A Beginner’s Guide to Your Metabolism
by: Trinh Le, MPH, RD for MyFitnessPal
Contrary to popular belief, the majority of your daily calorie burn doesn’t come from puddles of sweat after a tough workout. Calorie burn is actually driven by your metabolism.
Metabolism is the entire process of converting calories into energy to power all your bodily processes. And it isn’t just about calorie burning! It’s also about calorie storing. Your metabolism determines the number of calories you need daily to maintain your weight. While there’s only one way calories can enter your body (nom nom!), there are many ways for calories to leave it. Here are the three major factors that affect your metabolism and overall calorie burn.
1. BASAL METABOLIC RATE (BMR): Calories to Survive
Accounts for 60–70% of daily calories you burn.
Basal metabolic rate is the number of calories your body needs to support the vital functions that keep you alive (breathing, digesting, filtering waste) while at rest. These functions eat up a whopping 60–70% of your daily calorie intake, making BMR the largest contributor to your metabolism. Your BMR doesn’t include the calories you burn for normal daily activities or exercise. Here are the key factors that play into BMR:
BODY SIZE – A bigger individual requires more calories to sustain their body at rest and with any activity they do. Taller and heavier individuals have larger organs (muscles, brain, heart) that require more calories for upkeep.
BODY COMPOSITION – Muscle is more metabolically active than fat, meaning more calories must be burned to maintain a pound of muscle compared to a pound of fat. Two healthy individuals of the same age, height and weight can have very different BMR if they have significantly different percentages of lean versus fat mass. Since most women have more fat mass compared to men, they have correspondingly lower BMR compared to men of the same height and weight.
AGE – Your BMR is higher when you are younger, especially since calories are needed to supply your growing body. The trend is that as you age you slowly gain weight in the form of fat mass and lose weight in the form of muscle mass. Read 5 Ways to Cope with a Slowing Metabolism to learn more about how aging affects your metabolism.
GENETICS – You knew this was coming! Some people are born with higher (or lower) BMR than others, and this is completely normal. Your genes are not something you can fix, but if you suspect you have a genetic condition that slows down your metabolism (such as familial hypothyroidism), this is something you should consult a medical professional about.
HORMONES – They act like chemical dials allowing your body to turn your metabolism up or down depending on its needs. The two main hormones (thyroxine and triiodothyronine) directly responsible for turning up BMR come from your thyroid gland. Other hormones indirectly cause your thyroid gland to release more or less of these hormones, leading to a change in BMR.
HEALTH – Generally, your BMR is higher when you are fighting off an infection or healing from a major wound. This is because your body requires more calories to accomplish both of these tasks.
Because these factors introduce so much variability, calculating someone’s exact BMR is hard to measure accurately without sinking serious cash into fancy equipment. Instead, BMR is generally approximated using an equation called the Mifflin–St. Jeor, which has been shown to be most accurate in predicting BMR for healthy adults compared to other existing equations. This equation approximates your BMR using your gender, body size and age when it calculates your daily calorie goal.
2. FOOD THERMOGENESIS: Calories to Digest
Accounts for 10% of daily calories you burn.
Sure, it’s not an easy word to say, but at least the concept is somewhat simple. Food thermogenesis is the energy (calories) you need to digest and absorb food. Of all the macronutrients, protein requires the most work to digest followed by carbs and fat. About 10% of your daily calorie intake is used to digest and absorb a meal with mixed macronutrients, but here’s the breakdown in case you’re interested:
A high-protein, low-carbohydrate diet tries to leverage this phenomenon to burn calories, as it takes more energy to burn protein than carbs or fat. Since food thermogenesis only accounts for 10% of daily calorie burn, eating more protein will only have a small effect on your metabolic rate. While protein is still helpful for weight loss, you need to consider the cons of eating too much, including the wear and tear on your kidneys.
3. PHYSICAL ACTIVITY: Calories to Move
Accounts for 20% of daily calories you burn.
For most people physical activity accounts for 20% of daily calories burned, but this percentage can be higher on tough workout days. Keep in mind that it’s not just about the calories burned while working out; it’s also about the calories burned while working on the job (think: typing, carrying heavy loads, standing, fidgeting) and having fun (think: shopping, playing, singing). MyFitnessPal takes this into account by asking you to identify your usual activity level (sedentary, lightly active, active, very active) to more accurately predict your total daily calorie burn.
UNDERSTANDING YOUR MACRONUTRIENT FUEL
Our body is designed to extract energy from all three types of macronutrient fuel (carbohydrates, fat and protein), but that doesn’t mean a calorie is just a calorie. What our body is going through while it’s burning those calories determines which fuel it turns to the most. Here are some common conditions:
Additionally, certain diseases and conditions will affect the fuel your body uses the most. For example, if you’re suffering from a third-degree burn, you’ll need much more protein fuel to heal and rebuild tissues.
You can also change the fuel type your body prefers during exercise by training. As you train, your body becomes more efficient at using oxygen during exercise. This allows you to burn more fat during higher-intensity exercises rather than mostly carbs. For this reason, you can perform at a higher intensity (e.g., run, cycle and swim farther and faster) without feeling tired.
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