Whilst your metabolism is a complex and dynamic system where inputs affect outputs and outputs affect inputs you cannot escape the laws of thermodynamics which dictate that to lose weight the volume of energy you expend must exceed the volume of energy you consume. Otherwise known as creating a negative energy balance or calorie deficit. To visualise this flow of energy in and out of your body I like to use a bathtub analogy as pictured in the rather crude drawing below.
In this analogy controlling your weight is akin to controlling the level of water in the bathtub. An increase in the water level corresponds to an increase in weight. A decrease in the water level corresponds to a decrease in weight.
To reduce energy intake you can turn down one or more taps (breakfast, lunch, dinner, snacks and caloric drinks!) To increase energy output you can increase the size of the drain (physical exercise!) But remember… there are five taps in and only one drain out.
Losing weight is predominantly determined by energy intake!
Last but not least you can build a bigger bathtub by increasing levels of lean mass through resistance exercise - it’s not a perfect analogy but this means the same flow of energy into your body creates a smaller overall rise in weight as you are burning more calories at rest.