Understanding Metabolism
“Metabolism” describes the process of consuming, storing and using energy. When we eat a meal, we break down, absorb and store nutrients.
Carbohydrates are stored primarily as glycogen in the skeletal muscle and the liver. This is why the loss of muscle increases the risk of Type II diabetes — if we have less space to store glycogen, the glucose sticks around in the bloodstream.
Dietary fat is stored as triglycerides in fat cells (adipose tissue).
Protein is broken down into amino acids, which enter the blood and extracellular fluids.
We tap into nutrient stores to generate the energy required to sustain biological functions and perform physical activity. Colloquially this is often described as “burning carbs” or “burning fat.” Keep in mind that using fat as fuel is not fat loss. We use fat as fuel for less demanding tasks and carbs (or glycogen) for more energy-intensive activities.
“Energy expenditure” is the overall number of calories we burn over the course of a day.
Basal metabolic rate (BMR): BMR typically makes up about 60-70% of daily energy expenditure. This is the amount of energy required to sustain biological functions like breathing, circulating blood, regulating body temperature, maintaining brain and nerve function.
Thermic effect of feeding (TEF): TEF accounts for ~10% of daily energy expenditure. This is the number of calories used to ingest, absorb, metabolize, and store nutrients from food.
Exercise activity thermogenesis: Exercise activity typically accounts for the smallest proportion of total energy expenditure (~5%), but this is highly variable depending on the type, volume, intensity, and frequency of exercise.
Non-exercise activity thermogenesis (NEAT): NEAT is estimated to contribute roughly 15% of total daily energy expenditure. This is all non-exercise movement, including subconscious activity like fidgeting.
Energy intake (“calories in”) and energy expenditure (“calories out”) determine body weight maintenance. When energy intake chronically exceeds energy expenditure, we gain weight. When energy expenditure chronically exceeds energy intake, we lose weight.
Metabolic rate is not a fixed setting that gets damaged or broken; it is a dynamic process that responds to body composition, activity levels, and energy availability.
It decreases during sustained caloric restriction, but this is a physiological adaptation, not permanent damage. Energy expenditure increases with weight gain and decreases with weight loss. This is why when you start eating fewer calories, you’ll lose weight, but over time, weight loss stalls. The metabolism has “adapted” to the new energy intake: a smaller body requires less calories to “keep things running” and burns fewer calories during exercise.