When you are consuming calories at a deficit level level, you are at a negative energy balance. This means your energy expenditure is greater than your energy intake.
Two things happen when in a deficit:
1. Your body will mobilise and use its stored energy. Depending on the type of deficit and the amount of time you spend in it, it will use either fat or lean muscle tissue to make up the difference that isn’t coming from calories consumed.
2. As your body adapts, it will recognise the need for survival as your stored energy supply ‘runs out’ and will attempt to advert this by adjusting your energy expenditure. It will slow your BMR, NEAT and naturally your exercise activity will reduce as fatigue sets in.
This second point is what happens when you spend too long in a calorie deficit and you experience your weight loss plateauing.
A healthier approach to a deficit includes reverse dieting and cycling regularly from a deficit back to maintenance. This will allow the body to adjust and adapt without the stress of survival taking over.