, so taller and heavier individuals generally have higher absolute BMRs
• Hormonal status, thyroid hormones, cortisol, insulin, and sex hormones all influence metabolic rate
• Diet composition, the macronutrient profile of the diet affects TEF; a higher protein intake increases the thermic effect of eating
• Physical activity history, regular exercise increases muscle mass and can raise resting metabolic rate over time

"Metabolism is not a fixed switch you are born with. It is a dynamic system influenced by dozens of variables, and understanding those variables is the first step to supporting it intelligently."

What "Supporting Metabolism" Means for Supplements

When a supplement claims to "support metabolism," it is important to understand what that phrase can and cannot mean under EU food supplement regulations. Supplements cannot claim to increase metabolism or burn fat, these would be medicinal claims. What they can do is provide nutrients that have an established EFSA-approved role in normal metabolic processes.

This is the foundation of Halden Life Metabolic Support. Chromium Picolinate has an EFSA-approved role in normal macronutrient metabolism. Green Tea Extract standardised to 50% EGCG provides antioxidant support. Cayenne Extract at 40,000 HU contributes to thermogenic processes. L-Carnitine L-Tartrate supports the transport of fatty acids into the mitochondria for energy production. None of these ingredients claims to transform your metabolism, but together, they provide the nutritional substrate that your metabolic processes depend on, every single day.