We were originally taught that 3500 kcal equals a pound, and if you could get a deficit of that amount, that’s what you would lose. What we now know is that works in a lab, but our bodies are just too resilient and adaptable for this to work in reality. In reality, when you cut your calories for a significant amount of time, your body begins to adapt, and burn less energy to do the same tasks. Your basal metabolic rate slows.
Why? Because our bodies are adapted for famine, not excess, and they are designed to keep us alive. Your body interprets a deficit as a threat, and counters that threat by slowing down for survival. So, yes, you need a calorie deficit, but it should’t be the main focus.
The focus should be feeding the body healthfully, ideally in the way it wants to be fed, being as active as possible, minimizing and modulating stress, getting enough sleep, addressing any medical barriers, and hopefully, our bodies will respond in kind by approaching a more healthful weight. Hence, my focus on complete wellness, and tailoring a diet plan specific to the individual. I don’t believe in one-size-fits-all health or diet plans–we are individuals and should be treated as such. There are calorie guidelines to follow for healthy weight loss: they are 1400-1600 kcal for women and 1600-1800 kcal for men.
I typically don’t recommend anything under 1200 kcal for extended periods or unless instructed specifically by a healthcare professional. I feel these diets should be utilized for short term weight loss goals (i.e. prior to bariatric surgery to reduce the size of the liver) in those who have very large amounts of weight to lose. You should generally supplement your diet with a multivitamin, because it is difficult to get the full spectrum of required nutrients below 1000-1200 kcal.