Calorie Restriction is Not Sustainable, Here's Why
*As always, scroll to the very bottom for a TLDR synopsis*
Basically because you're body is REALLY smart and REALLY complex. Reducing your biochemical processes to a simple equation of calories in=calories out is 1) not true and 2) is a hard slap in the metaphorical face of your body. Not only is calorie restriction not sustainable, but it can really screw up your biochemistry in the long term. If you're looking to lose weight, there are much healthier and sustainable ways to do so (a post for another time). Calorie counting as a means of weight loss has place in very few situations and only for a short duration of time. Here's a quick overview of why;
In 2018, the weight loss industry in the U.S. alone made $72 billion. This is partly due to logical fallacies and partly due to a severe lack of regulation on "weight loss" and "diet" products sold.
Over millions of years, the body has developed mechanisms to resist starvation and not die in times of famine. You CANNOT bypass them simply by going on a diet. Dieting by calorie restriction doesn't result in weight loss long-term. Which is why if you want to experiment with any duration of fast, you must do so intelligently so that you can reap the benefits of cellular regeneration and not promote muscle degradation, metabolic slowing, or hormonal imbalances.
While there are any hormones involved in your weight and body composition, here's a quick rundown of three key ones...
1) Leptin (signals your brain that you are satisfied)
During a diet or in someone who has just undergone weight loss, leptin levels are reduced. Aka you feel less satisfied, so you feel the need to eat more to reach the same level of satisfaction.
2) Ghrelin (stimulates appetite)
When you calorie restrict, ghrelin levels increase. Ghrelin and leptin work together; hunger increases and satisfaction decreases. Studies have repeatedly shown that even after a year of weight loss and maintenance, these hormones do not return to their original levels before that weight loss.
3) Cortisol (stress hormone)
Calorie restriction increases cortisol levels. Research shows that even when weight is regained in dieters, cortisol levels are still elevated and much higher than in those who did not diet. As a result, the psychological reward of binge eating is substantially more appealing in dieters, as well as anxiety around food.
The general psychological response of calorie restricted dieters around food is an intensely high preoccupation with food. Interestingly, this bias towards food is only heightened as calorie restricted dieters also have an increased level of smell, further distracting them with thoughts of food.
On to the effects of metabolism. Metabolism refers to the range of biochemical reactions that occur in living organisms and is broken down as follows:
Calories burned or Total Daily Energy Expenditure (TDEE) = Resting Metabolic Rate (energy burned at complete rest in a 24hr period, typically 50-80% of TDEE) + Thermic Effect of Physical Activity (intentional movement and non-intentional movement) + Thermic Effect of Food (energy required to digest, process, and assimilate food).
In order, to maintain energy balance, when you decrease body weight, you also decrease TDEE. Calorie restriction leads to a drop in metabolic rate because you have less metabolically active tissue. Obviously this depends on what kind of tissue you have lost (as we know, the metabolic resting rate of muscle is much higher than that of fat- also why you want to be smart about fasting).
Physical activity: The energy required to move a lower bodyweight is decreased. In addition, any strength lost during calorie restriction will result in less strength to exert energy. Also, there is a lesser biological drive to move when at a less weight.
Food: When you eat less, you burn less digesting and absorbing nutrients.
This means that once your weight is lowered and you're back in energy balance, you will not continue to lose weight. Your TDEE will decrease to match your new calorie intake, which means that you cannot go back to eating the way you did before a "diet." Meaning as soon as you increase your calories again, there will be an immediate weight regain to put you back in balance. Maintaining that weight loss would mean sticking to a lower calorie intake inevitably.
Hormonal and psychological processes are trying to get you back to where you were before you started dieting. In addition, Adaptive Thermogenesis must be considered. Even if we reduce the TDEE model for the reduced muscle and fat mass effects, there's additional reduction in energy expenditure as our bodies become more metabolically efficient. We learn to do the same processes with less energy, which means that your body is putting you into a caloric surplus even with reduced energy intake.
There's a decrease in TDEE and an increase in appetite and hunger, meaning both sides of the equation are trying to get you back to your original weight. Change is possible, but the conversation must shift to body recomposition and proper education around nutrition is necessary. Not all weight loss is equal and the speed at which it happens is a determining factor in the type of weight lost. Most "diet" research only measures in the short term (due to limitations on funding duration...etc.).