How do we lose fat? Let’s go back to the basics.
A calorie deficit is required for fat loss. Where do we start with knowing what a calorie deficit is? There are basic math equations that have been used in the health industry for decades to provide an estimate of a person’s metabolic rate based on things like their height and weight… and then you factor in activity level to come up with a number that should be pretty close to a person’s “maintenance calories”. This of course is just an estimate, because there is a little bit of variability between people, and things like health issues should also be factored in. So then from that amount, you subtract calories to create what we call a calorie deficit. When you eat fewer calories than your body needs, you are putting it in a position where it is required to then draw from your fat stores in order to provide the energy your body needs to function. Yay, fat loss.
The question then becomes, how much of a deficit should you put yourself in? There are a few factors to consider. A “typical” deficit would consist of around 500 cal less than where you maintain your current weigh. But that’s only going to give you about a pound of loss per week on average, which if you have a lot of weight to lose would be painfully slow. Or, maybe you have an event coming up where you want to lose a larger amount in a short amount of time. For these circumstances and others, a larger deficit might be more appropriate so that faster loss can be achieved. The cons of this would be that of course you’re eating less food, so you’re going to be more hungry, have less energy, and perhaps have a higher chance of “falling off the wagon”/stabbing someone out of hanger. But, if you like seeing fast lost to keep you motivated, and feel psychologically able to deal with the hunger, then it might be a good option for you. (Note that if protein is kept high enough, and resistance training is utilized, loss of lean mass should be mitigated even in a larger deficit.)
What is NOT true is that not eating enough is going to cause you to not be able to lose fat or to gain fat (“starvation mode”).
There are lots of factors to consider when deciding if you were going to diet aggressively or more reasonably… both approaches have pros and cons and neither should be immediately discounted.