Metabolism is an awesome thing. Simply put, it's all the work your body does to keep your organs functioning.
And it's a lot of work. So with that, let's talk some science.
There's 4 main ways your body uses energy in the form of calories:
1) Basal Metabolic Rate (BMR): Imagine waking up in the morning and just laying there for the next 24 hours breathing. Your BMR is how many calories it takes to do that, and it's A LOT - accounting for 60-75% of the total calories your bod uses in a day to keep you running.
2) Thermic Effect of Food (T.E.F.): This is how many calories your body uses in a day to digest food. The less processed your diet is, the harder it is for your body to break down foods. The harder it is for your body to break down food, the more energy in the form of calories is required to get the job done (covered in THIS post ).
3) Non-Exercise Activity Thermogenesis (N.E.A.T.) : N.E.A.T. IS neat. No, seriously, it is. NEAT encompasses all of your movement outside of your workout. Things like brushing your teeth, washing the dishes, walking to the bathroom, and doing the macarena all count as NEAT.
NEAT is often times very underutilized. In fact, it's totally possible for someone who has a very active job where constant movement throughout the day is required (hello, my teacher friends) to "out-calorie-burn" someone who spends an hour at 5 A.M. doing a high intensity workout but sits for the rest of the 15 hours they are awake for the day (hello, my desk-job friends). I have prepared a home-made graphic to illustrate this (compare the middle bar to the bar on the far left). Snorts-as-she-pushes-her-glasses-to-her-nose.
4.) Exercise: Enough said. Wait, I'm not done. I change my mind. Look, I'm a die-hard fan of sweat sessions. I love it. It gives me endorphins, makes me strong and fast, and gets me closer to my pipe dream of being a stunt double for Dwayne Johnson. But exercise becomes pretty ineffective when you're down to that last dumb 10 pounds. This is because for most of us, exercise accounts for only 1 hour of our day and when we've already lost weight from moving more, our bodies become pretty dang efficient (burn less calories) when we adapt to physical activity. The calorie deficit you are able to create in 1 hour of physical activity is teeny weeny compared to the calorie deficit you can create by increasing your NEAT and decreasing your caloric intake. Fitness trackers overestimate calorie burn during workouts, too much exercise can stress the body to the point that it holds onto fat with a tighter grip (even if you feel like you've still got plenty to use), and high intensity workouts can make you hangry to the point to destroying your calorie deficit with one too many kind-bars, seeds/nuts, avocados, or any other "healthy" foods that we like to believe just aren't made of calories, but are. So rude.
Not to mention, the fitter your heart becomes, the more efficient you become during physical exertion. The more efficient you become, the fewer calories you will burn during your sweat sessions. For a lot of us, the positive internal changes from exercise show up before the external changes, causing us to believe that we aren't making any progress in our fitness because of the simple fact that we aren't losing any weight, which just isn't true.
Just because you aren't losing weight doesn't mean you aren't gaining fitness. It IS important to understand that the workout you were doing 20 pounds ago may have resulted in a high calorie burn, but as you become smaller and fitter, you will burn far less calories in the same workout. Further, because there's less of you, you require less calories to function (your BMR decreases slightly). But that's a whole other blog post about how your metabolism adapts to weight loss. I'll keep that in my back pocket for now.
To sum it up my fellow nerds, your metabolism is complicated, but understanding how it works doesn't have to be. Knowing how your body works is power when it comes to weight loss and it can save you a lot of headache and unnecessary stress having that knowledge to take with you as you continue getting healthier. So good on ya, friend, and keep up the good work.