Does exercise actually help you lose weight?
As a side effect.
"What does that mean?!"
Weight loss is incredibly simple. If you take in less calories than you burn you will lose weight. This is scientific fact. Your Total Daily Energy Expenditure (TDEE) is how many calories your body needs in a day to function. This is calculated by your Basal Metabolic Rate (BMR) which is determined by your weight and body fat and increased based on your activity. There's a full equation for it if you like, I prefer a simple calculator:
The simplest way to look at this is as follows:
I weigh 175 with 13.6% body fat and I exercise 4–5 hours per week.
My BMR is 1812, my TDEE is 2446.
If I desire to lose weight, I must take in less than 2446 calories per day. Only then will I lose weight. No matter what I do or eat, I will not lose weight if I take in more than 2446 calories.
Now, let's see what "As a side effect." means.
Let's cut out my entire exercise routine but keep eating the same diet. We'll be realistic and say you take stairs and move with a purpose a whole hour per week.
My BMR is still 1812, my TDEE is only 1993. Now I need to cut 500 calories from my diet every day to lose any weight!
As you can see, exercising a single hour a day (5 hours a week) increases my TDEE by 500/day.
Can you lose weight without exercise? Of course! Eat less. Way less. Or you can sacrifice one hour a day and enjoy (in my case) 500 more calories of food per day and still lose weight! (500 calories is almost a full meal)
Your body needs a certain amount of calories to function. If you eat less calories than this number, you will lose weight, Working out significantly increases caloric requirements and therefor improves weight loss immensely.